Unit 1
Unit 1
Title Goes Here
Part A: Review of Thermodynamics
If you have taken Chemistry 217: Chemical Principles I and Chemistry 218: Chemical Principles II from Athabasca University, you will find it helpful to reread Chapter 5 (pp. 132–166) and Chapter 19 (pp. 669–696) in Chemistry: The Central Science, 5th ed., by Brown, LeMay and Bursten. The following is a brief review of the basic thermodynamics you will need to complete Chemistry 330: Environmental Chemistry.
Energy
The changes in internal enery (DE) of a system can be represented by
DE + q ) w
where q is heat added to the system, and w is work done on the system.
Sign Convention
Often students have difficulty determining the sign (* or )) associated with work or heat. The sign determines the “direction” of the work or heat. Heat flowing into a system (from surroundings) and work done on a system (by surroundings) is always positive. The schematic shown in Figure A.1, below, should help you remember.
[pic]
Figure A.1: Direction of work or heat
Note: This sign convention also applies to DE and DH.
The table below will give you some examples to reinforce this idea.
|Event | |Work | |Heat |
| | |(w) | |(q) |
|Two chemicals react and the flask becomes warm | | | |* |
|Elevation of a weight | |) | | |
|A pot of water on a stove begins to boil | | | |) |
|A coiled/compressed spring expands | |* | | |
|An ice cube melts | | | |) |
|Garbage is compressed by a trash compactor | |) | | |
|[pic] | |* | | |
|A candle burns | | | |* |
Work
For ideal gases where
PV + nRT
and
V + volume
P + pressure
n + number of moles
R + ideal gas constant
T + temperature
work can be expressed as
w + *PEXTDV
where PEXT is the external pressure on the system and DV is the change in volume of the system.
Heat Capacity
The total heat flow (q) into or out of an object can be represented by
q + nCDT
where n is the number of moles, DT is the change in temperature and C is the molar heat capacity (dependent on the nature of the material in the object).
Enthalpy
At constant pressure DH can be expressed as
DH + DE ) PDV or DH + q
When DH is positive the reaction is said to be “endothermic.”
When DH is negative the reaction is said to be “exothermic.”
State Functions
E + internal energy
H + enthalpy (heat of reaction)
S + entropy (measure of disorder)
G + Gibb’s free energy (measure of spontaneity)
Because these are all “state functions,” we can use Hess’s law to calculate the overall state function of a reaction by adding the series of individual steps that will get us from reactant to product.
The heat of reaction (DH°RXN) can be calculated from the difference in the heats of formation (DHf°) of the products and reactants.
DH°RXN + SDHf° (products) * SDHf° (reactants)
In a similar fashion, the free energy of a reaction (DGRXN) can be calculated from the free energies of formation (DGf°).
DG°RXN + SDGf° (products) * SDGf° (reactants)
Finally, the change in entropy in a reaction (DS°RXN) is a function of the absolute entropies (S°) of the reactants and products.
DS°RXN + SS° (products) * SS° (reactants)
Spontaneity
A reaction is spontaneous (thermodynamically) if DG is negative (DG t 0). Gibb’s free energy can be represented by:
DG + DH * TDS
Under nonstandard conditions
DG + DG° ) RT lnQ where Q is the reaction quotient.
If the system comes to equilibrium (Q + K and DG + 0), then the above equation becomes
DG° + *RT lnK where K is the equilibrium constant.
Please keep the following definitions in mind.
For the general reaction:
xA ) yB → sC ) tD
For the equilibrium reaction
xA ) yB ↔ sC ) tD
Part B: Review of Chemical Kinetics
If you have taken Chemistry 217: Chemical Principles I and Chemistry 218: Chemical Principles II from Athabasca University, you will find it helpful to reread Chapter 14 (pp. 476–510) in Chemistry: The Central Science, 5th ed. by Brown, LeMay and Bursten. The following is a brief review of the basic chemical kinetics you will need to complete Chemistry 330: Environmental Chemistry.
The rate of a reaction is expressed as changes in concentration per unit of time (for solutions the units are M/s). This rate can be written as the appearance of
a product or the disappearance of a reactant. For the following reaction:
A → B
The relationship between reaction rate and concentration is expressed by the “rate law.” The general form of this rate law is as follows:
Rate + k[A]x[B]y[C]z . . .
for the general reaction
A ) B ) C . . . → Products
where k is the “rate constant, X, Y, Z . . . are the reaction orders and A, B, C . . . are the reactants. [Note: The rate law cannot be determined by the coefficients in the chemical equation. It is determined experimentally.] The rate constant
is specific to the nature of each reaction and will vary only with a change
in temperature. The order of a reaction can be determined by its rate law. Working through the examples in the table below will quickly clarify
this point.
|Rate law | |Order of | |Order of | |Order of | |Overall |
| | |“A” | |“B” | |“C” | |order |
|Rate + k[A] | |1 | |*0 | |*0.5 | |1.5 |
|Rate + k[A][B] | |1 | |*1 | |*0.5 | |2.5 |
|Rate + k[A][B][C] | |1 | |*1 | |*1.5 | |3.5 |
|Rate + k[A]2[B] | |2 | |*1 | |*0.5 | |3.5 |
|Rate + k[B]2[C]1/2 | |0 | |*2 | |*0.5 | |2.5 |
|Rate + | |2 | |*1 | |*0.5 | |1.5 |
|Rate + | |1 | |*1 | |*25 | |0.5 |
|Rate + k[A][B]2[C]2 | |1 | |*2 | |*2.5 | |5.5 |
As you can see, rate laws can become very complicated expressions. For this course, you will be primarily dealing with first–order, second–order and pseudo first–order reactions.
First–order Reactions
Rate + k[A]
or the integrated form
ln[A]t + *kt ) ln[A]0
where [A]t is concentration of A at time t and [A]0 is the initial concentration of A. You should note that a graph of ln[A]t versus time will give a straight line of slope k.
[pic]
First–order reactions also have a constant half–life (t1/2) expressed by:
Second–order Reactions
Rate + k[A]2
or the integrated form
In this case a graph of versus time (t) gives a straight line of slope k.
[pic]
Pseudo First–order Reactions
If a higher order reaction has reactants that remain constant in concentration, it can be approximated as first–order for calculation purposes. For example, for the following second–order rate law:
Rate + k[A][B] (second–order)
if [B] remains relatively constant, as in a steady–state situation, then [B] can be treated as a constant
[B] [ k__
so Rate + k[A][B] [ kk__[A] + K[A]
therefore Rate [ K[A] (pseudo first–order)
As long as [B] is constant, we can simplify our calculations by treating this reaction as pseudo first–order.
Activation Energy
The minimum energy required for a reaction to occur is known as the activation energy (Ea), and can be determined by the Arrhenius equation:
where k is the rate constant, A is the frequency factor (a constant dependent on the nature of the reaction), T is temperature and R is the ideal gas constant.
A graph of ln k versus gives a straight line whose slope is .
[pic]
A catalyst can lower the activation energy of a reaction by providing a different reaction mechanism. Essentially, the catalyst speeds up the reaction without undergoing net chemical change itself.
[pic]
Reaction Mechanisms
A reaction mechanism is merely a series of elementary reactions that take place in sequence. The slowest reaction in this multistep sequence is the rate determining step. Consider the overall reaction of:
A ) 2B → D
The detailed mechanism of this reaction occurs the two steps shown below:
A ) B [pic] C (fast)
C ) B [pic] D (slow)
In the example above, the second reaction is the rate determining step, and the rate law will take the form of that step:
Rate + k[C][B]
Note that from the equilibrium constant, K, in the first step, we can find an expression for [C].
K + or [C] + K[A][B]
If we substitute this value for [C] into the rate law dictated by the rate determining step, we would be able to write an expression for the rate law based on only the reactants (i.e., not any of the intermediates such as C).
Rate + k[C][B] + kK[A][B][B] + kK[A][B]2
Part C: Review of Photochemistry
If you have taken Chemistry 217: Chemical Principles I and Chemistry 218: Chemical Principles II from Athabasca University, you will find it helpful to reread Chapter 6 (Sections 6.1–6.3, pp. 168–182) and Chapter 18 (Sections 18.2–18.3, pp. 642–647) in Chemistry: The Central Science, 5th ed., by Brown, LeMay and Bursten. The following is a brief review of the basic photochemistry you will need to complete Chemistry 330: Environmental Chemistry.
Electromagnetic radiation is described with its wave–like properties in a single equation
nl + c
where n is frequency, l is wavelength and c is the speed of light. Various forms of radiant energy exist and are defined by the wavelengths that make up the various regions of the electromagnetic spectrum.
|Approximate wavelength (m) | |Region of spectrum |
|10*14 | |Cosmic rays |
|10*11 | |Gamma rays |
|10*9 | |X–rays |
|10*8 | |Ultra violet |
|10*7 | |Visible |
|10*6 | |Infrared |
|10*2 | |Microwaves |
| 1* | |Television |
|101* | |Radio |
The energy of this radiation is quantized into small packets of energy, called photons, which have particle–like nature. Elecromagnetic radiation can be pictured as a stream of photons. The energy of each photon is given by
where h is Plank’s constant (6.626 10*34 J · s). Note that the shorter the wavelength the higher the energy of each photon. Radiation in the ultraviolet region (high–energy end of the visible light spectrum) can cause chemical changes. Radiation in the infrared region (lower–energy heat radiation) can be absorbed and reradiated by some molecules. Both of these spectral regions are important to atmospheric chemistry.
Infrared Radiation
The vibrational frequency of molecules generally falls in the infrared region of the electromagnetic spectrum. A molecule can have various modes of vibration that exist at different energy levels (see Figure C.1, below).
[pic]
Figure C.1: Types of vibrational motions (not in order of energy level)
Such a molecule can absorb infrared radiation to move to a higher energy vibrational mode and then emit infrared radiation by going to a lower energy vibrational mode (see Figure C.2, below). Note that monoatomic molecules (e.g., He) and diatomic molecules of the form A–A (e.g., O2) do not absorb infrared radiation.
[pic]
Figure C.2: Absorption and emission of infrared energy
Ultraviolet Radiation
Ultraviolet radiation is higher energy (shorter wavelength) compared to infrared radiaiton. When a molecule absorbs a photon of ultraviolet radiation the molecule moves to a higher–energy electronic state. The molecule with this excess energy is said to be “excited.”
[pic]
Once the molecule is in this excited state, there are several processes that may occur depending on the nature of the excitation and the specific system involved. The three processes that will be discussed in this course are
1. radiationless deactivation.
2. photodissociation.
3. photoionization.
Radiationless Deactivation
The excited molecule moves to the ground state by loss of vibrational energy and physical interaction with other molecules.
Photodissociation
In this process, the bonds in an excited molecule break and generate radical components. For example:
These radicals formed in the atmosphere are extremely reactive, and form the base of a very rich and varied chemistry.
Photoionization
The molecule is so excited that it loses its highest–energy electron. For example:
This process requires a lot of energy (i.e., short–wavelength photons) and would only occur above 90 km in the atmosphere. These short wavelengths of high–energy ultraviolet radiation are completely filtred out by the time sunlight reaches ground level.
Part D: Selected Physical Constants and Data
Physical Constants
R + 8.314 J · K*1 · mol*1 or 0.08206 L · atm · K*1 · mol*1
h + 6.626 10*34 J · s
N + 6.023 1023 mol*1
e + 1.6021 10*19 coulomb
F + 96,485 C · mol*1 or 96,485 J · V*1 · mol*1
c + 3.00 108 m · s*1
Solubility Product Constants (Ksp)
AgBr 8 10*13 Cu(OH)2 2 10*20
Ag2CO3 6 10*12 CuS 1 10*36
AgCl 1 10*10 Fe(OH)3 1 10*36
Ag2CrO4 2 10*12 Hg2Br2 3 10*23
Ag[Ag(CN)2] 4 10*12 Hg2Cl2 6 10*19
AgI 1 10*16 HgS 1 10*52
Ag3PO4 1 10*19 KClO4 2 10*2
Ag2S 1 10*50 MgCO3 1 10*5
AgCNS 1 10*12 MgC2O4 9 10*5
Al(OH)3 2 10*32 MgNH4PO4 2 10*13
BaCO3 5 10*9 Mg(OH)2 1 10*11
BaCrO4 1 10*10 MnS 1 10*15
BaC2O4 2 10*8 PbCrO4 2 10*14
BaSO4 1 10*10 PbS 1 10*28
CaS 1 10*28 PbSO4 2 10*8
CaCO3 5 10*9 SrCrO4 4 10*5
CaF2 4 10*11 Zn(OH)2 5 10*18
CaC2O4 2 10*9 ZnS 1 10*24
Water Vapour Pressure (mm)
0°C 4.6 25°C 23.8
15°C 12.8 30°C 31.8
20°C 17.5 50°C 92.5
Average Bond Energies (kJ/mol)
C-H 413 C*C 348 C*N 293 C*O 358
C*F 485 C*Cl 328 C*Br 276 C__C 614
C__O 799 C__N 615 C5C 839 C5N 891
H*H 436 H*F 567 H*Cl 431 H*O 463
N*H 391 N*N 163 N__N 418 N5N 941
Ionization Constants—Acids (Ka)
Acetic 1.8 10*5 H2S K1 9 10*8
Arsenic K1 5.6 10*3 K2 1 10*15
K2 1 10*7 Oxalic K1 5.9 10*2
K3 3.0 10*12 K2 6.4 10*5
Benzoic 6.5 10*5 Phenol 1.3 10*10
Boric K1 5.8 10*10 Phosphoric K1 7.5 10*3
Carbonic K1 4.3 10*7 K2 6.2 10*8
K2 5.6 10*11 K3 4.2 10*13
Chromic K1 1.5 10*1 Succinic K1 7 10*5
K2 1 10*7 K2 2.5 10*6
Citric K1 3.5 10*4 Sulphuric K1 none (dissociates
K2 1.7 10*5 completely)
K3 4.0 10*6 K2 1.2 10*2
Formic 1.8 10*4 Sulphurous K1 1.7 10*2
Hydrocyanic 4.9 10*10 K2 6.4 10*8
Hydrofluoric 6.8 10*4 CCl3COOH 2 10*1
CHCl2COOH 5 10*2
Ionization Constants—Bases (Kb)
Acetate ion 5.3 10*10 Methylamine 4.4 10*4
Aminopyridine 5.0 10*8 Phosphate ion 2.5 10*2
Ammonia 1.8 10*5 Pyridine 1.7 10*9
Aniline 4.3 10*10 Triethylamine 6.4 10*5
Hydrazine 1.3 10*6 Urea 1.5 10*14
Hydroxylamine 1.1 10*8 Dimethylamine 6.4 10*4
Thermodynamic Quantities for Selected Substances (at 25°C)
Substance DHf° (kJ/mol) DGf° (kJ/mol) S° (J/mol)
AlCl3 (s) *705.6 *630.0 51.00
Br (g) 111.8 82.38 174.9
Br2 (g) 30.71 3.14 245.3
Br2 (l) 0 0 152.3
HBr (g) *36.23 *53.22 198.49
CaSO4 (s) *1434.0 *1321.8 106.7
C (g) 718.4 672.9 158.0
C (s, graphite) 0 0 5.69
CH4 (g) *74.8 *50.8 186.3
CCl4 (g) *106.7 *64.0 309.4
CF4 (g) *679.9 *635.1 262.3
CH3CH2OH (g) *201.2 *161.9 237.6
CH3CH2OH (l) *277.7 *174.76 160.7
CO (g) *110.5 *137.2 197.9
CO2 (g) *393.5 *394.4 213.6
Cl (g) 121.7 105.7 165.2
Cl* (aq) *167.2 *131.2 56.5
Cl2 (g) 0 0 222.96
F (g) 80.0 61.9 158.7
F2 (g) 0 0 202.7
H (g) 217.94 203.26 114.60
H) (aq) 0 0 0
H2 (g) 0 0 130.58
FeCl2 (s) *341.8 *302.3 117.9
Fe2O3 (s) *822.16 *740.98 89.96
I (g) 106.6 70.16 180.66
I2 (g) 62.25 19.37 260.57
I2 (s) 0 0 116.73
N (g) 472.7 455.5 153.3
N2 (g) 0 0 191.50
NH3 (aq) *80.29 *26.50 111.3
NH3 (g) *46.19 *16.66 192.5
NO (g) 90.37 86.71 210.62
NO2 (g) 33.84 51.84 240.45
O (g) 247.5 230.1 161.0
O2 (g) 0 0 205.0
O3 (g) 142.3 163.4 237.6
OH* (aq) *230.0 *157.3 *10.7
H2O (g) *241.8 *228.61 188.7
H2O (l) *285.85 *236.81 69.96
S (s, rhombic) 0 0 31.88
SO2 (g) *296.9 *300.4 248.5
SO3 (g) *395.2 *370.4 256.2
H2S (g) *20.17 *33.01 205.6
H2SO4 (aq) *909.3 *744.5 20.1
Unit 2
Stratospheric Chemistry:
The Ozone Layer
Introduction
Overview
Ozone in the stratosphere is our invisible shield against harmful high–energy radiation from the Sun. It is only recently that we have become aware of the existence and importance of the ozone layer—as it has been compromised.
In Unit 2, we consider the topic of stratospheric ozone, the chemical reactions involved in its creation, and the factors that contribute to its depletion.
The photochemical mechanisms that occur at low concentrations in the stratosphere when forming and destroying ozone are many and complex.
This course will discuss the principal reactions and the roles they play in detail.
Note: Unit 1 in this Study Guide consists of reference material you may need for the course. The numbers of the other units match those of the chapters in the textbook, Environmental Chemistry by Colin Baird.
Objectives
After completing this section, you should be able to
1. describe ozone’s role in filtering out harmful ultraviolet light.
2. state the normal range of overhead ozone on Earth in Dobson units.
3. describe in general terms the recent history of stratospheric ozone levels worldwide and specifically above the Antarctic.
Reading Assignment
Read pages 17–21 in the textbook.
Key Terms
ozone layer
ultraviolet (UV)
Dobson units (DU)
Study Notes
This section is a general introduction to the rest of Chapter 2 in the textbook.
It is important in this section to realize that ozone specifically filters out harmful ultraviolet radiation from the Sun (Objective 1). We will see in more detail in later sections how this is achieved.
Stratospheric ozone is sometimes referred to as the “ozone layer” and local depletions of ozone are sometimes referred to as “ozone holes.” Although these terms are picturesque, they can also be misleading. They seem to imply that there is a specific physical “layer” of ozone that encapsulates the earth and that can be punctured in a precise fashion to form “holes.” In reality, ozone is simply a minor gaseous component of our atmosphere, which is present in some concentration at almost any altitude around the globe. The amount of ozone directly overhead is usually measures about 350 DU, but can vary from 250 to 450 DU (Objective 2).
You are not required to know the history of the discovery and study of the stratospheric ozone layer in detail (Objective 3). Just remember that it is a relatively recent realization that worldwide levels of ozone are decreasing (keep Figure 2–4 on page 20 of the textbook in mind) and that there are local minimums at the poles—especially in the Antarctic. In addition, note that the ozone levels vary with season as shown in Figure 2–1 on page 18. The seasonal variation in the Antarctic is unique and dramatic, but there are special circumstances at play there. We will study the Antarctic system in greater detail in a subsequent section in Unit 2.
Note: The examinations in this course are based on the learning objectives. You are also responsible for a working knowledge of all of the key terms for every unit and should be able to define them in your own words.
Exercises
No exercises have been assigned for this section.
Regions of the Atmosphere and
Environmental Concentration Units for Gases
Objectives
After completing this section, you should be able to
1. list the major components, and the important minor components, of the atmosphere.
2. state the approximate altitudes where troposphere and stratosphere exist.
3. perform calculations using the ideal gas law.
4. interconvert absolute and relative gas concentrations.
Key Terms
troposphere
stratosphere
absolute concentration
relative concentration
mole fraction
partial pressureS
molecules per cubic centimeter (molecules cm*3)
parts per million by volume (ppmv)
ideal gas law (PV + nRT )
Reading Assignment
Read pages 21–23 in the textbook.
Study Notes
You should be familiar with the percentages given for atmosphere components (Objective 1) in the first paragraph of Regions of the Atmosphere in the textbook. Note that water vapour is considered an important minor component of the atmosphere, as you will see in our discussion on global warming. However, the textbook does not quote a water vapour percentage because its concentration fluctuates greatly around the globe.
One important point to note is that the troposphere and stratosphere (and other regions of the atmosphere) are defined by their temperature profile rather than their absolute altitude above the ground (see Figure 2–5b on p. 22 of the textbook). The size of the troposphere varies around the globe and is larger at the equator than at the poles. In the colder months of the year, the stratosphere can actually come down to ground level at the poles! Still, to state the approximate altitudes of the troposphere and stratosphere (Objective 2), use the altitudes quoted in the textbook (troposphere 0–15 km above sea level; stratosphere 15–50 km) as a rule of thumb.
This course builds on fundamental knowledge from first–year general chemistry. You are expected to be able to perform calculations for gases. This includes using the ideal gas law (Objective 3), as well as related concepts such as Dalton’s law of partial pressures
p(total) + p(Gas A) ) p(Gas B) ) p(Gas C) ) . . . etc.
where the mole fraction of any particular ideal gas can be given by its partial pressure over the total pressure of all the gases.
mole fraction of Gas A +
Finally, it is important that you understand the difference between absolute and relative concentrations (Objective 4). You should be able to express
gas concentrations in terms of “molecules per centimetre” or “parts per million by volume” or as a “partial pressure” given any one of the three.
This includes using a variety of related units associated with ppmv (e.g., ppbv and pptv) or with partial pressures (e.g., 1 atm + 760 Torr + 760 mm Hg + 101.325 kPa + 101325 Pa).
Warning: When working with gas mixtures, note that the unit of measurement “ppmv” (parts per million by volume) is sometimes referred
to as just “ppm.” One ppmv corresponds to p(total) + 10*6 (i.e., one one–millionth of that pressure or volume). So, for example, at sea level (where the pressure is 1 atm), 1 ppmv is equivalent to 10*6 atm.
Later in this course, when measuring liquids and solids, we will use parts per million by mass (also abbreviated as ppm). Make sure you recognize when “ppm” refers to parts per million by volume (for gases) and when it refers to parts per million by mass (for liquids and solids). Do not confuse the two units; they are not the same!
Exercises
Question 2–A
Neon makes up 0.0018% of the air in the atmosphere. Assume atmospheric pressure (temperature) is 1.0 atm (25°C) at sea level and 0.0030 atm (*10°C) at 40 km (high in the stratosphere).
a. Calculate the partial pressure of Ne (in atm) at each altitude.
b. Convert these partial pressures to units of ppmv.
c. Compare your values in Parts (a) and (b). What is the limitation of reporting relative concentrations in ppmv as compared to partial pressures?
Do Additional Problem 1, which can be found at the end of the chapter on page 76 of the textbook.
Note: There is a list of helpful physical constants in Unit 1 of this Study Guide that can be used in solving problems. Also, keep your first–year general chemistry textbook handy to jog your memory on basic formulae and calculations.
The Chemistry of the Ozone Layer—
Light Absorption by Molecules
Objectives
After completing this section, you should be able to
1. list the relative positions of x–rays, ultraviolet, visible, and infrared radiation by energy and wavelength.
2. list the various types of ultraviolet (UV) radiation and their corresponding spectral ranges.
3. state which atmospheric gas is primarily responsible for absorption of sunlight in the 120–220 nm, 220–320 nm, and 320–400 nm wavelength regions.
Key Terms
ultraviolet (UV)
infrared (IR)
absorption spectrum
UV–A
UV–B
UV–C
Reading Assignment
Read pages 23–26 in the textbook.
Study Notes
Figure 2–6 (p. 24 in the textbook) gives the relative order of x–rays, ultraviolet, visible, and infrared radiation by wavelength. Note that shorter wavelength implies higher energy, so that x–rays are of shorter wavelength and higher energy radiation than infrared radiation (Objective 1). Although you are required to know exact wavelength ranges for UV–A, UV–B, and UV–C (Objective 2), you are only required to know relative positions for the other regions shown in Figure 2–6. You can find a more complete electromagnetic spectrum in the Review of Photochemistry in Unit 1.
Molecules of oxygen, ozone, and nitrogen dioxide are primary absorbers of sunlight in the 120–220 nm, 220–320 nm, and 320–400 nm wavelength regions (Objective 3).
Exercise
Question 2–B
The Beer–Lambert law describes light absorbance (A) as a function of extinction coefficient (e), path length (b), and concentration of molecules (c).
A + ebc
The table below gives the extinction coefficients for ozone as a function of wavelength in the UV–B region of the spectrum.
|e (cm*1) | |l (nm) | | |
|100 | |280 | | |
|32 | |290 | | |
|10 | |300 | | |
|3 | |310 | | |
|0.8 | |320 | | |
One sample of air is exposed to light with a wavelength of 280 nm, while a second sample containing twice the ozone is exposed to a wavelength of 310 nm. What would the relative height of the two air columns have to be to absorb the same amount of light?
The Chemistry of the Ozone Layer—
Biological Consequences of Ozone Depletion
Objectives
After completing this section, you should be able to
1. state expected health and environmental results of reduced ozone levels in the stratosphere.
2. explain the variation UV radiation with latitude, time of day, altitude ozone thickness and cloud cover.
3. explain how UV radiation causes skin cancer and interferes with photosynthesis.
Key Terms
skin cancer
DNA molecules
malignant melanoma
sunscreen
basal cell carcinoma
cataract
photosynthesis
phytoplankton
Reading Assignment
Read pages 26–30 in the textbook.
Study Notes
Why is ozone depletion of such great concern? The less ozone between the sun and the earth’s surface, the more UV–B radiation (290–320 nm) can penetrate to ground level. The increase in UV–B and potential overexposure is of primary concern, because it can generate a wide range of detrimental biological effects such as skin cancer, eye cataracts, and interference with photosynthesis (Objective 1).
There is a strong positive correlation between incidence of skin cancer
and exposure to UV–B radiation. It is helpful to think of the various trends described in this section (i.e., geography, time of day, type of UV radiation)
in terms of how they affect the radiation dosages on the Earth’s surface. The larger the radiation dose the more incidences of skin cancer will appear in a given population. A concise summary of the trends is given below in Table 2.1 (Objective 2).
|Table 2.1: Trends in skin cancer and UV radiation dose |
|type of light | |low frequency | |high frequency | |
|latitude | |poles | |equator | |
|time of day | |early or late | |midday* | |
|incidences of skin cancer | |few | |many | |
|*Midday is considered to be 11:00 a.m. to 4:00 p.m. |
Note that for both time of day and latitude the angle of the Sun to the Earth is important. The distance that sunlight travels through the “ozone column” and high–energy radiation is absorbed is at a minimum when the Sun is directly overhead. At more oblique angles the sunlight must travel through more atmosphere before reaching the Earth’s surface.
The mechanism of the biological problems associated with UV–B exposure is the bond dissociation by absorbed high–energy UV–B radiation and subsequent mutations that occur in DNA (Objective 3).
Exercises
No exercises have been assigned for this section.
The Chemistry of the Ozone Layer—
Principles of Photochemistry
Objectives
After completing this section, you should be able to
1. describe the process of photolysis in a qualitative manner.
2. write balanced chemical equations for the photochemical cleavage of oxygen and ozone molecules in the stratosphere.
3. calculate the maximum wavelength of the photon involved given the D H (or D E ) of a photochemical cleavage reaction.
4. explain why a photon of sufficient energy to break a bond in a molecule may not necessarily photolytically dissociate that molecule.
Key Terms
photon
Plank’s constant (h)
enthalpy change (D H)
energy change (D E)
photochemical reaction
photolysis
ground state
excited state
Reading Assignment
Read pages 30–33 in the textbook.
Study Notes
In describing photolysis (Objective 1), you should include the role of the absorbed photon in changing the electronic state of a molecule from a ground to an excited state. In addition, mention how the excess absorbed energy can be lost as heat (mechanical motion) or lead to the dissociation of the molecule. If you need background information, read the Photochemistry Review in Unit 1.
The photolytic cleavage of molecular oxygen and ozone is given below (Objective 2):
O2 ) UV photon (l t 241 nm) ³ 2O
O3 ) UV photon (l t 320 nm) ³ O2 ) O
When given a bond dissociation energy (D E) or enthalpy (D H), use the relationship E + to determine the maximum wavelength (l) required to photolyze that bond (Objective 3). In this particular type of calculation, we can assume D E + D H. It is also important to note that D E and D H values (given in kJ mol*1 ) are macroscopic and the energy of one photon (defined by its wavelength) is microscopic. To relate these two types of values you need to use Avogadro’s number (6.023 1023 mol*1).
Re–read the paragraph at the top of page 33 and make sure you understand that a sufficiently energetic photon that is not absorbed by a molecule cannot possibly photodissociate that molecule (Objective 4).
Exercises
Do Problems 2–1, 2–2, and 2–3 within the chapter.
The Chemistry of the Ozone Layer—
The Creation and Noncatalytic Destruction of Ozone
Objectives
After completing this section, you should be able to
1. write balanced equations for the photochemical reactions involved in the production of ozone in the stratosphere.
2. write balanced equations for the noncatalytic reactions involved in the destruction of ozone in the stratosphere.
3. explain why ozone is concentrated in the stratosphere.
4. state the general variation in temperature in the troposphere and stratosphere.
5. explain the temperature profile observed for the troposphere and stratosphere.
Key Terms
ozone layer
temperature inversion
Chapman cycle
reaction mechanism
Reading Assignment
Read pages 33–36 in the textbook.
Study Notes
Four reactions are discussed in this section in detail. They are summarized in Figure 2–12 (p. 36) as a schematic of the so–called Chapman cycle. The Chapman cycle is also shown explicitly below in Equations 1–4 below. The first two (Eq 1 and 2) are ozone formation and the last two are noncatalytic ozone destruction (Eq 3 and 4). You must memorize these four equations representing this reaction mechanism (Objectives 1 and 2).
Chapman mechanism:
O2 ) hn (l t 241 nm) ³ 2O (1)
O ) O2 ) M ³ O3 ) M (heat released) (2)
O3 ) hn (l t 320 nm) ³ O2 ) O (3)
O ) O3 ³ 2O2 (heat released) (4)
(Note that M denotes a generic atom or molecule present in the gas phase that acts as a third body to stabilize products by absorbing excess energy from the reactants through collisions.)
Figure 2–5a (p. 22 in the textbook) shows that ozone peaks in the stratosphere at an altitude of about 25 km. Why is ozone concentrated in the stratosphere (Objective 3)? High–energy radiation (l t 241 nm) is required in the formation of ozone. This occurs in the upper part of the stratosphere and is directly proportional to the intensity of sunlight. At lower altitudes, where the atmosphere density is greater, less high–energy sunlight can penetrate to form ozone. Above the stratosphere the atmosphere becomes so thin that few molecules of ozone are able to form even though there is a greater intensity of energy radiation.
Figure 2–5b (p. 22 in the textbook) shows the temperature profile in both troposphere and stratosphere (Objective 4). The temperature in the troposphere is controlled by absorbed infrared radiation emitting from the Earth’s surface. Increased distance from the surface results in a lower temperature. However, in the stratosphere a temperature inversion occurs through increased occurrence of ozone formation (Eq 2 above), which is in turn controlled by incoming sunlight (l t 241 nm). Together these processes determine the temperature profile (Objective 5) and therefore the distinction between troposphere and stratosphere.
Exercises
Do Problems 2–4 and 2–5 within the chapter.
Do Additional Problem 3 at the end of the chapter on page 76.
The Chemistry of the Ozone Layer—
Catalytic Processes of Ozone Destruction
Objectives
After completing this section, you should be able to
1. calculate rate, rate constant or reactant concentration using the rate law for a given reaction.
2. perform calculations involving the general (or integrated) equation for first– and second–order reactions.
3. perform calculations that involve the Arrhenius equation.
4. identify a radical given the structure and charge of a molecule or atom.
5. list at least four radical catalysts that are known sinks for ozone.
6. state the difference between the terms equilibrium and steady state system.
7. write the two–step mechanism in which species X (e.g., hydroxyl radical) catalytically destroys ozone in the middle to high stratosphere.
8. explain why Mechanism II (Figure 2–14 on p. 44 in the textbook) is the dominant mechanism for catalytic destruction of ozone in the lower stratosphere.
Key Terms
overall reaction
catalyst
catalytic mechanism
free radical
nitric oxide radical (NO@)
nitrogen dioxide radical (NO2@)
nitrous oxide (N2O)
rate constant (k)
hydroperoxy radical (HOO)
hydroxyl radical (OH)
equilibrium
steady state
pseudo first–order
rate law
Arrhenius equation
activation energy
frequency factor
self–healing
Reading Assignment
Read pages 36–44 in the textbook.
Study Notes
This section covers several concepts and will require some additional study compared with some of the other sections. Do not panic! Take a deep breath and realize that you will need to spend more time on this section, but the material is manageable.
You will have covered the material you need achieve Objectives 1 through 3 in the kinetics portion of your first–year chemistry course. The following overview is designed to refresh your memory, but if you need more help, the Review of Chemical Kinetics in Unit 1 offers a more detailed discussion of first–year chemical kinetics.
The rate of first–order reactions is governed by a single reactant (A) raised to the first power. The general and integrated forms of this process are, respectively,
rate + k[A] and ln[A]t + *kt ) ln[A]0
where k is the rate constant, [A]t is the concentration of A at time t and [A]0 is the initial concentration of A. The more complex second–order reactions of the form rate + k[A]2 take the integrated form of + ) kt. If the rate law takes the second–order form of rate + k[A][B], where it is first–order with respect to both reactants A and B, it is sometimes helpful (in special circumstances) to treat it at a pseudo first–order reaction. That is, if [A] or [B] does not change substantially (e.g., an intermediate in a steady state situation) the rate law is essentially first–order. For example:
rate + k[A][B] (second–order reaction)
However, if [A] is constant then k[A] is also constant, so
rate + k__[B] (pseudo first–order reaction)
where k__ + k[A]
The rate of a reaction varies with temperature. Given various rates at different temperatures the activation energy, Ea, can be determined using the Arrhenius equation:
where k is the rate constant, T is the absolute temperature, R is the ideal gas constant (8.314 J K*1 mole*1) and A is a constant known as the “frequency factor.”
Review Box 2–1 (p. 38 in the textbook) because it is especially useful in introducing you to the art of identifying radicals by first thinking about the Lewis structure of a molecule or an atom (Objective 4).
Warning: The textbook is inconsistent in denoting radicals. In many cases it shows a “dot” to indicate the one unpaired electron. However, some examples in the textbook do not have the dot so the reader is left to assume the species is a radical. [dk, does the dot mean the species is or isn’t a radical?] In the Athabasca University materials we will not use the dot. You should know that species such as OH, CH3, ClO, H3COO, and others are radical species.
The production–destruction cycle (Chapman cycle) and the destruction of ozone by free radical catalysts (i.e., Cl, NO, OH and H) is a natural phenomenon (Objective 5). It is only when the concentrations of these radical catalysts become artificially high, through man–made routes, that there is a serious concern that the ozone sink will become too great. The following information will help you meet Objective 6. Most of the reactions occurring in the stratosphere, such as the Chapman mechanism, are driven by sunlight. Although the intermediates generated reach a constant concentration, it is not a closed system and therefore not an equilibrium system. In other words, in an equilibrium state the concentrations are constant because the rate of product formation (A) equals the rate of reactant formation (B). In a steady state situation, the concentration of the intermediate is constant because its rate of formation (A) equals its rate of destruction (B) in two separate reactions (see Figure 2.1).
Figure 2.1: Equilibrium versus steady state system
Finally, Mechanism I shown in Figure 2–14 (p. 44 in the textbook) summarizes the two–step catalytic destruction of ozone in the middle to upper part of the stratosphere (Objective 7). Mechanism II starts to dominate in the lower stratosphere where there is more catalyst (X) available and the concentration of oxygen radicals is lower (Objective 8).
Exercises
Do Problems 2–6, 2–7, 2–8, 2–9, and 2–10 in the chapter.
Do Additional Problems 4 and 6 at the end of the chapter on page 76.
The Chemistry of the Ozone Layer—
Atomic Chlorine and Bromine as X Catalysts
Objectives
After completing this section, you should be able to
1. write the two–step mechanism in which species X (where X + Cl or Br) catalytically destroys ozone in the middle to high stratosphere.
2. list at least two inactive or reservoir forms of Cl and Br in the stratosphere.
3. explain why stratospheric bromine can destroy ozone more readily than chlorine.
4. describe a natural mechanism by which both Cl and Br are removed from the stratosphere.
Key Terms
methyl chloride (CH3Cl, chloromethane)
catalytically inactive molecule (reservoir molecule)
chlorine nitrate (ClONO2)
endothermic
chlorofluorocarbon (CFC)
methyl bromide (CH3Br)
bromine nitrate (BrONO2)
Reading Assignment
Read pages 44–47 in the textbook.
Study Notes
Use Mechanism I shown in Figure 2–14 (p. 44 in the textbook) and replace X with either Cl or Br to obtain the two–step reaction for ozone destruction (Objective 1).
It is important to realize that several of the complex atmospheric systems we will study in this course involve some sort of temporary inactive reservoir of a normally active species. This concept becomes important in describing some phenomenon such as the Antarctic ozone hole, which we will discuss in the next few sections. In this section, neutral gaseous compounds like HX or XONO2 provide a temporary reservoir for the active X (where X + Cl or Br) species that can catalytically destroy ozone in the stratosphere (Objective 2).
Both CH3Cl and CH3Br are inert enough in the troposphere that they have enough time to migrate up to the stratosphere. Chlorine and bromine are next to each other in the same family of elements on the periodic table and therefore have very similar chemistry. However, their thermodynamic and kinetic behaviour vary somewhat and the extent to which they react is affected by the relative strength of their bonds. For example, the C–Cl bond is stronger than the C–Br bond so it is not surprising that CH3Cl is more resistant to reaction in the troposphere than CH3Br. In the stratosphere, Br forms weaker bonds than Cl in analogous reservoir compounds (HX or XONO2) and is readily photolyzed, so it exists to a smaller degree in nonactive forms than Cl (Objective 3).
Migration of HCl and HBr into the upper troposphere where it can be physically removed by precipitation is a slow, but major route to eliminating Cl and Br in the stratosphere (Objective 4).
Exercise
Question 2–C
Atomic fluorine is not regarded as an ozone depleter. However, bromine is considered to be more active in the stratosphere than Cl. Suggest a reason for this observation. [Hint: Calculate the enthalpies of reaction for HF, HCl, and HBr by the hydroxyl radical HX ) OH ³ H2O ) X]
The Ozone Hole and Other Sites of
Ozone Depletion—The Antarctic Ozone Hole
Objectives
After completing this section, you should be able to
1. describe the special polar winter conditions that lead to the Antarctic ozone hole.
2. explain the role of the crystals in polar stratospheric clouds (PSCs) in conversion from inactive to active chlorine.
3. write out the mechanism by which bromine or chlorine can catalytically destroy ozone under special polar weather conditions.
4. describe both seasonal and recent trends of ozone levels over the South Pole.
Key Terms
sulfuric acid (H2SO4)
nitric acid (HNO3)
carbonyl sulfide (COS)
vortex
polar stratospheric cloud (PSC)
Type I crystal
Type II crystal
denitrification
dichloroperoxide (ClOOCl)
Reading Assignment
Read pages 47–54 in the textbook.
Study Notes
Both the extreme cold (approximately *80°C) and the polar vortex are weather conditions that contribute greatly to Antarctic ozone hole (Objective 1). The severe cold stabilizes the ClOOCl dimer [dk is “dimer” a typo?] and more importantly allows for the crystal formation in PSCs. In the winter the Antarctic air circulation is circumpolar and not only isolates Antarctic air from the rest of the globe but allows almost no admixture of air from lower latitudes. This unique meteorological phenomenon is known as the polar vortex. The scheme below shows stratospheric circulation (adapted from The Antarctic Ozone Hole by Richard Stolarski, Scientific American 258, January 1988, 30–36). (CO2 forms “dry ice” at *78°C)
You should keep Figure 2–15 (p. 49 in the textbook) in mind when explaining the conversion of inactive forms of chlorine, such as HCl and ClONO2, to the active Cl (Objective 2). These reactions can also occur in the gas phase, but are so slow that they are virtually negligible. The crystals in the PSCs greatly hasten the production of active chlorine, while at the same time removing NO2 radicals (and therefore the potential to form inactive ClONO2) through gradual precipitation of larger crystals from the stratosphere to the troposphere. Mechanism II (summarized in Figure 2–14 on p. 44 in the textbook) is the major catalytic ozone destruction mechanism at work under these polar conditions. Review and memorize Steps 1, 2a, 2b, and 2c (pp. 50–51 in the textbook) for both chlorine and bromine atoms to achieve Objective 3.
At this point in the course, it is helpful to recall your basic astronomy and geography. The seasons in the northern and southern hemispheres of the globe are reversed. When it is winter in Canada (and the rest of the Northern Hemisphere) it is summer in Australia (and the rest of the Southern Hemisphere). Next to the reactions associated with the Antarctic ozone hole, this straightforward seasonal concept trips up many students. If need be, write the word “winter” on one side of your hand and “summer” on the other side as a quick reference.
The development of the Antarctic ozone hole has been a recent event (see Figures 2–2 and 2–3 on p. 19 in the textbook). The trend seems to be that the hole is growing larger, there is less overhead ozone available, and the hole remains longer during the year (Objective 4). The seasonal variation is dramatic and the hole is at its worst in the spring, especially in September and October. If you are surprised by the last sentence, you might consider writing “spring” and “fall” on your other hand.
Exercises
Do Problems 2–11, 2–12, 2–13, and 2–14 within the chapter.
Do Additional Problems 2 and 5 at the end of the chapter on page 76.
The Ozone Hole and Other Sites of
Ozone Depletion—Arctic Ozone Depletion
Objectives
After completing this section, you should be able to
1. explain why the ozone hole phenomenon is less severe in the Arctic than the Antarctic.
2. explain why full ozone holes have not yet been observed over the Arctic [dk is there a better word than full—how about complete—I have a hard time with a hole being described as full.]
Key Terms
No key terms have been identified.
Reading Assignment
Read pages 54–58 in the textbook.
Correction: On p. 55 in the textbook, in Figure 2–18 fourth to last line, the (g) after HNO3 should be (aq).
Study Notes
Take a few moments now and make a list of differences between the Arctic and Antarctic systems by going through the reading assignment carefully (Objectives 1 and 2). This will not only highlight the differences found in the Arctic, it will also reinforce the section above on the Antarctic.
Exercise
Do Additional Problem 7, at the end of the chapter on page 77.
The Ozone Hole and Other Sites of
Ozone Depletion—Global Decreases in Stratospheric Ozone
Objectives
After completing this section, you should be able to
1. explain the role of volcanoes in global ozone decrease.
2. write out the denitrification mechanism reactions that occur on a sulfuric acid droplet in the lower stratosphere.
3. list at least three contributing factors to the decline in mid–latitude ozone levels.
Key Terms
Mount Pinatubo (Phillipines)
El Chichón (Mexico)
nitrogen trioxide (NO3)
dinitrogen pentoxide (N2O5)
Reading Assignment
Read pages 58–60 in the textbook.
Study Notes
The injection of sulfuric acid into the lower stratosphere by volcanoes is a temporary sink for ozone globally (Objective 1). The sulfuric acid droplets in the lower stratosphere represent a route of denitrification and essentially a reduction in the formation of ClONO2. This necessarily means that more chlorine is available in its active form, which in turn can catalytically destroy ozone. The important chemical reactions for denitrification are shown in detail in this section (Objective 2).
The section also emphasizes that Mechanism II is dominant in the lower stratosphere. Please remember from our earlier discussion that this is due primarily to the availability of X and X__ species, as well as the lack of O radicals at that altitude.
Pay particular attention to the last two sentences of this section, as it lists several factors in lowering ozone levels (Objective 3).
Exercises
Do Problem 2–15 within the chapter.
Do Additional Problems 8 and 10 at the end of the chapter on page 77.
The Ozone Hole and Other Sites of Ozone Depletion—UV Increases at Ground Level
Objective
1. After completing this section, you should be able to describe and explain general ground level UV trends around the globe.
Key Terms
No key terms have been identified.
Reading Assignment
Page 60 in the textbook.
Study Notes
The reading is fairly explanatory for the most part. However, there is one small error in the last sentence that does make a major difference.
“. . .; however much of the decrease in UV–B occurred in the 1992–93 period . . .”
The word “decrease” should be “increase.”
Exercises
No exercises have been assigned for this section.
The Chemicals That Cause Ozone Destruction—
Chlorofluorocarbons (CFCs)
Objectives
After completing this section, you should be able to
1. define the group of compounds known as chlorofluorocarbons.
2. describe the physical and chemical properties of chlorofluorocarbons.
3. write the code number of a particular chlorofluorocarbon given its chemical formula and, conversely, write the chemical formula given its code number.
4. explain the particular environmental concern surrounding chlorofluorocarbons in the stratosphere.
5. define the term “ozone depleting potential.”
Key Terms
anthropogenic
sink
methyl chloride (CH3Cl)
methyl bromide (CH3Br)
chlorofluorocarbon (CFC)
freon
rule of 90
ammonia (NH3)
sulfur dioxide (SO2)
hydrogen fluoride (HF)
ozone–depleting substance (ODS)
ozone–depleting potential (ODP)
1,1,1–trichloroethane (CH3CCl3, methyl chloroform)
Reading Assignments
If you have had no previous experience in organic chemistry, read the Alkanes section in Background Organic Chemistry in Unit 1.
Read pages 60–64 in the textbook.
Study Notes
Chlorofluorocarbons (CFCs) are alkane–type compounds that contain only chlorine, fluorine, and carbon atoms (Objective 1). The Alkane section of Unit 1: Background Organic Chemistry will give you a better idea of the structural nature of CFCs.
The inert and volatile nature of CFCs makes them ideal for various applications (e.g., refrigerants, propellants for aerosol sprays and cleaning solvents), but it also makes them a long–term primary source of ozone depletion (Objective 2). Because of the long lifetime of these compounds, they build up in the atmosphere faster than they can degrade or escape. The dangers of long–term accumulation are a recurring theme in many areas of environmental chemistry. For example, in Unit 7 (Toxic Heavy Metals) we find that lead is a cumulative poison. Low dosages will not have an acute effect on the human body on a short–term basis. However, the lifetime of lead in the body is about six years, making prolonged low–level exposures a serious health hazard.
CFCs are still sometimes referred to by their old Du Pont trade name—Freon. However, Du Pont’s patents have long since expired and CFCs are available worldwide under various other trade names like Genetron (Allied–Signal). CFCs and some related families of compounds are identified and named by their code number. For example, CFC–11 (Freon–11) refers to CFCl3. The so–called “Rule of 90” will help you convert the old “freon” code to a chemical formula, or the other way around. This is described in Box 2–3 (p. 62 in the textbook). The following are explicit examples of how the “rule of 90” may be used to a chemical formula or code number for a CFC:
1. Given the code number CFC–13 (or Freon–13)
13 ) 90 + 103 (carbon/hydrogen/fluorine)
C + 1
H + 0
F + 3 (the rest is chlorine)
N the chemical formula is CClF3
2. Given the chemical formula C2F4Cl2
C + 2
H + 0
F + 4 (ignore number of chlorines)
204 * 90 + 114
N the code number is CFC–114 (or Freon–114)
Photolysis of CFCs in the stratosphere by UV–C light generates a chlorine atom. The chemical reactions involved in the actual destruction of ozone by Cl atoms through Mechanism I or II has been discussed in previous sections (Objective 4). Although existence of Cl atoms and its chemistry in the stratosphere is natural, it is the anthropogenic sources of Cl that further decrease the steady–state levels of ozone and subsequently increase ground–level UV–B that are of concern.
The term “ozone–depleting substance” (ODS) is introduced and explained in this section. You should be aware that the term “ozone–depleting potential” (ODP) is also commonly encountered when discussing and comparing CFCs and related compounds. Often you will see ODP as one of the values listed in tabulated data. ODP is a relative rating of the ability of a substance to destroy stratospheric ozone. It is based on CFCl3 as the standard with an ODP set at 1.0 (Objective 5). So, for example CFC–113 has an ODP + 0.8, which means it has 80% of the potential to deplete ozone as CFC–11. Alternative chemicals with a lower ODP are being sought to replace CFCs. This will be discussed in more detail in the next section.
Exercises
Do Problems 2–16, 2–17, and 2–18 within the chapter.
The Chemicals That Cause Ozone Destruction—
CFC Replacements
Objectives
After completing this section, you should be able to
1. explain why halogenated organics containing a C–H bond are being used as replacement compounds for CFCs.
2. explain why HFCs are considered the main long–term replacement for both CFCs and HCFCs.
3. write the code number of a particular HCFC or HFC given its chemical formula and, conversely, write the chemical formula given its code number.
4. list at least three primary areas or applications in which HCFCs are being used to replace CFCs.
5. state the problems associated with using HCFCs and HFCs to replace CFCs.
Key Terms
hydrochlorofluorocarbon (HCFC)
hydrofluorocarbon (HFC)
trifluoroacetic acid (TFA)
Reading Assignment
Read pages 65–66 in the textbook.
Correction: On page 65 in the textbook, the middle of the page in bold should read “HCFCs, hydrochlorofluorocarbon.”
Study Notes
Ideally, a replacement CFC compound should not only have the useful properties of that CFC and be “ozone friendly,” but also have a low toxicity, low flammability and not contribute to the “greenhouse” effect (Unit 3). However, with present technology only compromises can be achieved. In the case of HCFCs, the C–H bond is susceptible to attack by OH radicals in the troposphere, which should greatly reduce the amount that eventually migrates up to the stratosphere (Objective 1). Theoretically, HFCs would be ideal as CFC replacement compounds, because they contain no chlorine (Objective 2).
The conversion of HCFC and HFC codes to their respective formulae is identical to that of CFCs seen in the previous section (Objective 3). Use the rule of 90 as before to determine number of carbon, hydrogen, and fluorine atoms (the rest is assumed to be chlorine atoms). Take a moment to make sure you can see that the following codes (chemical formulae) are equivalent: HCFC–21 (CHFCl2) and HFC–134 (C2H2F4). Note that HFC–134 has two possible structural isomers that correspond to the empirical formula C2H2F4. HFC–134a is CH2F–CF3 and HFC–134b is CHF2–CHF2. You cannot deduce this from the “freon” code itself. However, the a, b, c, d, . . . after the numerical code merely denotes various isomers. It is only briefly mentioned here in case you run across this type of nomenclature in a question or a table of data.
These replacement compounds are intended to serve the same commercial purposes as the original CFCs, but with less potential damage to stratospheric ozone. Applications would include use as refrigerants, aerosol propellants, cleaning agents for electronics, insulating agents in plastic foams, and foam expansion agents to name a few (Objective 4). However, as mentioned above, these CFC replacement compounds are eventually compromises. There are still potential problems with HCFC flammability, potential toxicity (generation of TFA with both HCFCs and HFCs), as well as the non–zero ODP of the replacements themselves (Objective 5). Finally, it is worth mentioning that in addition to chemical substitutes, there are physical methods being developed that would make the use of CFCs unnecessary in several specific processes.
Exercises
No exercises have been assigned for this section.
The Chemicals That Cause Ozone Destruction—
Bromine– and Iodine–Containing Compounds
Objectives
After completing this section, you should be able to
1. compare the ODP of halons with their chlorine analogs.
2. state the main use of halons.
Key Terms
halon
methyl bromide (CH3Br)
Reading Assignment
Read pages 66–68 in the textbook.
Study Notes
A C–Br bond is weaker than a C–Cl bond, making photodissociation of bromine much more facile. This implies that the brominated CFC analogs or halons have the potential to destroy more ozone than the corresponding CFC compounds (Objective 1). Fortunately, a much smaller quantity of the brominated analogs is in use compared to their chlorinated counterparts.
The popularity of halons in firefighting was alluded to in this section (Objective 2). (DK EXPLAIN) As mentioned, one of the major reasons that halons are attractive for computer/electrical installations is that they are volatile and operate at moderate temperatures. This means that after a fire the halon evaporates and there is no clean up involved. The use of other common electrical fire fighting agents such as carbon dioxide or chemical powder extinguishers would either damage equipment with extreme cold or make such a mess that it would be impossible to clean out all the particulates.
Exercises
Do Problems 2–19 and 2–20 within the chapter.
The Chemicals That Cause Ozone Destruction—
International Agreements on the Production of CFCs and Other ODSs
Objectives
After completing this section, you should be able to
1. state what gases are being phased out by the Montréal Protocol and its related amendments.
2. explain how injection of ethane into the Antarctic stratosphere could potentially “heal” it.
3. explain why anthropogenic sources (CFCs in particular) of chlorine are thought to be the cause of increased stratospheric ozone depletion, despite the fact that natural sources of chlorine have a higher emission to the atmosphere.
Key Terms
Montréal Protocol (1987)
London Amendment (1990)
Copenhagen Amendment (1992)
Vienna Amendment (1995)
Reading Assignment
Read pages 68–71 in the textbook.
Study Notes
With the Montréal Protocol, industry, governments, and environmental groups around the world played important roles in the issue of environmental protection and exhibited unprecedented cooperation. Many of the more recent amendments to the Protocol are much more rigorous than the original agreement. However, it is historically important, because it was the first such international agreement of its kind. Currently, there are over 120 signatory countries to the Montréal Protocol.
You are not expected to know the history of the Montréal Protocol and subsequent amendments in a detail. You should know the original agreement was nominally signed in 1987 and that substances like CFCs, HCFCs, halons, and methyl bromide have been determined to be ODSs and are either banned or are being phased out in the near future (Objective 1).
Some quick fixes to ozone depletion have been and continue to be proposed. These would include reducing the chlorine radical to the nonactive chloride anion or simple scavenging of the chlorine radical by a volatile alkane such as ethane, as shown in the chemical equation on page 70 of the textbook (Objective 2). Remember that because of the complicated nature of ozone depletion, proposed solutions such as this could cause more problems than they solve.
The key to recent increased ozone depletion is the availability of active chloride radicals in the stratosphere. Carefully reread the second last paragraph of this section and note the scientific evidence that points clearly
to anthropogenic sources such as CFCs rather than natural sources (Objective 3).
[Dk, should you say anything about the Kyoto thingy?]
Exercises
Do Problems 2–21 and 2–22 within the chapter.
Systematics of Stratospheric Chemistry: A Review
Objective
After completing this section, you should be able to use the concept of the “loose oxygen” to predict expected reaction mechanisms for chemical reactions in the stratosphere.
Key Term
loose oxygen
Reading Assignment
Read pages 71–74 in the textbook.
Study Notes
Remember there are several ways to look at predicting likely reactions in a systematic way. The aim of this section is to get you away from wholesale memorization of entire series of reactions that occur in the stratosphere and adopt a better general understanding of what is happening. Whether you use differences in heats of formation between products and reactants or the “bonds–broken–minus–bonds–formed” concept makes little difference.
In this case, we are asked to identify loose oxygens so we can see what might be abstracted and what fate awaits a given species in the stratosphere.
Exercises
Do Problems 2–23, 2–24, 2–25, 2–26, and 2–27 within the chapter.
Do Additional Problem 9 at the end of the chapter on page 77.
Extra Exercise Answers
Note: the following are answers to extra questions posed in this Study Guide. Short answers are available to in–chapter problems can be found at the end of the textbook. In addition, detailed solutions for all problems in the textbook can be found in the accompanying Solutions Manual for Environmental Chemistry by Colin Baird.)
Answer 2–A
a. p(Ne sea level) + (1.0 atm)(0.000018) + 1.8 10*5 atm
p(Ne 40 km) + (0.0030 atm)(0.000018) + 5.4 10*8 atm
b. ppmv +
ppmv(Ne sea level) + + 18 ppmv
ppmv(Ne 40 km) + + 18 ppmv
c. Air becomes thinner with increasing altitude. If the “mixing ratio” of a gas remains the same at different altitudes, the relative concentrations are constant [Part (b)]. However, the partial pressure and therefore the absolute concentration of a gas may vary as shown in Part (a).
Answer 2–B
For the first air sample let A + ebc and for the second sample let A__ + e__b__c__
If A + A__ then
ebc + e__b__c__
Relative height of air columns +
+ + + 16.67
N the second sample of air at 310 nm must be almost 17 times longer than the first to absorb the same amount of light as the first sample at 280 nm.
Answer 2–C
Like chlorine or bromine, the fluorine radical reacts with CH4 (or water) to form HF. This is a temporary reservoir of F. To release the active fluorine atom a reaction occurs with the hydroxyl radical.
HX ) OH ³ H2O ) X (where X + F, Cl, Br)
However, the H–F bond energy is 567 kJ mol*1 compared with H–Cl and H–Br, which are 431 and 366 kJ mol*1, respectively. The enthalpies of reaction by hydroxyl attack as shown by the chemical equation above are 104, *32 and *97 kJ mol*1 for HF, HCl, and HBr, respectively. Whereas HCl and HBr can slowly release Cl and Br, HF is essentially inert.
Other points to consider: Both HBr and BrONO2 are readily decomposed photochemically, so most of the bromine remains in active forms such as Br and BrO. Also, Br is better at abstracting O from ozone (stronger X–O bond) than either Cl or F.
Review Procedure
1. Review the unit objectives and make sure that you can define, and use in context, the key terms introduced in this unit.
2. Go over the Review Questions (pp. 74–76 in the textbook) to test your factual knowledge of the material covered this unit.
3. Do some of the Supplementary Exercises for Chapter 2 for additional practice or examination preparation. Supplementary Exercises (with answers) can be found on the resource web pages that accompany the textbook at:
4. Proceed to Unit 3.
Unit 3
Ground–Level Air Chemistry
and Air Pollution
Overview
We live in air, it is all around us, and in fact we are quite literally immersed in the atmosphere. However, it has become so much a part of our lives that it becomes very easy to ignore. If I cup my hand (palm up) and ask what is in my hand; most people would say there is nothing in my hand. This is not absolutely true, because there is air in my hand.
The air around us is part of the troposphere and affects us more immediately than any other system in our environment. Unit 3 concentrates on four environmental concerns that occur in the troposphere: photochemical smog, fine particulate matter, indoor air pollution, and acid rain. We will explore the chemistry of both clean and polluted air within the troposphere, as well as some of the effects this has on the environment and human health.
Objectives
After completing this section, you should be able to
1. describe smog in general terms.
2. state the correlation between industrial development and air quality.
3. describe the overall natural mechanism for cleaning air.
Key Terms
smog
oxidizing environment
diatomic
pollutant
Reading Assignment
Read pages 85–86 in the textbook.
Study Notes
This section is a general introduction to the rest of Chapter 3 in the textbook.
Exercises
No exercises have been assigned for this section.
Concentration Units for Atmospheric Pollutants
Objective
After completing this section, you should be able to interconvert and perform calculations with various concentrations of substances in air.
Key Terms
molecules per cubic centimeter (molecules cm*3)
micrograms per cubic meter (mg m*3)
moles per litre (mol L*1)
parts per million (ppm)
Reading Assignment
Page 86 and Box 3–1 (pp. 87–89).
Study Notes
Make sure you are comfortable with interconverting the units mentioned in this section, before continuing on with the rest of the unit. The use of the “parts per . . .” system will be a review of some of the calculations encountered in Unit 2. Note that Box 3–1 (pp. 87–89) is extremely helpful and emphasizes the use of the ideal gas law (i.e., PV + nRT).
Warning: Experience has shown that unit conversions are a prime source of calculation error on examinations in this course.
Exercises
Question 3–A
The concentration of helium at sea level was measured to be 5.23 ppmv at 25°C. Convert this value to:
a. partial pressure in atm
b. the molarity scale
c. the molecules per cm*3 scale
Do Problems 3–1, 3–2, 3–3, and 3–4 within the chapter.
Corrections:
Problem 3–2, change the exponent from *4 to )14 (p. 89 in the textbook).
Problem 3–2, answer, on line 3 change 1 mole to 1 cm3 (p. 18 in the solutions manual).
Problem 3–3, change ppm to ppb (p. 89 in the textbook).
Problem 3–3, answer, replace 9.1 with 9.3 (p. AN2 in the textbook).
Urban Ozone: The Photochemical Smog Process—
The Origin and Occurrence of Smog
Objectives
After completing this section, you should be able to
1. describe the general appearance and major constituents of photochemical smog
2. explain the general chemistry involved in the production of photochemical smog
3. identify primary and secondary pollutants that occur in photochemical smog
4. describe the conditions required for photochemical smog to develop
5. perform simple kinetic and thermodynamic calculations for tropospheric chemical reactions
6. state the maximum allowable ozone concentration standard established by WHO
Key Terms
photochemical smog
hydrocarbon
volatile organic compound (VOC)
primary pollutant
secondary pollutant
free radical
NOx (NO ) NO2)
World Health Organization (WHO)
long–range transport
Reading Assignment
If you have had no previous experience in organic chemistry, read the Alkenes and their Chlorinated Derivatives and Symbolic Representations of Carbon Networks sections in the Background Organic Chemistry component of Unit 1.
Read pages 86–93 in the textbook.
Study Notes
Photochemical smog is a chemical soup that forms a brownish–yellow haze over urban areas and includes ground level ozone, NOx, PAN (peroxyaceyl nitrate; an organic nitrate), SOx, particulate matter, acidic aerosols and gases (e.g., HNO3), VOCs, and carbon monoxide (Objective 1). Some of these constituents and their chemistry will be discussed in more detail in subsequent sections of this unit. At this point, you should merely know the general chemical equation shown on page 89.
Smog is a very regional problem with differing compounds (relatively) and degrees of severity. The basic components of photochemical smog are illustrated in Figure 3.1 below and reflect the general reaction shown on page 89 (Objective 2). Species generated from the initial reaction between the primary pollutants NOx and VOCs are considered secondary pollutants (Objective 3).
Figure 3.1: Basic components of photochemical smog
The temperature requirement should be used cautiously when determining the requirements for smog. For example, Edmonton’s worst smog episodes occur in the cold winter months (when temperature inversions are common), but contain mostly NOx and particulate matter rather than ozone. Alberta ozone levels are discussed in more detail in the Study Notes of the next section. However, you should be able to describe the general required conditions as mentioned in the textbook and/or predict whether photochemical smog would likely occur in a given region using those conditions as criteria (Objective 4).
For example:
Would photochemical smog occur in the following locations?
1. Blind River, Ontario in summer
2. Edmonton, Alberta in winter
3. Toronto, Ontario in summer
4. Vancouver, British Columbia at night
Location Smog Expected Reasoning
Blind River No Rural area therefore little NOx
Edmonton Yes Temperature inversions, however not all
smog by products will be produced
Toronto Yes Meets criteria
Vancouver No No sunlight
Reminder: You should review Appendices A, B, and C, as needed and keep your first–year chemistry textbook close at hand to assist you with some of the kinetic and thermodynamic calculations encountered in this course (Objective 5).
Finally, the WHO standard for ground–level ozone ranges from 75–100 ppb, depending on length of exposure (Objective 6).
Exercises
Do Problem 3–5 in the chapter.
Correction: In the answer to Problem 3–5, replace exponents *9 and *17 with *8 and *16, respectively (p. AN2 textbook).
Do Additional Problems 1 and 2 at the end of the chapter on page 165.
Urban Ozone: The Photochemical Smog Process—
Reducing Ground–Level Ozone and Photochemical Smog
Objectives
After completing this section, you should be able to
1. explain why in most cases a variation in NOx concentration has a greater effect on ozone production than a variation in VOC concentration
2. describe how NO and NO2 can be quantitatively detected in air using the technique of chemiluminescence
3. state at least three strategies to reduce urban ozone levels
4. write out the fundamental chemical reactions involved in a basic three–way catalytic converter
Key Terms
anthropogenic hydrocarbon
chemiluminescence
photomultiplier tube (PMT)
catalytic converter
two–way converter
three–way converter
air/fuel ratio
selective catalytic reduction
Reading Assignment
Read pages 93–101 in the textbook.
Study Notes
In most cases of photochemical smog, there is an excess of VOCs present compared with NOx. This means NOx behaves as the limiting reagent and so a variation in NOx concentration can affect the steady state ozone level. Similar variations in concentration of VOCs would have little effect on ozone level (Objective 1). Figure 3–2 (p. 94) and the discussion surrounding it is helpful in visualizing this idea.
You should be able to reproduce the two chemical equations (p. 96) that that are the foundation for NO and NO2 detection by chemiluminescence (Objective 2). Strategies for ozone reduction have included legislating restrictions for VOC emissions, as well as reducing NOx emmisions through lowering combustion temperatures, using catalytic converters in automobiles, and selective catalytic reduction in some commercial operations (Objective 3).
You need to have Figure 3–3 (p. 99) with all reactions clearly in your mind to be able to describe a three–way catalytic converter (Objective 4). It is important to remember which reactions are reductive in nature and which are oxidative. To be efficient, the fuel/air mixture must be rigorously controlled on an on–going basis.
Aside: It used to be that almost anyone could do mechanical repairs on their own automobiles. However, advances such as the computer–controlled fuel/air ratio have made newer cars so sophisticated that few people these days would attempt much more than adding air to the tires or the occasional oil change.
Exercises
Do Problems 3–6, 3–7, and 3–8 within the chapter.
Do Additional Problem 3 at the end of the chapter on page 165.
Acid Rain—The Sources and Abatement of
Sulfur Dioxide Pollution
Objectives
After completing this section, you should be able to
1. state the pH requirement that defines acid rain.
2. write balanced chemical equations for the reaction of carbon dioxide, sulphur dioxide, and nitrogen oxides in water.
3. state one natural and two anthropogenic sources of SO2 in the atmosphere.
4. write the general chemical equation for roasting metal sulphides.
5. explain how hydrogen sulfide is removed from natural gas giving the chemical equation.
6. perform calculations to determine the amount of SO2/NOx emitted or extracted from various industrial processes.
7. describe at least three strategies to reduce sulfur dioxide emissions.
Key Terms
acid rain
sulfuric acid (H2SO4)
nitric acid (HNO3)
carbonic acid (H2CO3)
sulfur dioxide (SO2)
inclusion
hydrogen sulfide (H2S)
Claus reaction
total reduced sulfur
point source
roasting process
calcium carbonate (CaCO3, limestone)
calcium oxide (CaO, lime)
scrubber process
flue–gas desulfurization
clean coal technology
precombustion cleaning
combustion cleaning
fluidized bed combustion
postcombustion cleaning
SNOX™ process
coal conversion
Reading Assignment
Read pages 101–106 in the textbook.
Study Notes
Acid rain is defined as any precipitation having a pH of 5.0 or less (Objective 1). Normal unpolluted rainwater has a pH of about 5.6.
It is suggested that for this section that you review acids and bases in your first–year chemistry textbook. If you took Chemistry 218 from Athabasca University refer to Chapters 13, 14 and 15 of Chemistry: Molecules, Matter, and Change, 3rd ed., by Atkins and Jones. You should have some basic knowledge of oxyacids and their aqueous formation (Objective 2). The chemistry of acid rain will be dealt with in much more detail in another section later in this unit.
Remember that it is important to realize the difference between a weak and strong acid. An acid (HA) resides in aqueous equilibrium with its ionic species H) and A*:
Ka +
where Ka is the acid dissociation constant.
If Ka is small and the acid exists mostly in the HA form, then it is said to be a “weak” acid. If Ka is large (u 1) and the acid exists mostly in its ionic form (H) and A*), it is said to be a “strong” acid.
To illustrate this, if we wanted to prepare a 1.0 L water solution with a pH of 3.0, we would require 56.6 mL of 1.0 M acetic acid (CH3COOH, a weak acid) and dilute it to 1.0 L. To prepare a solution with the same pH using a strong acid such as 1.0 M hydrochloric (HCl) would require only 1.0 mL of acid!
Volcanoes and plant decomposition account for much of the natural atmospheric sources of SO2, but combustion of coal and other fossil fuels, as well as commercial roasting of metal ores, makes up the bulk of anthropogenic sources (Objective 3). You should be able to write out the reaction of any metal sulfide with oxygen to form SO2 similar to the chemical equation for roasting nickel sulfide shown on page 103 (Objective 4). The equation for the Claus reaction to remove H2S is also found on page 103 (Objective 5).
You are also responsible for knowing how to carry out simple stoichiometric calculations for industrial processes in which acidic emissions are generated and/or trapped (Objective 6). The exercises will give you an idea of what type of questions to expect to find on the examination.
Have a careful look at pages 104–105 and make sure you know some of the strategies used to limit SO2 emissions (Objective 7). In some cases, these methods of waste gas removal are not just treated as an added expense, but can sometimes be an integral part of the entire commercial operation of a process.
Warning: Note that the term SNOX™ process (SO2/NOx removal) is a commercial name of a particular process of SO2/NOx removal. It is not a general term or process for postcombustion cleaning.
Exercises
Do Problems 3–9, and 3–10 within the chapter.
Do Additional Problem 8 at the end of the chapter on page 166.
Acid Rain—The Ecological Effects of
Acid Rain and Photochemical Smog
Objectives
After completing this section, you should be able to
1. explain why there is a difference between the prevalent type of acid rain found in eastern versus western North America.
2. describe the difference between wet and dry deposition.
3. explain the effects of acid rain on vegetation, release of toxic metals, and fish populations.
4. write the balanced chemical equation describing the dissolution of limestone by acidic precipitation.
5. describe what is meant by a “buffered lake.”
6. state two methods of rejuvenating an acidified lake.
Key Terms
high–sulfur coal
dry deposition
wet deposition
neutralize
buffer
dissolved organic carbon (DOC)
deciduous tree
Reading Assignment
Read pages 106–112 in the textbook.
Study Notes
Acid rain has caused a great deal of environmental concern in Canada and other parts of the globe and has had a great deal of publicity. Whether the major component of acid rain is sulphuric or nitric acid depends on the availability of SO2 and NO2 emissions in a particular region (Objective 1). The word “rain” in the term acid rain seems to imply that this is problem solely associated with wet deposition or removal of acidic emissions from the atmosphere through aqueous precipitation. However, the role of dry deposition is highlighted in this section and the two should be considered together (Objective 2).
Acidic emissions have several adverse effects on biological life. You should not only be able to describe the effects of acid rain on plant and animal life, you should also be able to explain some of the mechanisms (e.g., release of aluminum ions, reduction in DOC) involved (Objective 3).
To achieve Objective 5 it may be helpful to remember the Henderson–Hasselbach Equation, which is often used to calculate the pH of a buffered solution.
pH + pKa )
In a buffered lake, which has underlying limestone (CaCO3), the acid system would be carbonic acid (H2CO3). The above equation would become:
pH + 6.37 )
The pH of the water is dependent on the ratio of to [H2CO3]. If we were making a buffered solution in the lab of + 0.10 M and [H2CO3] + 0.12 M the pH would be 6.29. We could also make the same pH solution using + 1.0 M and [H2CO3] + 1.2 M. However, the second solution would have a greater “buffering capacity” that is, it would take more acid or base in the second solution to change the ratio and hence the pH.
In a buffered lake the is kept even more stable, because there is a continued supply of ions from the limestone (CaCO3) every time more acid is added.
CaCO3 ) H) → Ca2) )
Finally, the best way to reduce acidity of natural water systems by acid rain is to reduce acidic emissions. In addition to this wholesale solution, one can help neutralize a lake in the short–term by addition of limestone, lime or in some cases phosphate (Objective 6).
Exercises
Do Problems 3–11, 3–12, and 3–13 within the chapter.
Do Additional Problems 4 and 9 found at the end of the chapter on page 165.
Particles in Air Pollution
Objectives
After completing this section, you should be able to
1. describe common types of suspended particles.
2. identify whether a particle is fine or coarse by its diameter.
3. perform calculations using Stoke’s law.
Key Terms
particulate
suspended particle
diameter
coarse
fine
dust
soot
mist
fog
aerosol
Stoke’s law
Reading Assignment
Read pages 112–114 in the textbook.
Study Notes
You should be able to identify the various descriptive terms (e.g., fog, soot, mist, etc.) as being particulates and that anything below 2.5 ìm is considered a “fine” particulate (Objectives 1 and 2).
Caution: A lot of students associate the term “aerosol” with a liquid. Please note that we will use aerosol to mean BOTH solid and liquid droplets.
There is only a qualitative description of Stoke’s law in the textbook. The actual formula is given below. You are not required to memorize the formula, but you should be able to use it in calculations (Objective 3). Stoke’s law describing partial sedimentation is given by
Rate +
where g acceleration due to gravity, Dò is the density difference between particles and the air, h is air viscosity and d is the particle diameter. It is important to remember that Stoke’s law only applies to particles of greater than a 1 ìm in diameter. Smaller particles are too light and possess chaotic motion so they do not settle according to the equation above.
Exercise
Question 3–B
Given Stoke’s law above in the Study Notes assume g + 9.81 m s*2 is gravitational acceleration, the density of air (20°C and 1.0 atm) is
1.1 10*3 g cm*3, and air viscosity is h + 1.76 10*4 g cm*1 s*1.
1. Assume 35 mm particles have a density of 2.65 g cm*3. What is their rate of descent?
2. Are these particles considered fine or coarse?
3. If these particles were released from an 80 m smokestack, how long would it take for the particles to reach the ground?
Particles in Air Pollution—
Sources of Atmospheric Particles
Objectives
After completing this section, you should be able to
1. state the common source of fine and coarse particulates.
2. state the components of a sulfate aerosol.
3. write the balanced chemical equation for the reaction of nitric or sulfuric acid with ammonia.
Key Terms
aluminum silicate
pollen
particle trap (filter)
ammonium sulfate ((NH4)2SO4)
ammonium nitrate (NH4NO3)
sulfate aerosol
Reading Assignment
Read pages 114–116 in the textbook.
Study Notes
Although you should read this section in detail, a good summary differentiating the usual origins of fine and coarse particles is given in the
last paragraph (Objective 1). Please note the trend in acidity between fine
and coarse particles based on their origin. Fine particles that are usually composed of nitrates or sulphates are acidic, while coarse particles with soil content tend to be more alkaline. Table 8–1 (p. 435 textbook) is a shortlist of various common oxidation states of sulphur. Those species listed under )4 and )6 oxidation states are the type of species found in sulphate aerosols (Objective 2). You are responsible to write chemical equations similar to the one at the top of page 116 for the formation of sulphates and nitrates from ammonia (Objective 3).
Exercises
No exercises have been assigned for this section.
Particles in Air Pollution—
Air Quality Indices for Particulate Matter
Objectives
After completing this section, you should be able to
1. describe the cause of haziness in the troposphere.
2. differentiate between respirable and ultrafine particles.
3. use the PM and TSP indices correctly.
Key Terms
PM index (e.g., PM10)
inhalable (or respirable) particle
ultrafine particle
total suspended particulates (TSP)
Reading Assignment
Read pages 116–117 in the textbook.
Study Notes
You should also note that particles in the 0.4 to 0.8 ìm range can scatter light. This phenomenon is known as Mie scattering. Visual air quality (VAQ) is an increasingly important issue in Canada. This is especially true in our arctic and mountain park environments. For example, the presence of SO2, NH3 and the right humidity can generate (NH4)2SO4 · xH2O, which produces a haze. This is a common effect in the mountain parks of Alberta and British Columbia.
Since only fine particles (0.4 to 0.8 mm range) can scatter light, haze is usually an aerosol of nitrates or sulphates and in some cases fine carbon compounds (Objective 1). Inhalable (or respirable) particles are less than 10 ìm in diameter, while ultrafine particles are less than 0.05 mm in diameter (Objective 2). In a later section we will see that certain particulate sizes also have a direct effect on health.
The PMx (PM + particulate matter) index actually reports a concentration (in mg m*3) of particles containing a diameter equal to or smaller than x (in mm). The TSP (total suspended particulates) is also reported as a concentration (in mg m*3), but it includes particles of any diameter (Objective 3).
Exercises
Do Problem 3–14 within the chapter.
Do Additional Problem 6 found at the end of the chapter on page 166.
Particles in Air Pollution—
The Distribution of Particle Sizes in an Air Sample
Objectives
After completing this section, you should be able to
1. explain the origin of the three “modes” of particles in a particle numbers versus diameter plot.
2. describe the difference between adsorption and absorption.
Key Terms
nuclei mode
accumulation mode
coarse particle mode
residence time
absorbed
adsorbed
Reading Assignment
Read pages 117–121 in the textbook.
Study Notes
It is important that you have a sense the range of particle sizes, as well as how they are removed (wet removal, sedimentation) or transformed (coagulation) into other particles. If you can explain the source of the three modes shown in Figure 3–7 (Objective 1) and can relate that figure with Figures 3–8 and 3–9; you will have grasped the essential concept of this section. Finally, you should be able to relate particle size and surface area and correlate that with the relative ability to absorb or adsorb (Objective 2) molecules.
Exercises
Do Problems 3–15 and 3–16 within the chapter.
Does Additional Problem 5 found at the end of the chapter on page 166?
The Health Effects of Outdoor Air Pollutants—
The Effects of Smog
Objectives
After completing this section, you should be able to
1. describe at least two major health concerns associated with smog.
2. compare and differentiate London–type smog with photochemical smog.
Key Terms
chronic
threshold pollutant
photochemical smog
London smog
Reading Assignment
Read pages 121–125 in the textbook.
Study Notes
This section recites the many deleterious effects of smog on human health (Objective 1). Not surprising many of these are respiratory problems. However, the effects are by no means limited to respiration. You should make a list for yourself and commit a few to memory, including the mechanism involved.
The table below will assist you with a comparison between London–type smog with Los Angeles–type smog (Objective 2). The sulphur dioxide generated from burning sulphur containing coal is not only a respiratory irritant, but is a primary pollutant in the formation of acid rain.
| | |Photochemical Smog | |London–type Smog |
|Chemical nature | |oxidizing (O3, PAN) | |reducing (SO2) |
|Source | |automobile emissions | |Buring coal |
|Particulate | |liquid aerosol | |smoke and soot |
Exercises
No exercises have been assigned for this section.
The Health Effects of Outdoor Air Pollutants—
The Effects of Particulates
Objectives
After completing this section, you should be able to
1. list at least three reasons why coarse particles are less of a human health concern than fine particles.
2. describe at least two major health concerns suspected to be caused by particulates.
3. describe the Six City study of mortality rates versus fine particle concentration.
Key Terms
electrostatic precipitator
baghouse filter
EPA (Environmental Protection Agency, USA)
“Six Cities” study
correlation coefficient
sudden infant death syndrome (SIDS)
Reading Assignment
Read pages 125–130 in the textbook.
Study Notes
The list provided at the bottom of page 125 is a good summary of reasons why coarse particles are of less concern to human health when compared with fine particles (Objective 1). Although the role of particulates is strongly linked to several issues of human health such as respiratory problems, lung cancer, and cardiopulmonary diseases (Objective 2), it is still a controversial connection. The evidence is mostly statistical in nature. You should be able to describe the Six City study shown in Box 3–2 and understand both the findings and the possible limitations of those findings (Objective 3). Later in the course, we will see that particles like asbestos fibres, tobacco smoke and soot from coal have been related to specific diseases.
Exercises
No exercises have been assigned for this section.
Detailed Chemistry of the Troposphere—
Trace Gases in Clean Air
Objectives
After completing this section, you should be able to
1. list at least five gases released into the troposphere from “natural sources.”
2. write the two–step mechanism for hyroxyl radical production in the troposphere.
3. describe in general terms the role of OH (and the related species HOO) in tropospheric chemistry.
4. list two factors that control the steady state concentration of OH.
Key Terms
hydrogen halide (e.g., HF, HCl, HBr)
hydroxyl free radical (OH)
endothermic
activation energy
hydride
Reading Assignment
Read pages 130–132 in the textbook.
Study Notes
Table 3–2 provides an excellent summary of natural trace gases (Objective 1). You should be familiar with both the gas and its natural source. Note that many of these gases also have potential anthropogenic sources.
The two chemical equations on page 131 describe the production mechanism for OH radical (Objective 2). Keep in mind that the source of O3 in the troposphere shown in the first equation is also a result of another photochemical two–step mechanism.
NO2 ) hn → NO ) O
O ) O2 → O3
Therefore, OH production is a direct result of photolysis reactions—it is driven by sunlight. The textbook also mentions that OH is the vacuum cleaner of the troposphere. It reacts with many pollutants and oxidizes them (Objective 3). As a result, the more oxidizable species (pollutants) there are in the troposphere, the more quickly OH will get used up. Essentially the concentration of OH radical is directly proportional to solar flux (i.e., rate of photolysis) and inversely proportional to the concentration of oxidizable substrates in the atmosphere (Objective 4).
[OH] +
Note: Please be aware that there are several other minor routes to OH production in the troposphere. The excited oxygen radical (O*) can abstract a hydrogen from an alkane like methane or photolysis products from nitric acid, nitrous acid, and hydrogen peroxide can generate OH. You are not responsible to know these, but you should realize that reactions in the atmosphere are very complex and numerous.
O* ) CH4 → OH ) CH3OH
HNO2 ) hn → OH ) NO
HOOH ) hn → 2OH
Exercises
Do Problems 3–18 and 3–19 within the chapter.
Detailed Chemistry of the Troposphere—
Principles of Reactivity in the Troposphere
Objectives
After completing this section, you should be able to
1. list the two general types of chemical reaction that the OH radical can undergo.
2. explain why OH can abstract hydrogen more readily from other molecules than HOO.
3. state the most common fate of peroxy radicals in the troposphere.
4. describe the general requirement for an oxygen molecule to successfully abstract a hydrogen atom from radicals that contain non–peroxy oxygen.
Key Terms
Lewis structure
d orbital
abstraction
peroxy radical (ROO)
hydroperoxy radical (HOO)
endothermic
exothermic
thermoneutral
Reading Assignment
If you have had no previous experience in organic chemistry, read the Common Functional Groups section of the Background Organic Chemistry component in Unit 1.
On third to last line delete bond dash after CH3 (p. AP8 in the textbook)
Change the –NH2 group furthest to the right in the cysteine structure to –OH (p. AP10 in the textbook).
Read pages 132–137 in the textbook.
Study Notes
The two general types of reactions that an OH radical can undergo (Objective 1) are:
OH addition
e.g., H2C__CH2 ) OH → H2(HO)C__CH2
hydrogen abstraction
e.g., H2C__CH2 ) OH → H2C__CH ) H2O
As the examples above show both abstraction and addition can occur with the same compound.
For the remaining three objectives you should start to think about the thermodynamics of any potential reaction. If the products formed are more thermodynamically stable than the reactants the reaction will usually be driven forward (see Figure 2–13(a) in Box 2–2 p.41 textbook). Water molecules are much more energetically stable than peroxides, so hydrogen abstraction by the OH radical is much more favourable than similar abstractions by peroxy radicals (Objective 2). Formation of the very stable NO2 radical by transfer of the “loose” oxygen (Unit 2) from the peroxy radical drives that reaction and therefore makes it a common fate of peroxy radicals in the troposphere (Objective 3). Finally, multiple bonds are always stronger than single bonds and so reactions involving oxygen molecules abstracting a hydrogen atom to form multiple bonds is preferred (Objective 4).
Exercises
Do Problems 3–20, 3–21, and 3–22 within the chapter.
Detailed Chemistry of the Troposphere—
The Tropospheric Oxidation of Methane
Objectives
After completing this section, you should be able to
1. write down the series of chemical reactions that represent the oxidation of methane to carbon dioxide.
2. apply similar principles and chemical equations for the oxidation of other hydrogen–containing molecules.
3. identify the rate determining step in the oxidation of hydrogen–containing molecules in the troposphere.
4. explain why molecules like methane and methyl chloride react slowly in the troposphere.
Key Terms
anaerobic biological decay
formaldehyde (H2C__O)
hydride (e.g., CH4, H2S, and NH3)
nonmethane hydrocarbons (NMHC)
Reading Assignment
Read pages 137–141 in the textbook.
Study Notes
The general overall reaction of any hydrocarbon with OH in the troposphere results in the ultimate conversion of that hydrocarbon to water and carbon dioxide. The overall reaction for methane is shown at the bottom of page 138 and includes the production of NO2 and regeneration of OH. It is important to remember that this overall reaction is driven by sunlight. You need to memorize all the steps of this mechanism summarized in Figure 3–14 (Objective 1).
Take some time now to learn the tropospheric oxidation of methane well. Later you will encounter other hydrocarbons and hydrogen–containing compounds that undergo quite similar reactions and methane serves as an excellent learning model. A little effort at this stage will save you a tremendous amount of time in the following sections. To assist you with this, you are also required to understand each step of the mechanism well enough to apply them to analogous reactions with other hydrogen–containing molecules (Objective 2). You may wish to revisit this section again when you get up to page 149. After exposure to more related tropospheric reactions, the principles of methane oxidation with be easier to grasp.
Finally, the slowest step (rate determining step) of this mechanism is the initial abstraction of the hydrogen atom (Objective 3). It is not a surprise then that a strong C–H bond (found in methane and methyl chloride) would slow down the rate of hydrogen abstraction and therefore the overall tropospheric oxidation of that molecule (Objective 4).
Exercises
Do Problems 3–23, 3–24, 3–25, 3–26, and 3–27 within the chapter.
Detailed Chemistry of the Troposphere—
Photochemical Smog: The Oxidation of Hydrocarbons
Objectives
After completing this section, you should be able to
1. state the major source for ground level ozone.
2. write out the stepwise mechanism for oxidation of RCH + CHR.
3. predict likely reaction products of analogous tropospheric species applying the principles outlined in the textbook.
4. explain, using chemical equations, the change in concentration of hydrocarbons, aldehydes, ozone, NO2, and NO during a photochemical smog episode.
Key Terms
ethene (ethylene, H2C__CH2)
diurnal
urban ozone
aldehyde
autocatalytic
Reading Assignment
Read pages 141–145 in the textbook.
Study Notes
You will find this section somewhat challenging, because it tries to describe a process that has many interrelated parts. You may want to reread this section several times to become familiar with the many aspects of the photochemical smog process.
Careful examination of the reaction equations on page 143 will show you that photodissociation of NO2 in the presence of molecular oxygen is the source of ozone in the troposphere (Objective 1) and that increased NO concentration depresses the ozone concentration by reforming NO2 and molecular oxygen.
A summary of the oxidation of a general ethene molecule is given in Figure 3–17 (Objective 2). You should be able to apply the principles used in that mechanism to similar molecules (Objective 3). The bulk concentrations of gases shown in Figure 3–16 roughly follow the mechanism under discussion (Objective 4). Note that the peak of hydrocarbons at 9 am reflects emissions from morning automobile traffic.
Exercises
Do Problems 3–28, 3–29, and 3–30 within the chapter.
Correction: Problem 3–29 answer in first line delete 8 in front of NO and in third line add an 8 in front of NO (p. AN–2 textbook).
Do Additional Problem 7 found at the end of the chapter on page 166.
Detailed Chemistry of the Troposphere—
Photochemical Smog: The Fate of the Free Radicals
Objectives
After completing this section, you should be able to
1. identify at least three pollutants associated with photochemical smog.
2. write the mechanism for the formation of PAN.
3. describe the fate of OH radicals that react with NO, NO2, and other OH radicals.
4. explain how a decrease in NO2 concentration can actually increase ozone concentration when VOC levels are lower than normal.
Key Terms
peroxyacetylnitrate (PAN, CH3C(__O)OONO2)
nitrate radical (NO3)
Reading Assignment
Read pages 145–150 in the textbook.
Study Notes
It is important to realize that the troposphere’s generation of reactive species such as OH, O3, and O radicals is a natural part of its mechanism to clean itself. The health and environmental problems occur when there is an unnaturally high concentration (usually anthropogenic in origin) of hydrocarbons (NMHCs) and NOx. The tropospheric cleaning mechanism then generates harmful intermediates and products in high enough concentration to be considered pollutants. The three major pollutants generated are ozone, PAN and formaldehyde (Objective 1); although there are others associated with photochemical smog such as carbon monoxide, nitrogen oxides, nitric acid, and hydrocarbons. The formation of PAN is detailed on pages 147 to 148 (Objective 2).
The eventual fate of most radicals is in termination reactions with other radicals to form stable neutral species. OH radicals are involved in termination reactions with NO, NO2, and other OH radicals to form nitrous acid, nitric acid, and hydrogen peroxide, respectively (Objective 3).
Carefully reread the final paragraph of this section and make sure you understand how a decrease in NO2 concentration can actually increase ozone concentration when VOC levels are lower than normal (Objective 4). Conversely, you should also understand that if there is an excess of VOCs that reduction in NO2 reduces ozone. Figure 3–2 seen earlier in this unit is helpful in visualizing these two seemingly contractitory observations relating NO2 and ozone concentrations.
Finally, at this point you may want to revisit the section describing the oxidation of methane to convince yourself you can meet Objective 2. That is, can you apply similar principles and chemical equations shown for methane oxidation to the oxidation of other hydrogen–containing molecules.
Exercises
Do Problems 3–31, 3–32, 3–33, and 3–34 within the chapter.
Detailed Chemistry of the Troposphere—Oxidation of Atmospheric SO2: The Homogeneous Gas–Phase Mechanism
Objective
After completing this section, you should be able to describe the gas–phase oxidation of sulfur dioxide and eventual formation of sulfuric acid in the troposphere.
Key Terms
sulfur dioxide (SO2)
sulfur trioxide (SO3)
sulfuric acid (H2SO4)
homogeneous
heterogeneous
Reading Assignment
Read pages 150–151 in the textbook.
Study Notes
Homogeneous gas–phase oxidation, homogeneous aqueous–phase oxidation and heterogeneous oxidation on particles are the three general routes for oxidation of SO2. We will discuss the first two routes in detail in this course.
You should know the chemical mechanism of the gas–phase oxidation and understand that this is not the predominate route to removal from the troposphere; especially in clean air.
Exercises
No exercises have been assigned for this section.
Detailed Chemistry of the Troposphere—
Aqueous–Phase Oxidation of Sulfur Dioxide
Objectives
After completing this section, you should be able to
1. perform calculations on dissolved aqueous gases using Henry’s Law equation.
2. predict a shift in species concentration within an equilibrium system by applying Le Châtelier’s Principle.
3. explain how homogeneous aqueous–phase oxidation of sulfur dioxide occurs.
Key Terms
Henry’s Law
hydrogensulfite (bisulfite, )
Le Châtelier’s Principle
Reading Assignment
Read pages 151–154 in the textbook.
Study Notes
Both Henry’s Law and Le Châtelier’s Principle are concepts that you should have been introduced to in first–year general chemistry (Objectives 1 and 2). However, this may be the first time you have used them together to solve environmentally based problems. If you find you are having difficulties with the chapter problems (3–35, 3–36, and 3–37), you should go back and review equilibrium and acids and bases sections of your freshman chemistry course.
Henry’s Law is given by: [X (aq)] + KH P
Given any two of the above values you should be able to calculate the third using Henry’s Law.
The concentration of CO2 in a soft drink is 0.12 mol L*1. Calculate the CO2 pressure in the bottle over the liquid at 25°C. The Henry’s Law constant for CO2 in water at this temperature is 3.1 10*2 mol L*1 atm*1.
Henry’s Law [X (aq)] + KH P
(0.12 mol L*1) + (3.1 10*2 mol L*1 atm*1) P
P + 4.0 atm
Homogeneous oxidation in water via dissolved ozone or hydrogen peroxide (Objective 3) represents the major oxidation pathway of SO2 (except in very dry conditions).
Exercises
Do Problems 3–35, 3–36, and 3–37 within the chapter.
Do Additional Problem 10 found at the end of the chapter on page 166.
Indoor Air Pollution—Formaldehyde
Objectives
After completing this section, you should be able to
1. list at least three sources of indoor formaldehyde.
2. state the symptoms of sick building syndrome.
3. perform calculations involving rate of emission, air changes per hour, building volume and outdoor concentration of a pollutant to determine the steady state concentration of an indoor pollutant.
Key Terms
formaldehyde (H2C__O)
allergy
asthma
carcinogen
Reading Assignment
Read pages 154–157 in the textbook.
Study Notes
Formaldehyde is a component of various resins like urea–formaldehyde and phenol–formaldehyde and can be slowly released as “free formaldehyde” from these resins. Products like particleboard, plywood, panelling, glass fibre insulation, carpeting, clothing, and drapery are a few common indoor items that can emit formaldehyde (Objective 1).
Since most people living in industrialized nations will spend most of their lifetime indoors, and given that more office buildings are being built as “closed” ventilation systems, indoor air quality has become an area of increased concern. Concentration of gases like formadehyde, carbon monoxide, carbon dioxide, nitrogen dioxide, radon, or exposure to particulates like smoke, pollen, dust, mold, bacteria, viruses or even a lowering of oxygen levels are all related to poor indoor air. The group of illnesses caused by inferior indoor air has been dubbed sick building syndrome (SBS). Symptoms of SBS can include headaches, fatigue, lack of concentration, and nausea (Objective 2). In recent years, SBS has become well publicized both in the scientific and medical literature as well as the news media. The causes of SBS and building related illness are complex. The Environmental Protection Agency (EPA) lists the following key contributing factors:
1. inadequate ventilation
2. pollutants emitted inside of buildings
3. contaminations form outside sources
4. biological contamination
These factors often couple with other conditions such as inadequate temperature, uncomfortable humidity and poor lighting. SBS is also occupant–dependant. Personal health, environmental tolerance, purpose of being in the building and psychological aspects often influence the perception of the occupants’ well being in a building.
The steady state indoor concentration of an air pollutant Ci is given by:
Ci + Co )
where Co is the outdoor concentration, R is the rate of emission of the pollutant, k is the first order rate constant, and V is the volume of the indoor space (Objective 3). The rate constant (k) is usually given in units of inverse hours (h*1) and are sometimes referred to as air changes per hour (ach), which is the number of times the volume (V) of the indoor space is exchanged with outdoor air in one hour.
Warning: When doing these steady–state indoor air calculations make sure that units used are consistent with each other. For example, if emission rate (R) is given in mg h*1 and the volume (V) of the indoor space is in m3, then the concentrations (Ci and Co) must necessarily be in units of mg m*3 for this equation to work.
Exercises
Question 3–C
Your office has a volume of 36 m3 and the new carpet off gases1.60 mg h*1
of formaldehyde. If the outdoor concentration of formaldehyde is
9.2 10*3 mg m*3, what is the minimum air changes per hour (ach) needed to keep the concentration in your office below 0.136 mg m*3?
Do Additional Problem 12 found at the end of the chapter on page 166.
Indoor Air Pollution—
Nitrogen Dioxide and Carbon Monoxide
Objectives
After completing this section, you should be able to
1. state the common source(s) of indoor carbon monoxide and nitrogen dioxide.
2. describe the potential health effect of long–term exposure to nitrogen dioxide.
3. describe the toxicology of acute exposure to carbon monoxide.
Key Terms
oxygenated substance
hemoglobin
carboxyhemoglobin
Reading Assignment
Read pages 156–157 in the textbook.
Study Notes
Any source of combustion could potentially generate either nitrogen dioxide from the heat produced or carbon monoxide through incomplete combustion (Objective 1). Common sources include gas and kerosene appliances like space heaters, stoves, and water heaters. Exposure of nitrogen dioxide is suspected of causing respiratory problems in children (Objective 2).
Carbon monoxide is toxic to humans. Oxygen binds to hemoglobin in the blood and is transported throughout the human body for respiration. Carbon monoxide binds much more strongly with hemoglobin to form carboxyhemoglobin. Carbon monoxide can readily and almost irreversibly displace oxygen on hemoglobin and will suffocate a person exposed to it (Objective 3). This is a serious health hazard in poorly ventilated areas where CO is produced in large quantities (e.g., closed garages with running vehicles). The symptoms of carbon monoxide poisoning usually include yawning, sleepiness and a cherry–red colouring of the skin. The person will fall asleep and eventually die.
Exercises
No exercises have been assigned for this section.
Indoor Air Pollution—
Environmental Tobacco Smoke
Objectives
After completing this section, you should be able to
1. state at least four major health hazards associated with smoking tobacco.
2. describe the components of tobacco smoke.
Key Terms
environmental tobacco smoke (ETS)
tar
nicotine
passive smoking
carcinogen
Reading Assignment
Read pages 157–158 in the textbook.
Study Notes
More than 3,000 years before the discovery of America, tobacco was a sacred plant used by priests and medicine men. It was used to communicate with the spirits and soothe pain. Christopher Columbus discovered tobacco smoking as well as America in 1492 when landing on Cuba. In 1560, tobacco established itself in Europe thanks to Jean Nicot, who was convinced of the plant’s healing properties. Initially the plant (Nicotiana Tabacum) and then the active ingredient (nicotine) carried this entrepreneur’s name.
Nicotine is usually present in its protonated form so it absorbs slowly through the mouth. Hence the need to breathe the smoke into the lungs. Nicotine is suspended on minute particles of tar and absorption from the lungs to the blood occurs in seconds. Remarkably smokers can unknowingly titrate their own nicotine hit. Puffing by smokers seems to subconsciously slow with cigarettes that have higher nicotine content and increases with low–nicotine cigarettes.
Chronic smoking has been shown to cause various unpleasant symptoms such as eye irritation, as well as numerous serious diseases (e.g., heart disease, lung cancer). As you read through this section make yourself a list and commit at least four to memory (Objective 1).
You should be able to identify tar as a tobacco particulate and name several of the gases found in tobacco smoke including carbon monoxide (Objective 2).
Exercises
No exercises have been assigned for this section.
Indoor Air Pollution—Asbestos
Objectives
After completing this section, you should be able to
1. name the two forms of asbestos.
2. list at least three commercial uses for asbestos.
3. explain why asbestos is of environmental concern.
Key Terms
white asbestos (chrysotile)
blue asbestos (crocidolite)
synergy
Reading Assignment
Read pages 158–159 in the textbook.
Study Notes
You may be surprised to find out that asbestos is natural substance. Perhaps because of its widespread use as a fireproofing material or the discovery that it is a human carcinogen, many people assume that it is a synthetic material. You are not responsible to know the chemical formulae for the different forms of asbestos, but you should know that it is a family of silicate minerals. You should also know about white and blue forms of asbestos and the characteristic nature of their fibres (Objective 1). Since asbestos is essentially a fibrous rock, you might realize that the commercial uses listed in this section are not that surprising (Objective 2).
It is interesting to note that it is the structure of the fibres that cause cancer and not their chemistry. The short crocidolite fibres are particularly dangerous because they penetrate deeper into the lungs than the chrysotile fibres. Since asbestos had such widespread use before it was know to be a health hazard, it is an environmental concern by its massive availability to the population. In addition, its removal from buildings often makes it more hazardous than just leaving it alone (Objective 3).
Exercises
No exercises have been assigned for this section.
Indoor Air Pollution—
Radioactivity from Radon Gas
Objectives
After completing this section, you should be able to
1. describe radioactivity and name the type of particles that are emitted from the nuclei during decomposition.
2. describe how radon gas is produced naturally and where it is commonly found.
3. explain the health hazard associated with radon gas.
4. perform basic half–life and nuclear chemistry calculations.
Key Terms
radioactive
alpha particle (a)
nuclear reaction
beta particle (b)
atomic number
mass number
gamma “particle” (g)
half–life (t1/2)
exponential
radon gas
daughters of radon
Reading Assignment
Read pages 159–163 in the textbook.
Correction: In Problem 3–38(c) replace the product 214Pb with 210Pb (p. 160 textbook).
Study Notes
You should be able to describe and carry out calculations for reactions in which alpha, beta, or gamma particles are emitted (Objective 1). The term “particle” is in quotation marks for gamma radiation, because it has historically been thought of as a “wave.” This has occurred for other forms of radiation like light. Many people prefer to think of light as a wave rather than quantized packets of energy or particles (i.e., photons).
You should be able to describe in general terms how radon gas is formed and how it finds its way into buildings (Objective 2). You are not responsible to memorize the decay series describe on page 162, but you should know why the daughters of radon are more dangerous than radon gas itself.
In Unit 2 we discussed how biological problems could be initiated by exposure to UV–B light which can break bonds and cause mutations in DNA. Radioactive nuclei emit particles that can do exactly that. The mutations generated in the DNA can and does initiate cancer (Objective 3). In the case of radon, the cancer is obviously localized in the lungs.
Radioisotopes can undergo a variety of nuclear reactions. Note in nuclear reactions the sum of mass numbers (superscript) and of the atomic numbers (subscript) is the same on both sides of the equation. Here are some examples of typical reactions.
1. alpha decay
238U → 234Th ) 4He
2. beta radiation
131I → 131Xe ) 0e
Note that 0e is a b–particle (essentially an electron)
You also should remember two important equations to help you with some of the problem sets (Objective 4).
1. half–life of decaying radioactive material
t1/2 +
where t1/2 + half–life and k + rate constant
2. energy change in a nuclear reaction
D E + Dmc2
where D E + change in energy, D m + change in mass, and c + speed of light
Exercises
Question 3–D
A 10.0 g sample of gallium–68 decays with a half–life of 68.3 min. How much of this material remains after 12 h?
Do Problem 3–38 within the chapter.
Extra Exercise Answers
The following are answers to extra questions posed within this Study Guide. Short answers are available to in–chapter problems can be found at the end of the textbook. In addition, detailed solutions are available in the accompanying Solutions Manual for Environmental Chemistry by Colin Baird for all problems found in the textbook.
Answer 3–A
1. At sea level assume total pressure is 1.0 atm.
Rearrange equation:
2. Use ideal gas equation
PV + nRT
Rearrange equation
3. Convert molarity to molecules cm*3
Answer 3–B
1. Use Stoke’s Law
2. The particles are greater than 2.5 ìm, therefore they are considered coarse particles.
3.
N it would take 2.8 105 s or about 3.2 days to settle.
Answer 3–C
Use Ci + C0 )
and rearrange equation to solve for air changes per hour
N a minimum of 0.35 ach would be needed
Answer 3–D
Use the half–life equation to determine the rate constant (k)
Rate + k[A]
or the integrated form (see Appendix B: Review of Chemical Kinetics)
N
Review Procedure
1. Review the unit objectives and make sure that you can define, and use in context, the key terms introduced in this unit.
2. Go over the Review Questions (pp. 164–165 in the textbook) to test your factual knowledge of the material covered this unit.
3. Do some of the Supplementary Exercises for Chapter 3 for additional practice or examination preparation. Supplementary Exercises (with answers) can be found on the resource Web pages that accompany the textbook.
4. Do the tutor–marked assignment for Units 2 and 3 (TMA 1), make a photocopy for yourself and send the original to your tutor. Then proceed to Unit 4.
Unit 4
The Greenhouse Effect
and Global Warming
Overview
Classifications such as arctic, tropical, or desert conjure up vivid environmental images as well as defining weather patterns typical of these geographical regions. The climate of any particular region is simply the representative or characteristic weather that is found there from year to year. Although climate is by no means a static phenomenon over more extended periods of time, the relatively recent global warming trend has received a tremendous amount of attention. The atmospheric mechanism for climate change is the major focus of Unit 4. We will examine the nature and chemistry involved in the greenhouse effect and discuss how it might be artificially enhanced through anthropogenic sources of so–called greenhouse gases.
Introductory Section
Objective
After completing this section, you should be able to describe in general terms the link between greenhouse effect and global warming.
Key Terms
greenhouse effect
greenhouse gas
global warming
Reading Assignment
Read page 173 in the textbook.
Study Notes
This section is a general introduction to the rest of Chapter 4 in the textbook.
Exercises
No exercises have been assigned for this section.
The Mechanism of the Greenhouse Effect—
The Earth’s Energy Balance
Objectives
After completing this section, you should be able to
1. list the three types of incoming light from the Sun.
2. state the wavelength ranges for UV, visible, and IR light coming from the Sun.
3. state the wavelength range for thermal IR.
4. describe the greenhouse effect and explain what causes it.
5. differentiate between greenhouse effect and enhanced greenhouse effect
Key Terms
visible light
infrared (IR) light
ultraviolet (UV) light
thermal infrared region
enhanced greenhouse effect
Reading Assignment
Read pages 174–177 in the textbook.
Study Notes
Examine Figure 4–1 (p. 174 textbook) to see both the radiation coming
in from the Sun and the radiation emitted by the Earth’s surface. You are responsible for knowing that a mixture of UV, visible, and IR light make
up the solar radiation (Objective 1), as well as the wavelength ranges each type of light and the thermal IR radiation emitted by the Earth (Objectives 2 and 3). A more complete electromagnetic spectrum is shown in the Review of Photochemistry in Unit 1.
The textbook does an excellent job of describing the greenhouse effect. You should be able to explain how the thermal IR radiation is redirected back to Earth by greenhouse gas molecules (Objective 4). You should also be aware of the greenhouse analogy. A greenhouse is essentially a glass house used to grow plants in cool climates. The glass of the greenhouse allows sunlight to come in, but prevents wholesale mixing of warmed greenhouse air with the cooler outside air. Although the mechanism of trapping heat is slightly different between the atmosphere and a real greenhouse, it is a striking parallel.
It is important to realize that the so–called “greenhouse effect” is a natural phenomenon produced by infrared absorbing gases that are found naturally in the earth’s atmosphere. However, “enhanced greenhouse effect” is the correct term for the anthropogenic increase in the greenhouse gases. As used in the media, the term “greenhouse effect” refers to the additional temperature increase as a direct result of human–made sources of infrared absorbing gases. Even though the term greenhouse effect is often used to mean both greenhouse effect and enhanced greenhouse effect, you should keep them straight in your own mind (Objective 5).
Exercises
No exercises have been assigned for this section.
The Mechanism of the Greenhouse Effect—
Molecular Vibrations: Energy Absorption by Greenhouse Gases
Objectives
After completing this section, you should be able to
1. state the three general types of molecular vibrations.
2. explain the correlation between molecular vibrational frequency and the frequency of light that can be potentially absorbed by a molecule.
3. predict which molecules and/or vibrational modes will absorb infrared light.
Key Terms
molecular vibration
bond stretching
bending vibration
electron cloud
dipole moment
symmetric stretch
antisymmetric stretch
collinear geometry
Reading Assignment
Read pages 177–179 in the textbook.
Study Notes
The three molecular vibrations are symmetric and antisymmetric stretches, as well as bending vibrations (Objective 1). The strongest IR absorption occurs when the molecular vibrational frequency matches the frequency of the light (Objective 2). However, a match in frequency is not enough to ensure light absorption. An absolute requirement for IR light absorption is that there be a net dipole moment change during the vibration (Objective 3). You should be able to identify whether a molecule has a net dipole moment based on its structure. Furthermore you should be able to recognize whether the net dipole moment will change with various types of vibrations.
Exercises
Do Problems 4–1 and 4–2 within the chapter.
Do Additional Problems 1 and 2 at the end of the chapter on pages 218–219.
Correction: Additional Problem 1 (a) answer to “Symmetric, antisymmetric, and bending vibrations for both.” (b) answer add “Only the SO2 symmetric stretch will contribute much.” (p. AN–4 in the textbook).
The Major Greenhouse Gases—
Carbon Dioxide: Emissions and Trends
Objectives
After completing this section, you should be able to
1. perform calculations relating energy and wavelength of light.
2. explain both the annual and general p(CO2) trends seen at the Mauna Loa Observatory.
3. list the sources and sinks of carbon dioxide in the atmosphere.
Key Terms
rotational energy
vibrational energy
ice core sample
polymeric CH2O
fixed carbon
fossil fuel
temporary sink
permanent sink
CO2 fertilization
Reading Assignment
Read pages 179–188 in the textbook.
Study Notes
The information in the Review of Photochemistry component of Unit 1 will help you to relate energy and wavelength of light (Objective 1). The reason for the long term increase in CO2 concentration is somewhat controversial and is not completely understood. However, there is concern that increased human activity in recent years is contributing to the rise in CO2 levels. Such activities include the direct release of CO2 through burning of fossil fuels, as well as deforestation and the release of other greenhouse gases that can in turn impede natural CO2 sinks. The smaller cyclic fluctuations in p(CO2) come about from increased photosynthetic activity in the warmer months of the year, thus temporarily reducing CO2 levels (Objective 2).
Photosynthesis and ocean uptake are carbon dioxide sinks, while respiration, plant decay, and combustion are sources of carbon dioxide (Objective 3). The absorption of CO2 by bodies of water globally is massive. If you remember Henry’s law ([X (aq)] + KH P) from previous units you will recall that the amount of gas dissolved in a liquid is temperature dependent. Essentially, cooler water can absorb more carbon dioxide. Ice core data has always shown a direct correlation between CO2 levels and temperature. However, the cause and effect is not all that clear. Increased carbon dioxide leads to increased temperature through greenhouse effect; but increased temperature leads to increased release of CO2 from water into the atmosphere. Which came first?
Exercises
Do Problems 4–3, 4–4, and 4–5 within the chapter.
Do Additional Problem 8 at the end of the chapter on page 219.
The Major Greenhouse Gases—Water Vapor
Objectives
After completing this section, you should be able to
1. explain what is meant by “window” in relation to the IR emission spectrum from the Earth’s surface.
2. state the wavelength range of this window.
Key Terms
low–altitude cloud
window
Reading Assignment
Read pages 188–189 in the textbook.
Study Notes
One can clearly see the window in Figure 4–7 (p. 181). You should be able to describe this window (Objective 1) and know its approximate range (Objective 2). Note that an absorption spike due to ozone falls within this window at about 9.8 ìm.
Exercises
Do Additional Problems 4 and 5 at the end of the chapter on page 219.
Other Substances That Affect Global Warming—
Trace Gases: Atmospheric Residence Time
Objectives
After completing this section, you should be able to
1. list at least four greenhouse gases.
2. calculate residence time (Tavg), total atmospheric amount (C), and rate of input or output (R), given any two of these values.
Key Terms
pollutant gas
free atom
homonuclear diatomic molecule
heteronuclear diatomic molecule
steady state
residence time
Reading Assignment
Read pages 189–191 in the textbook.
Study Notes
You are not required to memorize Table 4–1 (p. 190). However, you should know that carbon dioxide, methane, and nitrous oxide are greenhouse gases. In addition, you should be able to give one or two specific examples of chloro– or fluorocarbon compounds that are trace greenhouse gases (Objective 1).
The simple formula Tavg + can be used to achieve Objective 2.
Exercises
Do Problems 4–6 and 4–7 within the chapter.
Other Substances That Affect
Global Warming—Methane
Objectives
After completing this section, you should be able to
1. list the six sources and three sinks of methane.
2. explain how the concentration of atmospheric methane can be determined analytically using GC/FID.
3. explain what is meant by positive and negative feedback and give an example of each as it affects global warming.
4. perform radiocarbon dating calculations.
5. describe a clathrate compound.
Key Terms
anaerobic decomposition
swamp gas or marsh gas (methane)
ruminant animal
gas chromatography (GC)
flame ionization detector (FID)
retention time
chromatogram
injector
detector
carbon–14 (14C) (C14)?
radiocarbon dating
positive feedback
negative feedback
clathrate compound
Reading Assignment
Read pages 191–200 in the textbook.
Study Notes
Data obtained from ice core samples in Greenland and Vostok show that CO2, CH4, and temperature are closely related: the fluctuations have followed each other for the past 200,000 years (a relatively short time on the geological time scale). There is still much scientific controversy over what is driving this system. Do the level of greenhouse gases (CO2 and CH4) control the temperatures or do the increased temperatures generate more greenhouse gases? Either way, the fluctuations are abrupt and we are now in a relatively stable temperature/CO2/CH4 flux compared to fluctuations over the past 200,000 years.
The six sources of methane are natural wetlands, fossil fuels, landfills, ruminant animals, rice paddies, and biomass burning (Objective 1). The ranking in the textbook taken from a 1996 analysis by Stern and Kaufman shows only the anthropogenic sources of methane. Natural sources of methane should not be ignored. For example, the amount of methane from natural wetlands is greater than that from livestock. The sinks for methane are destruction by OH in the troposphere, reaction with soil, and loss to the stratosphere (Objective 1).
You do not need to know about all the details of the GC/FID instrumentation itself. You do need to know that a mixture of organics in an air sample can be separated on a GC column and that the resulting chromatogram can be used to identify and determine the amount of methane in that sample (Objective 2).
The concept of negative and positive feedback loops is seen in many natural systems and will be dealt with again in later parts of this course. In general, these natural systems can be very complex. However, it is important to understand how a change in one component may affect another component positively or negatively (Objective 3).
You may wish to review the description of first–order reactions in Unit 1’s Review of Chemical Kinetics to help you with radiocarbon dating calculations (Objective 4). Pay particular attention to the equation describing half–life.
Finally, you will have encountered hydrates (water–containing compounds) in your first–year chemistry course. For example, copper sulfate is commonly found as a brilliant blue crystal that is actually a pentahydrate compound (CuSO4 ⋅ 5H2O), but the anhydrous copper sulfate (CuSO4) is a colourless compound. The water molecules co–crystalize with CuSO4 to form a very energetically stable compound. When a molecule is caged by other molecules it is called a clathrate. Ionic compounds other than copper sulfate, as well as some gases like carbon dioxide, sulfur dioxide, noble gases, and methane, also readily form hydrate clathrates (Objective 5).
Exercises
Do Problems 4–8 and 4–9 within the chapter.
Do Additional Problem 3 at the end of the chapter on page 219.
Other Substances That Affect
Global Warming—Nitrous Oxide
Objectives
After completing this section, you should be able to
1. list two natural and two anthropogenic sources of nitrous oxide.
2. state the sink for nitrous oxide.
3. identify the oxidation state of nitrogen in various nitrogen species.
4. explain the processes of nitrification and denitrification.
Key Terms
nitrous oxide (laughing gas, N2O)
aerobic (oxygen–rich)
anaerobic (oxygen–poor)
denitrification (reduction)
nitrification (oxidation)
nitrate fertilizer
ammonium fertilizer
adipic acid
nylon
Reading Assignment
Read pages 200–202 in the textbook.
Study Notes
Natural sources of nitrous oxide include release by oceans, as well as normal nitrification and denitrification processes in soils. Anthropogenic sources include fossil–fuel combustion, biomass burning, and above all use of agricultural fertilizers (Objective 1). The only real sink for nitrous oxide is eventual destruction in the stratosphere, either photochemically or in reaction with excited state oxygen atoms (Objective 2).
Given any of the nitrogen species shown in Figure 4–14 or compounds such as NO, NO2, and HNO3 you should be able to identify the oxidation state of the nitrogen atom (Objective 3). To achieve this, you should recall how to establish formal charge on an atom in a molecule. We assume that the H atom carries a )1 formal charge and the O atom carries a *2 formal charge. Every atom charge in any given molecule or ion or radical add together to give the total charge of that species. Simple algebra will allow you to deduce the formal charge on any remaining atoms.
For example, to determine the formal charge on nitrogen in nitric acid (HNO3) and hence the oxidation state of nitrogen in that compound, we note that the molecule carries no total charge—it is neutral. We then add up the formal charges of all the atoms and set them to a total of zero.
1H()1) ) 1N(x) ) 3O(*2) + Total Charge (0)
Now solve for x. Before you read further, confirm that x + )5 and therefore the oxidation state of the nitrogen in nitric acid is )5.
You are not responsible for detailed knowledge of biological nitrification and denitrification processes that occur in soils. However, if shown a series of nitrogen species you should be able to identify whether it is being oxidized or reduced and consequently whether it is undergoing nitrification or denitrification. Keep in mind that nitrification is oxidation of the nitrogen and occurs in aerobic soils, while denitrification is a reduction in anaerobic soil (Objective 4). Do not confuse the two processes.
Exercises
Do Problem 4–10 within the chapter.
Do Additional Problem 9 at the end of the chapter on page 220.
Other Substances That Affect Global Warming—
CFCs and Their Replacements and Tropospheric Ozone
Objectives
After completing this section, you should be able to
1. explain why most CFC replacement compounds pose less of a greenhouse threat than CFCs.
2. explain why the amount of IR absorption by tropospheric ozone varies with geographical area.
Key Terms
window region
tropospheric ozone
Reading Assignment
Read pages 202–204 in the textbook.
Study Notes
A recent television advertisement selling 30% fat–reduced chocolate bars has the spokesperson in workout attire on an exercise bicycle, smiling at the camera while assured us we can enjoy even more of our favorite treat. Presumably, the reduction in fat will remove enough guilt to allow us to eat two bars instead of just the one. From a greenhouse gas perspective CFC replacement compounds such as HCFCs and HFCs are a “lite” version of CFCs. A close examination of Table 4–1 reveals that the replacement CFCs do not heat as efficiently as CFCs. They do not absorb as much thermal IR in the window region as their CFC analogs do (Objective 1). However, in absolute terms, replacement CFCs are greenhouse gases and their use and potential release to the atmosphere is still a genuine concern.
Tropospheric ozone is concentrated in areas where air pollution is prevalent (see Unit 3) and is therefore considered a local phenomenon (Objective 2).
Exercise
Do Problem 4–11 within the chapter.
Other Substances That Affect Global Warming—
The Climate–Modifying Effects of Aerosols
Objectives
After completing this section, you should be able to
1. describe the two mechanisms by which light interacts with atmospheric compounds.
2. explain the role of tropospheric sulfate aerosols in altering the surface air temperature of the Earth.
3. list two natural sources of aerosols and the major anthropogenic source.
Key Terms
aerosol particle
reflect
scatter
absorb
albedo
Mount Pinatubo
direct effect
indirect effect
dimethylsulfide (DMS, (CH3)2S) [dk what is the ( doing in the latter?]
phytoplankton
methanesulfonic acid (CH3SO3H)
Reading Assignment
Read pages 204–209 in the textbook.
Study Notes
There are several terms used to describe the interaction of light with particulate aerosols. However, Figure 4–15 (a) illustrates the two mechanisms (light reflection and absorption) that occur with airborne particulates (Objective 1). The terms light scattering and albedo are just alternative words to describe reflection of light. Sulfate aerosols reflect some of the incoming sunlight and have a cooling effect (Objective 2). Sulfates also provide nucleation sites to form fine droplets, which can further increase the amount of sunlight reflected.
The major natural sources of sulfates are volcano eruptions and oxidized volatile sulfur compounds (e.g., DMS) released into the air over oceans by marine phytoplankton. Other plants and microbes also emit a wide variety of sulfur compounds, including methylmercaptan (CH3SH), hydrogen sulfide (H2S), and dimethyldisulfide (CH3SSCH3) to name a few. However, these are minor compared with DMS emissions over the open ocean. The major anthropongenic source of aerosols is through combustion; namely use of fossil–fuels and biomass burning (Objective 2).
Exercise
Do Additional Problem 7 at the end of the chapter on page 219.
Global Warming to Date
Objectives
After completing this section, you should be able to
1. describe the controversy in attributing global warming to anthropogenic sources of greenhouse gases.
2. explain the effect of including aerosols in global temperature modeling.
3. explain the concept of Effective CO2 concentration.
Key Terms
Intergovernmental Panel on Climate Change (IPCC)
Effective Carbon Dioxide concentration
global warming potential (GWP)
Reading Assignment
Read pages 209–217 in the textbook.
Study Notes
You may need to read this section a couple of times to get a sense of the controversy surrounding all the issues (Objective 1). The direct measurement of global temperature is almost useless in determining its correlation to increased greenhouse gas emissions, because of the small fluctuations being measured and the fact that they may be part of a larger climatic change. To get a better handle on this problem scientists have designed computational models to account for observed temperature changes. However, these models are complex and many sources and sinks are neither known nor included in the calculations. Still, the evidence is now strong enough that the controversy is no longer an issue of whether global warming is occurring, but how fast it will happen.
Box 4–1 nicely illustrates the effect of including cooling by aerosols into a simple model (Objective 2). Also, Figure 4–20 on the preceding page shows a good comparison of model (with and without aerosol sulfates) and actual data.
Effective (or Equivalent) Carbon Dioxide concentration is a convenient convention to lump together all greenhouse gases and look at their total effect (Objective 3). In other scientific literature, you may see the term global warming potential (GWP) referred to in describing greenhouse gases. This is used to compare the effectiveness of different greenhouse gases. A potential of 1.0 GWP has been assigned to carbon dioxide (CO2). All other gases are then compared to CO2. For example, a typical CFC may have a GWP of about 2000. This means that it would take 2000 times more CO2 to have the same greenhouse effect as the CFC.
Exercise
Do Additional Problem 6 at the end of the chapter on page 219.
Review Procedure
1. Review the unit objectives and make sure that you can define, and use in context, the key terms introduced in this unit.
2. Go over the Review Questions (pp. 217–218 in the textbook) to test your factual knowledge of the material covered this unit.
3. Do some of the Supplementary Exercises for Chapter 4 for additional practice or examination preparation. Supplementary Exercises (with answers) can be found on the resource Web pages that accompany the textbook.
4. Arrange a time and place for your mid–term examination through the Office of the Registrar.
5. Proceed to Unit 5.
Unit 5
Energy Use, CO2 Emissions, and
Their Environmental Consequences
Overview
Imagine what your life would be like without all the energy you use every day. How would you get to work, have a hot shower, light your way in the dark, cook your food, or communicate by telephone or e–mail? Modern civilization has almost taken energy–driven conveniences for granted. We are in fact extremely dependent on a number of energy sources. However, as we saw in the previous unit, the continued buildup of greenhouse gases resulting from the generation of this energy points strongly to global warming. Unit 5 begins with a survey of global energy uses, trends, and projections, followed by a more detailed look at various conventional and alternative energy sources including fossil fuels, solar energy, and nuclear energy. In each case, we will discuss the chemistry involved and examine the limitations and issues surrounding that source of energy.
Predictions about Future Global Warming, Energy Use, and CO2 Levels—The Potential Consequences of Global Warming
Objectives
After completing this section, you should be able to
1. list at least three potential environmental consequences of global warming.
2. state at least two human health concerns that could result from global warming.
Key Terms
Intergovernmental Panel on Climate Change (IPCC)
positive fertilization effect
malaria
dengue
yellow fever
cholera
Reading Assignment
Read pages 223–226 in the textbook.
Study Notes
As you reread this section, make a note of both environmental (Objective 1) and human health (Objective 2) consequences that may arise from global warming.
The unpredictability of weather systems caused by global warming should be emphasized. It is often underestimated how much we depend on consistent annual weather patterns. Most of us assume that if global temperatures increase, they will do so in evenly around the globe. This is not true. You should appreciate that if weather becomes too erratic from year to year, there are serious implications. For example, a farmer needs to know what time of year to plant and harvest. In some places a change of as little as a week in planting or harvesting can lead to low yields or outright crop failures.
Exercises
No exercises have been assigned for this section.
Predictions about Future Global Warming, Energy Use, and CO2 Levels—Energy Use
Objectives
After completing this section, you should be able to
1. describe the factors which determine the amount of commercial energy consumption in a country.
2. perform calculations involving exponential growth.
Key Terms
commercial energy
Q (stands for quint, which is approximately 1.05 1021 J)
gross national product (GNP)
developing country
developed country
exponential growth
Reading Assignment
Read pages 226–228 in the textbook.
Study Notes
Commercial energy use can vary greatly between countries. For example, in Canada factors such as much industrial activity, expectations of a high standard of living, a cool climate, large distances to travel, and inexpensive energy make the usage per capita relatively high compared with other countries. You should be able to discuss factors such as these when predicting the energy usage of any country (Objective 1). Use the equation V + V0ekt shown in Problem 5–1 to carry out calculations involving exponential growth (Objective 2).
Note: The value Q is equal to exactly 1018 BTU (British thermal units, the imperial measurement for energy).
Exercise
Do Problem 5–1 within the chapter.
Predictions about Future Global Warming, Energy Use, and CO2 Levels—Energy Reserves
Objectives
After completing this section, you should be able to
1. list the major proven energy reserves.
2. identify the major fossil–fuel reserve.
3. state the fundamental origin of coal, oil, and natural gas.
Key Terms
proven reserve
aromatic
polymeric
lignin
Reading Assignment
Read pages 228–231 in the textbook.
Study Notes
Table 5–1 (p. 229 in the textbook) gives a short list of the major proven energy reserves globally, with coal being the clear winner (Objectives 1 and 2). This section does a good job of describing the ultimate sources of the so–called fossil fuels (Objective 3). From a greenhouse gas perspective you can view these fuels simplistically as trapped CO2 and energy.
Exercises
No exercises have been assigned for this section.
Predictions about Future Global Warming, Energy Use, and CO2 Levels—CO2 Emission Scenarios
Objective
After completing this section, you should be able to describe the three emission scenarios relating CO2 emission, resultant atmospheric CO2 levels, and temperature.
Key Terms
emission scenario
climate stabilization
Reading Assignment
Read pages 231–236 in the textbook.
Study Notes
There are no study notes for this section.
Exercises
No exercises have been assigned for this section.
Predictions about Future Global Warming, Energy Use, and CO2 Levels—CO2 Allocation Schemes
Objective
After completing this section, you should be able to describe two potential carbon dioxide emissions allocation schemes and their foreseen consequences.
Key Terms
Rio Environmental Summit (1992)
Kyoto Agreement (1997)
Reading Assignment
Read pages 236–238 in the textbook.
Study Notes
There are no study notes for this section.
Exercises
Do Additional Problems 1 and 3 at the end of the chapter on page 285.
Predictions about Future Global Warming, Energy Use, and CO2 Levels—CO2: Minimizing Future Emissions
Objectives
After completing this section, you should be able to
1. explain the concept of a carbon tax.
2. perform calculations to predict energy and CO2 release from the combustion of a given fossil fuel.
3. describe at least two methods by which carbon dioxide emissions can be sequestered.
Key Terms
stoichiometry
carbon tax
carbon sequesterization
iron fertilization
Reading Assignment
Read pages 238–242 in the textbook.
Study Notes
The carbon content of a fossil fuel determines the amount of carbon dioxide that is eventually released. To encourage use of fuels that generate less CO2 a so–called carbon tax policy has been suggested (Objective 1). You should be able to carry out basic thermodynamic and stoichiometric calculations based on complete and incomplete combustion reactions (Objective 2).
Reread this section and make note of the various CO2 sequestering schemes. You should be aware of all of these, but for examination purposes you must be able to describe only two in detail (Objective 3).
Note: You ought to be aware that a certain number of proposals, such as iron fertilization of oceans, carry some potential danger. Humans know relatively little about Earth’s natural systems and environment, yet often we do not hesitate to effect huge global changes based on a small amount of information. For example, in Unit 2 we saw that CFCs initially seemed chemically inert so they were used as “safe” compounds in refrigerants, solvents, and blowing agents. No one knew that CFCs were serious ozone depleters and greenhouse gases until there was a serious problem. Based on incidences such as this, one should have a healthy skepticism of proposed quick fixes—especially when they involve altering natural systems to any large degree.
Exercises
Do Problems 5–2 and 5–3 within the chapter.
Do Additional Problem 5 at the end of the chapter on page 285.
Solar Energy
Objective
After completing this section, you should be able to list all the major renewable energy sources.
Key Terms
renewable energy
hydroelectric power
wind power
wind farm
biomass
alcohol fuel
geothermal power
Reading Assignment
Read pages 242–244 in the textbook.
Study Notes
This section provides a general introduction to solar energy. The author of your textbook considers many of the renewable energy sources, such as hydroelectric power, wind power, and biomass–related power, as forms of stored solar energy. The only exception is geothermal power, which is truly Earth–based. You should be aware of all these renewable energy sources for the examination.
Exercises
No exercises have been assigned for this section.
Solar Energy—
The Direct Absorption of Solar Energy
Objectives
After completing this section, you should be able to
1. state the two general mechanisms for obtaining energy from sunlight.
2. explain the difference between passive and active systems.
3. describe solar thermal electricity.
Key Terms
thermal conversion
photo–conversion
passive solar heating
active thermal conversion
heat exchanger
endothermic dehydration
hydrated sodium sulfate (Na2SO4 ⋅ 10H2O)
solar thermal electricity
cogeneration of energy
Reading Assignment
Read pages 244–246 in the textbook.
Study Notes
There are two mechanisms for obtaining energy from sunlight (Objective 1). High–energy components of sunlight can be used to change the electronic state of an absorbing compound to generate electricity directly (photo–conversion). We will discuss this in more detail in a subsequent section. Thermal conversion uses sunlight to increase the vibrational, rotational, and transitional modes of a compound (by heating it up), so that it can eventually perform thermodynamic work.
For the exam, you will be expected to give an examples of passive and active solar systems to help explain what they are (Objective 2). In addition, given an example you should be able to identify whether it is a passive or an active system. Finally, you should be able to give a brief description of solar thermal energy is, and comment on the importance of achieving high temperatures and the benefit of cogeneration of energy (Objective 3).
Exercises
No exercises have been assigned for this section.
Solar Energy—Limitations on Energy Conversions: The Second Law of Thermodynamics
Objectives
After completing this section, you should be able to
1. state the Second Law of thermodynamics.
2. calculate the maximum fraction of original heat that can be converted to electricity.
Key Terms
entropy (S)
heat energy (q)
Reading Assignment
Read pages 246–248 in the textbook.
Study Notes
This section provides a quick review of the Second Law of thermodynamics (Objective 1) and develops a simple useful formula to allow you to calculate maximum yield of electricity converted from heat energy (Objective 2). You should commit this formula (p. 247 in the textbook) to memory.
Exercises
Do Problems 5–4 and 5–5 within the chapter.
Solar Energy—Solar Cells
Objectives
After completing this section, you should be able to
1. explain the photovoltaic effect.
2. describe how doping of silicon can induce a directional current in that semiconductor when exposed to sunlight.
Key Terms
photovoltaic effect
valence shell
semiconductor
insulator
conductor
band gap
doping
negative–charge (n–type) semiconductor
positive–charge (p–type) semiconductor
Reading Assignment
Read pages 248–251 in the textbook.
Study Notes
Recall from your first–year physics or chemistry that light shining on a metal surface can cause that surface to emit electrons. This is known as the photoelectric effect. It was explained by Albert Einstein using quantum theory and gained him a Nobel Prize in physics (1921). Only photons of sufficient energy (radiation of high enough frequency) striking the metal can induce the release of electrons. This means if the light is too low in energy, no amount of this light (intensity or brightness) will release electrons from the metal surface. This phenomenon is used in a photovoltaic cell to create a charge potential or separation of negative and positive charges (Objective 1).
At this stage, it is also helpful to understand the concept of energy bands within solid materials and their effect on a material’s conductivity. A material with an energy band that is partially filled by electrons is a metallic conductor, because excited electrons can freely move to neighbouring molecular orbitals. A material is an insulator if an energy band is completely filled and the next empty band is too high in energy for the electrons to conduct. In between a metallic conductor and an insulator we have a semiconductor. A semiconductor also has a completely filled band, but unlike an insulator the next empty band is close enough in energy that an excited electron can jump into the orbitals of that band and move freely. The various types of materials are summarized below in Figure 5–1.
Figure 5.1: Band structures of a variety of compounds (a) metallic conductor, (b) insulator, (c) semiconductor, (d) n–type semiconductor, and (e) p–type semiconductor.
Doping affects the electron occupancy of the energy bands and allows for the conduction of some current. In an n–type semiconductor there is an excess of negative charge and we slightly fill the higher energy band. In a p–type semiconductor we partially empty the lower energy band and there is an overall excess of positive charge. Use Figure 5–7 (p. 250 in the textbook) to visualize how a photo–induced directional current is generated in silicon by doping various parts of the material (Objective 2).
Exercise
Do Additional Problem 2 at the end of the chapter on page 285.
Solar Energy—
Advantages and Disadvantages of Solar Energy
Objective
After completing this section, you should be able to list at least three advantages and three disadvantages of solar energy.
Key Terms
No key terms have been identified for this section.
Reading Assignment
Read pages 251–252 in the textbook.
Study Notes
There are no study notes for this section.
Exercises
No exercises have been assigned for this section.
Conventional and Alternative Fuels and Their Environmental Consequences—Gasoline and
Its Variations
Objectives
After completing this section, you should be able to
1. describe the major types of organic compounds that make up petroleum.
2. explain what is meant by engine knocking.
3. describe the octane number rating scale.
4. list three gasoline additives that can reduce engine knocking.
Key Terms
petroleum (crude oil)
alkane
cycloalkane
aromatic hydrocarbon
BTX (benzene ) toluene ) xylene)
engine knock
octane number
isooctane (2,2,4–trimethylpentane)
n–heptane
tetramethyl lead (Pb(CH3)4)
tetraethyl lead (Pb(CH2CH3)4)
MTBE (methyl tert–butyl ether)
reformulated gasoline
Reading Assignment
If you have had no previous experience in organic chemistry, read the Rings of Carbon Atoms and Benzene sections in the Background Organic Chemistrycomponent of Unit 1.
Read pages 252–256 in the textbook.
Study Notes
Petroleum is a mixture of various organic compounds including mostly alkanes, cycloalkanes, and aromatic hydrocarbons (Objective 1). One of the major steps in petroleum refining is to separate the crude into fractions based on boiling point ranges (Table 5–1). Note that within each range one still has a mixture of hydrocarbons.
|Table 5.1: Hydrocarbon fractions from petroleum |
|Fraction | |Hydrocarbons | |Boiling–point range (°C) | |
|gas | |C1 to C4 | |*160 to 30 | |
|gasoline | |C5 to C12 | |* 30 to 200 | |
|kerosene, fuel oil | |C12 to C18 | |*180 to 400 | |
|lubricants | |C17 to C22 | |*350 and up | |
|paraffins | |C23 and up | |low–melting solids | |
|asphaltenes | |C35 and up | |soft solids | |
Engine “knock” (or “pinging”) is a spontaneous and uncontrolled rapid ignition of the fuel–air mixture in the engine cylinder that occurs during the compression stroke and ahead of the normal flame front (Objective 2). Remember that the octane rating is merely a reflection of a fuel’s resistance to knocking. The higher the rating the more smoothly burning and effective the fuel is. The octane rating is obtained experimentally and reported as a mixture of isooctane and n–heptane (Objective 3). For example, an octane rating of 87 implies that the fuel has the same knocking characteristics as a blend of 87 percent isooctane and 13 percent n–heptane.
To increase octane ratings petroleum also undergoes a variety of catalytic processes during refining such as isomerization (to produce more branched alkanes), cracking (to shorten the more abundant long–chain alkanes), and reforming (to convert alkanes to cyclic aromatics). Additives to reduce knocking are also employed and include tetramethyl and tetraethyl lead, organomanganese compounds, BTX, and MTBE (Objective 4).
Note: To reduce the amount of lead being introduced to the environment, a number of alternative antiknock agents have been investigated over the years, including several organometallic compounds of thallium, cerium, selenium, tellurium, iron and manganese. The most successful of these has been the manganese compound methylcyclopentadienyl manganese tricarbonyl (MMT), which has been used as a replacement for tetramethyl and tetraethyl lead in gasoline in Canada since 1977. Ironically MMT it is more toxic than tetraethyl lead and can interfere with modern catalytic converters. In light of this, Canada has been trying to legislate a reduction in its use. In a strange twist of fate, on July 20, 1998, the Canadian government announced that a regulation previously imposed in 1996 on the importation and inter–provincial trade of this substance was being lifted. This sudden change in policy was precipitated by a multi–million dollar lawsuit by MMT’s manufacturer in the United States under the NAFTA Agreement.
Exercise
Do Problem 5–7 within the chapter.
Conventional and Alternative Fuels and Their Environmental Consequences—Natural Gas
and Propane
Objective
After completing this section, you should be able to state the advantages and disadvantages of using natural gas as a vehicular fuel.
Key Terms
compressed natural gas (CNG, methane ) trace ethane and propane)
liquified petroleum gas (LPG, propane)
energy dense fuel
Reading Assignment
Read pages 256–257 in the textbook.
Study Notes
There are no study notes for this section.
Exercises
No exercises have been assigned for this section.
Conventional and Alternative Fuels and Their Environmental Consequences—Oxygenated Fuels and Additives: Methanol, Ethanol, and Ethers
Objectives
After completing this section, you should be able to
1. write out the water gas shift reaction.
2. use the Mxx and Exx notation for alcohol fuels.
3. state the pollution advantages and disadvantages of using alcohol fuels.
4. describe how ethanol fuel is produced in large quantities.
5. explain why ethanol is not a fully renewable fuel.
6. draw the chemical structure of MTBE.
7. explain why MTBE is used as a gasoline additive.
Key Terms
methanol (CH3OH)
ethanol (ethyl alcohol or grain alcohol, CH3CH2OH)
energy–dense fuel
synthesis gas (hydrogen ) carbon monoxide)
water gas shift reaction
neat
gasahol (gasoline ) alcohol)
tertiary butyl alcohol (2–methyl–2–propanol)
cold start
oxygenated fuel
dimethyl ether (CH3OCH3)
MTBE (methyl tert–butyl ether)
ETBE (ethyl tert–butyl ether)
volatile organic compound (VOC)
Reading Assignment
Read pages 257–266 in the textbook.
Study Notes
Please remember the water gas shift reaction (shown on page 259) is an equilibrium—it can go either forward or backward depending on reaction conditions (Objective 1).
The Mxx (methanol at xx percent) and Exx (ethanol at xx percent) notation is explained at the bottom of page 262 (Objective 2). Go through this section carefully and take note of both the advantages and disadvantages of using alcohol as a fuel (Objective 3). For example, one advantage is that alcohol fuels are energy–dense fuels (compared with natural gas and hydrogen). However, they have slightly less energy density than gasoline (disadvantage). It would require more alcohol than gasoline to propel a vehicle the same distance and its fuel tanks would have to be much larger.
Ethanol can be obtained in large quantities from fermentation of plant material (Objective 4). Although this appears to be a renewable source, the amount of additional energy that must be put into obtain fuel–grade ethanol by way of biomass transport and refining through distillation makes this a losing proposition (the textbook estimates an input of energy that exceeds the eventual output by 25%). This may be less of an issue if biomass burning is used to carry out the distillation instead of fossil fuels. Still, ethanol fuel from biomass is not a fully renewable source of energy (Objective 5).
The structure of MTBE can be found on page 265 (Objective 6). MTBE is used to increase octane number and to oxygenate the fuel so that less CO is produced during combustion. In addition, it is also being used to lower the volatility of the fuel to help reduce problems like vapour lock in fuel lines (Objective 7).
Exercises
Do Problems 5–8, 5–9, 5–10, 5–11, and 5–12 within the chapter.
Do Additional Problems 4, 6, and 7 at the end of the chapter on page 285.
Conventional and Alternative Fuels and Their Environmental Consequences—Hydrogen—
Fuel of the Future?
Objectives
After completing this section, you should be able to
1. describe the advantage of a fuel cell over hydrogen combustion.
2. explain how a fuel cell works giving the balanced equations for the half reactions.
3. explain why operating an electric car may not insure zero emissions of pollutants into the environment.
Key Term
fuel cell
Reading Assignment
Read pages 266–270 in the textbook.
Study Notes
Burning hydrogen is a less efficient way to transfer energy than using a fuel cell. A fuel cell has the added advantage that it does not emit any pollutants during its operation (Objective 1).
You need to commit Figure 5–9 (including the balanced half reactions) to memory (Objective 2). At this point you may wish to go back to your first–year chemistry textbook and quickly review the chapter on electrochemistry.
Zero emissions occur when the electric car is in operation. However, the electricity used to recharge the battery might be generated using fossil fuels. If the electricity were generated from other sources (e.g., wind, nuclear, solar, hydro, and geothermal) then the car would be fossil–fuel–free in its operation and truly have zero emissions (Objective 3).
Exercises
Do Problems 5–13 and 5–14 within the chapter.
Do Additional Problem 8 at the end of the chapter on page 286.
Conventional and Alternative Fuels and Their Environmental Consequences—Hydrogen: Storage
Objective
After completing this section, you should be able to describe three ways in which hydrogen can be stored.
Key Terms
liquid hydrogen
pressurized tank
metal hydride
Reading Assignment
Read pages 270–272 in the textbook.
Study Notes
The three hydrogen storage technologies to date are liquid hydrogen, compressed hydrogen, and metal hydrides. You should be able to list these three storage methods, and indicate advantages and disadvantages associated with each one.
Finally, the textbook mentions the development of graphite fibres to store hydrogen gas. These cylinder–like allotropes of carbon, known as nanotubes, have been widely researched since their discovery in 1991. These nanotubes (1–3 nm in diameter) have high tensile strength, high electrical conductivity, and a high surface area. Hydrogen can readily adsorb onto these carbon fibres.
Exercises
Do Problems 5–15, 5–16, and 5–17 within the chapter.
Conventional and Alternative Fuels and
Their Environmental Consequences—
Hydrogen: Production
Objectives
After completing this section, you should be able to
1. differentiate between an energy source and and energy vector (carrier).
2. describe how hydrogen gas can be generated by electrolysis.
3. explain why hydrogen gas cannot be directly generated from water using sunlight.
4. describe how hydrogen gas can be generated physically using solar energy and chemically using fossil fuels.
Key Terms
energy source
energy vector (energy carrier)
photovoltaic cell
solar tower
Reading Assignment
Read pages 272–274 in the textbook.
Study Notes
Hydrogen gas is essentially a synthetic fuel and as such is not technically a direct energy source (Objective 1). You should be able to make a distinction between energy source and energy carrier (vector) for both hydrogen and any other form of energy. For example, coal is an energy source, but electricity (unless a bolt of lighting hits you) is an energy vector (carrier).
The net reaction for the electrolysis of water is given at the bottom of page 272. In explaining the electrolysis of water you should know the half reactions that occur at both the anode and cathode (Objective 2).
Anode: H2O(l) → 2e* ) 2 H)(aq) ) 1/2 O2(g) (oxidation)
Cathode: 2H2O(l) ) 2e* → H2(g) ) 2 OH*(aq) (reduction)
Although sunlight at ground level carries wavelengths of light capable of cleaving the O–H bond in water, the water molecule itself does not absorb that particular radiation (Objective 3). This section concludes by describing one physical method of generating hydrogen gas using a solar tower and several chemical methods of producing hydrogen from fossil fuels like coal and methane (Objective 4). You should keep at least one of the chemical equations in mind for examination purposes.
Exercise
Do Problem 5–18 within the chapter.
Conventional and Alternative Fuels and
Their Environmental Consequences—
Conclusions Concerning Alternate Fuels
Objective
After completing this section, you should be able to contrast and compare the various alternative fuels now under study and consideration.
Key Terms
International Energy Agency (IEA)
biofuel
Equivalent Carbon Dioxide emission
biodiesel
LPG (propane)
CNG (natural gas)
Reading Assignment
Read pages 274–275 in the textbook.
Study Notes
There are no study notes for this section.
Exercises
No exercises have been assigned for this section.
Nuclear Energy—Fission Reactors
Objectives
After completing this section, you should be able to
1. describe in general terms nuclear fission and nuclear fusion.
2. describe how fission is used in a power generating station.
3. write a typical nuclear reaction involving the fission of uranium–235.
4. distinguish between a fission reaction in a controlled reactor and in an atomic bomb.
5. explain how plutonium is produced in a nuclear power reactor.
6. describe how mining uranium can pollute the local environment.
7. list at least three advantages and three disadvantages of using nuclear power (fission).
8. perform basic half–life and nuclear chemistry calculations.
Key Terms
nuclear energy
fission
fusion
heavy nuclei
isotope
uranium–235 (235U)
proton
neutron
electron
a–particle (helium atom)
b–particle (fast–moving electron)
g–particle (high–energy photon)
fuel rod
decay
radioactive
half–life
weapons–grade uranium
Three Mile Island (1979)
Chernobyl (1986)
Reading Assignment
Read pages 275–279 in the textbook.
Study Notes
The energy release in nuclear reactions, whether they are splitting heavy nuclei (fission) or combining small nuclei (fusion), is tremendous compared with chemical reactions such as combustion (Objective 1). This section primarily deals with the use of fission to generate power (fusion is discussed in a later section). A nuclear power station harnesses the heat energy from these reactions to produce steam, which in turn is used to produce electricity (Objective 2). You should memorize the fundamental fission reaction for uranium–235 isotope at the top of page 276 (Objective 3). Note that a neutron initiates the fission and eventually three neutrons are released. Each of the released neutrons has the potential to split another uranium–235 atom in a so–called chain reaction. If the concentration of uranium–235 is very low, there is a good chance that the generated neutrons will not hit another uranium atom before leaving the mass. In a nuclear power station this is further controlled by separating the fuel into rods and placing moderating material (to stop neutrons) between the rods. However, in a mass with a high concentration and amount of uranium–235 (i.e., weapons–grade uranium) the total number of atoms undergoing fission can double or triple each for each cycle to cause an explosion (Objective 4).
Note: You may sometimes hear the term “critical mass.” In nuclear science, this means the minimum amount of fissionable material that has to be brought together to have a nuclear explosion (or meltdown). The term has also crept into common usage, so that critical mass can mean the minimum number of people or resources that would have to come together to achieve a common goal.
You are not required to memorize the nuclear reactions shown on page 277 for the generation of plutonium. However, you should be able to give a general description of the process and know that uranium–239 is the starting material (Objective 5).
There has been much concern over the use of nuclear energy. In the ore, uranium and other radioactive substances are usually immobile and therefore harmless. The processes of mining the ores and extraction however make radioactive substances labile and thus able to incorporate into biological systems (Objective 6). Note the advantages and disadvantages of nuclear energy are neatly summarized for you in Table 5–6 (Objective 7). You should be aware that risk assessments show that a nuclear power station has a much lower total cost in human life than the building, operation, and dismantling of a similar sized coal–powered station.
The Study Notes for the section entitled Indoor Air Pollution—Radioactivity from Radon Gas in Unit 2 will help you meet Objective 8.
Exercises
Question 5–A
1. Identify the missing species in the following nuclear reactions.
a. 24Na → _____ ) b
b. 150Gd → _____ ) a
c. 57Zn → 56Cu ) _____
d. 7Be ) _____ → 7Li
e. 244Am → 134I ) _____ ) 3n
f. 235U ) n → 135Te ) _____ ) n
g. 226Ra → 222Rn ) _____
h. 43Sc → 42Ca ) p ) g
Nuclear Energy—Plutonium
Objectives
After completing this section, you should be able to
1. explain why spent fuel rods from fission reactors are more radioactive than the original rod.
2. explain the purpose of breeder reactors.
3. describe the process of reprocessing.
4. describe two methods that have been proposed to dispose of excess plutonium.
Key Terms
plutonium–239 (239Pu)
spent fuel rod
reprocessing
weapons–grade plutonium
vitrify
mixed oxide fuel (MOX)
deep geological disposal
Reading Assignment
Read pages 279–282 in the textbook.
Study Notes
It seems counterintuitive that as uranium–235 fuel rods undergo fission reactions more radioactive material (plutonium–239) is produced (Objective 1). Yet, this is exactly what occurs. In fact, breeder reactors have been set up to maximize this process and produce plutonium–239. In most cases the intent is to produce weapons–grade plutonium (Objective 2). One can form compounds (usually salts) of the metals in a fuel rod and effectively separate the various components from each other by using differences in the solubility of these salts. This is called reprocessing (Objective 3).
Finally, disposal of excess plutonium is controversial. Two major methods proposed are noted at the bottom of page 280 (Objective 4).
Exercise
Do Problem 5–19 within the chapter.
Nuclear Energy—Fusion Reactors
Objectives
After completing this section, you should be able to
1. write the nuclear reaction for two fusion reactions.
2. state what radioactive products might be created by a fusion reactor.
3. explain why tritium is particularly dangerous to human health.
Key Terms
hydrogen bomb
deuterium (2H or D)
tritium (3H or T)
Reading Assignment
Read pages 282–283 in the textbook.
Study Notes
As the sun and stars are powered by fusion, some have considered hydrogen the ultimate fuel of the future. The two reactions you need to keep in mind are (Objective 1):
D ) D → He ) n
D ) D → T ) H
Note that these fusion reactions produce radioactive tritium and neutrons. Although neutrons are not radioactive, they can create radioactive substances if absorbed by some other atoms (Objective 2). This means that in stark contrast to fission, fusion produces radioactive substances that are either short–lived or in small amounts. Please note that although tritium is a relatively low energy radioactive isotope of hydrogen, it poses a serious human health risk. Any part of the body that has H2O could potentially incorporate tritium, because it is chemically the same as the non–radioactive isotopes of hydrogen. Once in the body and close to DNA for longer periods of time, tritium can cause substantial damage (Objective 3).
Although we have been able to build nuclear fusion bombs (i.e., hydrogen bombs) investigation still continues on a commercial fusion reactor. Research into controlled nuclear fusion is being carried out in several countries. The Tokamak fusion reactor at the Princeton Plasma Physics Laboratory was one of the more famous and is an important part of the International Thermonuclear Experimental Reactor (ITER) project. It is a huge taurus–shaped reactor that uses powerful electromagnets and a combination of high–tech heating methods to create a contained plasma that fuses hydrogen nuclei.
In 1998 the United States pulled out of the ITER endeavour, which effectively shut down the Tokamak reactor and fusion research. In January 2000 the remaining ITER partners—the European Community, Japan, and Russia—agreed on a design for a smaller machine that is supposed to cut the original cost in half, to about $4 billion. If a smaller, cheaper version of ITER goes ahead, the US might rejoin the project.
Exercises
No exercises have been assigned for this section.
Extra Exercise Answers
The following are answers to extra questions posed within this Study Guide. Short answers to in–chapter problems can be found at the end of the textbook. In addition, detailed solutions for all problems in the textbook can be found in the Solutions Manual for Environmental Chemistry by Colin Baird.
Answer 5–A
1. a. 24Na → 24Mg ) b
b. 150Gd → 146Sm ) a
c. 57Zn → 56Cu ) n
d. 7Be ) b → 7Li
e. 244Am → 134I ) 107Mo ) 3n
f. 235U ) n → 135Te ) 107Zr ) n
g. 226Ra → 222Rn ) a
h. 43Sc → 42Ca ) p ) g
Review Procedure
1. Review the unit objectives and make sure that you can define, and use in context, the key terms introduced in this unit.
2. Go over the Review Questions (pp. 283–285 in the textbook) to test your factual knowledge of the material covered this unit.
3. Do some of the Supplementary Exercises for Chapter 5 for additional practice or examination preparation. Supplementary Exercises (with answers) can be found on the resource Web pages that accompany the textbook.
4. Do the tutor–marked assignment for Units 4 and 5 (TMA 2), make a photocopy for yourself and send the original to your tutor.
5. Prepare to write the midterm examination by reviewing Units 2–5. A sample practice mid–term examination has been provided for you in the Student Manual.
6. After writing the mid–term examination, proceed to Unit 6.
Unit 6
Toxic Organic Chemicals
Overview
“The dose makes the poison” is familiar saying that is quite applicable in this unit. What does this saying mean? As an example consider potassium, which is a mineral that helps to regulate the electrolyte system in the human body. We need to ingest food containing potassium to survive (minimum serum level of 3.5 mmol L*1). Yet, if we consume too much and potassium levels in the body become too high (serum level above 5 mmol L*1) we die. Therefore potassium is both essential and toxic to humans—depending on amount. This Unit 6 will introduce you to some of the basics of toxicology (study of poisons).
We will also look at the chemistry and environmental problems associated with insecticides and pesticides with special emphasis on chlorinated organic compounds such as DDT, PCBs and dioxins. Much like the CFCs studied in Unit 2, many organochlorine compounds are too inert to be readily destroyed, so they become quite persistent in the environment. In addition, biomagnification and bioaccumulation are characteristic problems associated with chloroorganics. We will see a similar problem again for other pollutants such the mercury and lead compounds studied in Unit 7. Unlike heavy metals, chloroorganics can be destroyed or chemically altered to become compounds, which are not harmful.
Introduction
Objectives
After completing this section, you should be able to
1. use the expressions chemical and synthetic chemical correctly.
2. describe hydrophobic in terms of the solubility of a substance.
Key Terms
synthetic chemical
toxicology
organochlorine
hydrophobic
lipophilic
Reading Assignment
Read pages 293–294 in the textbook.
Study Notes
To a scientist all substances are made up of chemicals. However, many people use the term “chemical” when they actually mean “synthetic chemical” (Objective 1). This can sometimes be a source of miscommunication between the scientific community and the general public or vice versa. For example, when people announce that they wish to eat food free of chemicals, they are not saying they wish to have food free of fats, carbohydrates, proteins, sugars, salt, minerals, and water (i.e., an empty plate). They are actually saying they wish to have food free of artificial or synthetic additives.
Molecules with similar polarities dissolve well within each other. So, a nonpolar compound such as oil dissolves well in pentane, but poorly in water. In this case, the oil is said to be hydrophobic (literally, afraid of water) and forms a separate layer on top of the water (Objective 2). Organochlorine compounds are also nonpolar and indeed hydrophobic, but we can also look at them in a slightly different manner. Organochlorine compounds dissolve well in nonpolar substances and are therefore said to be lipophilic (literally, loving fat). This point will become more important as we move through subsequent sections of this unit.
Note: The term “synthetic chemical” also implies there are natural chemicals. This is indeed true. However, many people believe that being synthetic is inherently bad or conversely that being natural is always good. In extreme cases, some also go as far to claim that anything that is natural cannot harm you, because humans are part of the natural ecosystem. This viewpoint is not only simplistic, but also wrong. Socrates took a hemlock tea to end his own life, asbestos (a natural mineral) is one of the few proven carcinogens, and a tracheotomy is needed to keep someone breathing after their throat has swollen shut from accidentally chewing on a small dieffenbachia (a common house plant) [one or two fs?] leaf. Whether a chemical is synthetic or natural has no bearing on its toxicity or danger to human health. Also, the history of any given compound has absolutely no effect on its properties. Vitamin C extracted from an orange is the same compound and has the same properties as Vitamin C synthetically produced in a laboratory.
Does this mean we can use synthetic chemicals without thinking about the consequences? Certainly not! The philosophy that we should avoid the excessive and unnecessary use of any chemical (both synthetic and natural) is absolutely sound. By the way, eating oranges is better for you than swallowing pills, not because the Vitamin C is any different, but because oranges contain several other substances that are nutritionally good for you.
Exercises
No exercises have been assigned for this section.
Pesticides—Types of Pesticides
Objectives
After completing this section, you should be able to
1. explain that pesticides are generally classified by the target organism affected.
2. state the three main categories of pesticides.
Key Terms
pesticide
insecticide
herbicide
fungicide
Reading Assignment
Read pages 294–295 in the textbook.
Study Notes
Table 6–1 gives a good indication of the various pesticides and their classifications based on their target organism (Objective 1). You only need to realize that these various pesticides exist. You need not need remember them all, but you should know the three main categories—insecticides, herbicides, and fungicides (Objective 2).
Exercises
No exercises have been assigned for this section.
Pesticides—Traditional Insecticides
Objectives
After completing this section, you should be able to
1. state the two principal motivations for using insecticides.
2. explain why organic pesticides have replaced inorganic pesticides.
Key Terms
malaria
yellow fever
bubonic plague
fumigant
sodium fluoride (NaF)
boric acid (B(OH)3)
Reading Assignment
Read pages 295–296 in the textbook.
Study Notes
Disease prevention and securing food crops are the two principal motivations for the initial use of pesticides (Objective 1). This section describes some of the history of insecticide use. You are not responsible for knowing specific examples, but you should realize that many of the really nasty pesticides used were metal or metalloid based. They were usually quite persistent in the environment and often just as dangerous to humans as their intended target organism. For these reasons organic pesticides have largely displaced many of the older metal–based pesticides (Objective 2).
Exercises
No exercises have been assigned for this section.
Organochlorine Insecticides
Objectives
After completing this section, you should be able to
1. list the properties characteristic of organochlorines.
2. interconvert concentration values between the parts per scale (e.g., ppm) and a mass per volume scale (e.g., mg L*1).
Key Terms
organochlorine
hexachlorobenzene (HCB, C6Cl6)
ppm (parts per million)
Reading Assignment
Read pages 297–298 in the textbook.
Study Notes
A listing of notable organochlorine properties is given at the top of page 297 (Objective 1).
Earlier in Unit 2 in the section entitled “Regions of the Atmosphere and Environmental Concentration Units for Gases,” we introduced the unit parts per scale by volume used for gases. Take a moment now to make sure you understand how to use the parts per scale for mass (Objective 2). It is slightly different for masses, so do not confuse the two.
Some helpful conversions to remember:
1 ppm + mg kg*1 + mg g*1 (in general)
1 ppm + mg L*1 + mg mL*1 (for water where 1 L + 1 kg)
1 ppm + 103 ppb + 106 ppt
Exercise
Do Problem 6–1 within the chapter.
Organochlorine Insecticides—DDT
Objectives
After completing this section, you should be able to
1. draw the structure of DDT and DDE.
2. state the main uses of DDT.
3. explain why DDT has been banned in many developed countries.
Key Terms
DDT (para–dichlorodiphenyltrichloroethane)
mosquito
World Health Organization (WHO)
DDE (dichlorodiphenyldichloroethene)
insufficient shell (calcium carbonate) thickness
metabolite
nondegradable
fat–soluble
malaria
typhus
human breast milk
Reading Assignment
Read pages 298–302 in the textbook.
Study Notes
The structures of DDT and DDE can be found on page 299 (Objective 1). You should know that the primary use of DDT was as an insecticide against mosquitoes, lice, and fleas to prevent transmittal of disease (Objective 2). Reading the history of its introduction, what insecticides it replaced, and the countless lives it saved will give you an appreciation of its importance.
It has been said that the chemical substance that has saved more human lives than any other is DDT. Conversely, the chemical substance that has claimed more human lives than any other is H2O (presumably drownings). However, most people would still prefer a glass of water instead of a glass of DDT solution.
You should be able to describe how the DDT derivative DDE comes about and its effect on the environment and human health. The combination of these risks couple with the availability of other pesticides and the DDT resistance built up by insects has lead to elimination of its use in many Western industrialized countries (Objective 3).
A mosquito was heard to complain
That a chemist had poisoned his brain
The cause of his sorrow
Was paradichloro
Diphenyltrichloroethane.
Anonymous
Exercise
Do Problem 6–2 within the chapter.
Organochlorine Insecticides—The Accumulation and Fate of Organochlorines in Biological Systems
Objectives
After completing this section, you should be able to
1. explain what is meant by bioconcentration and bioconcentration factor (BCF).
2. differentiate between bioconcentration and biomagnification.
3. explain how partition coefficient (Kow) is used to predict BCF.
4. perform calculations using the partition coefficient equation
Kow + .
Key Terms
bioaccumulation
bioconcentration
bioconcentration factor (BCF)
partition coefficient (e.g., octanol–water is Kow)
food chain
food web
biomagnification
Reading Assignment
Read pages 302–305 in the textbook.
Study Notes
There are terms in this section that are related and should be kept clear in your own mind. Bioconcentration is merely the concentration of a substance in an organism compared with its surroundings. The numerical value associated with this is called the bioconcentration factor (BCF), where
BCF + (Objective 1). Biomagnification is the increase of the concentration of a substance in organism along a food chain (Objective 2). Please do not confuse these terms.
The BCF is calculated using direct measurements substance concentrations [S] from the organism and surroundings. To get an estimate of this value without doing direct measurements, one can determine the partition coefficient (Kow) in the laboratory by a simple standard two–layer extraction (Objective 3). The amount of substance is measured in each layer (octanol and water) and the value of Kow is calculated from their ratio. Given the required information and using the equation on page 303 you should also be able to perform related calculations (Objective 4).
Exercise
Do Problem 6–3 within the chapter.
Organochlorine Insecticides—Analogs of DDT
Objectives
After completing this section, you should be able to
1. explain how DDT works as an insecticide emphasizing the role of molecular shape.
2. give an example of a DDT analog that functions as an insecticide, but does not bioaccumulate.
Key Terms
molecular shape
Na)–initiated nerve impulse
DDD (para–dichlorodiphenyldichloroethane, aka TDE)
bioaccumulate
methoxychlor
Reading Assignment
Read pages 305–307 in the textbook.
Study Notes
You should be able to explain the nerve–binding mechanism, the role of DDT’s molecular shape, and the fact that this toxicological mechanism does not occur in humans (Objective 1). You should also be able to state the differences between methoxychlor and DDT (Objective 2).
Exercises
Do Problems 6–4 and 6–5 within the chapter.
Organochlorine Insecticides—
Other Organochlorine Insecticides
Objectives
After completing this section, you should be able to
1. explain in general terms what toxaphene is and why it was banned.
2. explain in general terms what benzenehexachloride is and why it became restricted.
Key Terms
toxaphene (chlorinated camphene)
International Joint Commission for the Great Lakes (IJC)
1,4–dichlorobenzene
insecticidal fumigant
benzenehexachloride (BHC, 1,2,3,4,5,6–hexachlorocyclohexane)
Reading Assignment
Read pages 307–309 in the textbook.
Study Notes
Toxaphene is a family of compounds created from replacing various hydrogen atoms on camphene (shown below) with chlorine atoms. Toxaphene bioaccumulates, is persistent, and is extremely toxic to fish (Objective 1).
camphene (2,2–dimethyl–3–methylene–bicycle[2.2.1]heptane)
A representative structure of benzenehexachloride is given on page 309. It is also a family of compounds in which the various isomers differ in relative orientations of the chorine atoms on the ring. It was not as widely used or toxic it does bioaccumulate and so its use has been restricted (Objective 2). Note that both toxaphene and BHC are mixtures of related compounds or congeners. We encounter congeners again later in this unit in our discussion of dioxins.
Exercises
No exercises have been assigned to this section.
Organochlorine Insecticides—
Chlorinated Cyclopentadienes
Objectives
After completing this section, you should be able to
1. draw the structure of perchloropentadiene.
2. name three cyclodiene pesticides.
Key Terms
cyclopentadiene
perchlorocyclopentadiene
Persistent Organic Pollutant (POP)
cyclodiene pesticide
aldrin
dieldrin
endosulfan
leachate
mirex
Reading Assignment
Read pages 309–313 in the textbook (exclude Environmental Instrumental Analysis 6–1: Electron Capture Detection of Pesticides).
Study Notes
The structure of perchlorocyclopentadiene is clearly shown at the bottom of page 309 (Objective 1). What is not so clear from the discussion is how the so–called cyclodiene pesticides are formed from perchlorocyclopentadiene or how they are related to each other. Cyclodiene pesticides are formed from the reaction of an alkene and perchlorocyclopentadiene, as shown in the general reaction below.
This general reaction has generated a series of related compounds known as cyclodiene pesticides. Examples of the more well know pesticides in this family are shown below. Note the similar structural feature in each case. You should be able to name at least three of these (including mirex) for examination purposes (Objective 2).
In the early part of 2001, officials from 90 countries signed the Stockholm Convention on Persistent Organic Pollutants (POPs). This is a landmark UN treaty that controls the production, import, export, disposal, and use of these toxic chemicals. It establishes tough international controls on an initial cluster of 12 chemicals, of which most are subject to an immediate ban. These compounds have come to be called the “dirty dozen” and are listed in Table 6–3 (p. 308 textbook). They comprise eight pesticides (aldrin, chlordane, DDT, dieldrin, endrin, heptachlor, mirex and toxaphene); two industrial compounds (polychlorinated biphenols (PCBs) and hexachlorobenzene (HCB), which is also a pesticide); and two byproducts of combustion and industrial processes (dioxins and furans).
Exercises
No exercises have been assigned to this section.
Principles of Toxicology—
Dose–Response Relationships
Objectives
After completing this section, you should be able to
1. differentiate between the methodology in toxicology and epidemiology.
2. identify the key features in typical linear and logarithmic dose–response curves.
3. explain the difference between LD50 (LOD50) and NOEL.
4. calculate a dose of a substance for different body weights.
5. describe the Ames test.
Key Terms
toxicology
epidemiology
acute toxicity
environmental toxicology
chronic exposure
chloracne
extrapolation
dose
dose–response relationship
LD50
LOD50
threshold
NOEL (no observable effects level)
Ames test
Reading Assignment
Read pages 313–318 in the textbook.
Study Notes
The major difference between toxicology and epidemiology is the use of animal tests versus using passive human data to determine the effect of a toxin on human health (Objective 1). You should be able to sketch the approximate shape of both types of dose–response curves and identify where LD50 and NOEL (threshold) occur in the curves (Objective 2). In addition, you should be able to make a distinction between LD50 and NOEL (threshold) and describe what these values represent (Objective 3).
After completing Problem 6–6 within the section, you should have a better idea of what is required to achieve Objective 4.
The Ames test (also known as the Ames assay) is used as a quick analysis to predict whether a substance is likely to be a human carcinogen. A mutant strain of bacteria (Salmonella typhimurium) is treated with the test chemical and then placed on a nutrient–deficient plate that does not support growth of that particular mutant form. If the test chemical causes mutation back to the wild strain of the bacteria (i.e., reversion) a colony of the wild bacteria will thrive in the media on the plate and grow. One merely counts the number of colonies that form on the plate and this is a reflection of mutagenic activity of the particular chemical under investigation (Objective 5).
Exercises
Do Problem 6–6 within the chapter.
Do Additional Problem 2 at the end of the chapter on page 374.
Principles of Toxicology—
Risk Assessment and Management
Objectives
After completing this section, you should be able to
1. state the three pieces of information that need to be known about a chemical in order to perform a risk assessment.
2. estimate the ADI or RfD given the NOEL value or vice versa.
Key Terms
risk assessment
hazard evaluation
dose–response
human exposure
ADI (acceptable daily intake)
MDD (maximum daily dose)
US EPA (US Environmental Protection Agency)
RfD (toxicity reference dose)
Reading Assignment
Read pages 318–320 in the textbook.
Study Notes
A listing of information needed for completing a risk assessment of a chemical is given at the bottom of page 318 (Objective 1).
The ADI or RfD is usually set at 100 times the value of the NOEL (Objective 2). For the purpose of calculations in this course use the 100 factor. However, you should be aware that when data available is potentially more unreliable (e.g., using test animals that do not correlate well with effects in humans) another factor of 10 might be added. That means the ADI or RfD is estimated as being 1000 times the NOEL. Conversely, in some rare cases if human data is available the estimation may be reduced by a factor of 10 and ADI and RfD would be 10 times the NOEL.
Exercises
Do Problem 6–7 within the chapter.
Do Additional Problem 1 at the end of the chapter on page 374.
Other Types of Modern Insecticides—
Organophosphate and Carbamate Insecticides
Objectives
After completing this section, you should be able to
1. draw the general structure of each of the subclasses of organophosphates and give an example of each.
2. draw the general structure of a carbamate and give one example.
3. state one advantage and one disadvantage of carbamate and organophosphate insecticides over organochlorine pesticides.
Key Terms
organophosphate
nonpersistent
pentavalent phosphorus
dichlorvos
parathion
malathion
acetylcholine
carbamate
carbofuran
carbaryl
aldicarb
Reading Assignment
Read pages 320–323 in the textbook.
Correction: In parathion structure, change P__O to P__S (p. 321 textbook)
Study Notes
The general structures for the three subclasses of organophosphate insecticides with an example of each is given in Figure 6–5 (Objective 1). The general structure of a carbamate pesticide is given at the bottom of page 322 and examples are mentioned on page 323 (Objective 2). For the first two objectives you need to know only the general structure and names of specific examples. You are not required to know the structures of the specific examples.
The carbamate and organophosphate insecticides are more toxic in the short term, but are relatively nonpersistent in the environment compared with organochlorine pesticides (Objective 3).
Exercises
No exercises have been assigned for this section.
Other Types of Modern Insecticides—Natural Insecticides and Integrated Pest Management
Objectives
After completing this section, you should be able to
1. identify pyrethrins as a class of pesticides that are natural.
2. state at least five pest control methods used in pest control management.
Key Terms
pyrethrin
integrated pest management
Reading Assignment
Read pages 323–326 in the textbook.
Study Notes
You are not responsible to know the general structure for pyrethrins, but you should know they are a naturally occuring pesticides (Objective 1). Synthetic pyrethrin analogs have been developed by chemists over the years with the intent to produce an effective insecticide that might be more “natural.”
The methods shown for integrated pest management are shown at the bottom of page 326 (Objective 2). In many cases, chemical control ends up being the least costly (financially) method of pest control, especially when dealing with larger areas. This means that quite often the use of chemicals is reduced, but not completely eliminated in any overall strategy combination.
Exercises
No exercises have been assigned to this section.
Herbicides—Triazine Herbicides
Objectives
After completing this section, you should be able to
1. describe the purpose of a herbicide.
2. state an example of a metal–containing herbicide.
3. explain why organic herbicides have replaced metal–containing herbicides.
4. draw the general structure of a triazine herbicide and give one specific example.
Key Terms
defoliate
sodium arsenite (Na3AsO3)
sodium chlorate (NaClO3)
copper sulfate (CuSO4)
triazine
atrazine
photosynthesis
nontoxic metabolite
simazine
metribuzin
Reading Assignment
Read pages 327–329 in the textbook.
Study Notes
Herbicides are used to destroy unwanted plants (Objective 1). Some of the early metal–containing compounds are listed in the second paragraph on page 327 (Objective 2). Modern herbicides are organic because they are less harmful to mammals, less persistent in the environment, and more specific to certain types of plants (Objective 3).
You should be able to reproduce the general structure shown at the bottom of page 327 and know that one R–group is chlorine and the other two are amino groups (Objective 4). You only need to know one of atrizin, simazine, or metribuzin by name as a specific example of a triazine herbicide.
Exercises
No exercises have been assigned for this section.
Herbicides—Other Organic Herbicides
Objectives
After completing this section, you should be able to
1. state two other general classes of organic herbicides.
2. list at least two other specific examples of organic herbicides.
Key Terms
chloroacetamides
alachlor
metolachlor
phosphonate
glyphosate
Reading Assignment
Read pages 329–330 in the textbook.
Study Notes
Chloroacetamides (e.g., alachlor and metolachlor) and phosphonates (e.g., glyphosate) are two general classes of additional herbicides with respective examples of each (Objectives 1 and 2).
Exercises
No exercises have been assigned to this section.
Herbicides—Phenoxy Herbicides
Objectives
After completing this section, you should be able to
1. explain that phenoxy herbicides are synthesized from phenols.
2. draw the chemical structures of 2,4–D and 2,4,5–T.
Key Terms
phenoxy herbicide
phenol
2,4–D (2,4–dichlorophenoxyacetic acid)
2,4,5–T (2,4,5–trichlorophenoxyacetic acid)
MCPA (4–chloro–2–methylphenoxyacetic acid)
Hodgkin’s lymphoma
Reading Assignment
Read pages 330–331 in the textbook.
Study Notes
Although you should be aware that phenoxy herbicides are synthesized from phenols, you are not responsible to know the synthetic route in detail (Objective 1). Both 2,4,5–T and in particular 2,4–D are well–known and important herbicides. You should be able to reproduce their structures shown on page 331 (Objective 2).
Exercises
No exercises have been assigned for this section.
Herbicides—Dioxin Contamination of
Herbicides and Wood Preservatives
Objectives
After completing this section, you should be able to
1. name chlorinated phenols and dibenzon–p–dioxins.
2. draw the chemical structure of 2,3,7,8–TCDD.
3. write the chemical reaction for the production of 2,3,7,8–TCDD from 2,4,5–T.
4. deduce the possible combinations of chlorinated phenols that would generate a given chlorinated dibenzon–p–dioxin.
5. describe what Agent Orange was and how it was used.
6. draw the chemical structure of pentachlorophenol.
7. state the major dioxin congener that would be generated from pentachlorophenol.
8. state the main use of pentachlorophenol.
Key Terms
1,4–dioxin (para–dioxin or p–dioxin)
Agent Orange (1:1 mixture 2,4–D and 2,4,5–T)
2,3,7,8–TCDD (2,3,7,8–tetrachlorodibenzo–p–dioxin)
congener
pentachlorophenol (PCP)
OCDD (octachlorodibenzo–p–dioxin)
Reading Assignment
Read pages 332–337 in the textbook.
Study Notes
In naming both chlorinated phenols and dibenzon–p–dioxins you need to know the standardized numbering for the rings so that you can assign the chlorine positions (Objective 1). In the case of phenols, the oxygen takes position at ring carbon number 1. You then assign chlorines to minimize the position number of the first chlorine. For, example the structure shown below should be named 3,4–dichlorophenol not 4,5–dichlorophenol. The numbering scheme for the dibenzon–p–dioxin ring is shown explicitly in the structure of 2,3,7,8–TCDD, which you should also be able to draw, is given on page 333 (Objective 2). We shall see that this specific dioxin is particularly toxic.
The reaction describing the formation of a dioxin from phenols is shown
at the top of page 332 (Objective 3). In addition, you should be able to elucidate which specific chlorinated phenols might generate a given a dibenzon–p–dioxin (Objective 4). Box 6–2 is an excellent guide to help
you with this particular objective.
As mentioned n the textbook, Agent Orange is a mixture of 2,4–D and 2,4,5–T (Objective 5). You should be able to reproduce the chemical structure of pentachlorophenol shown on page 334 (Objective 6) and know that its only dioxin congener it can form in theory is OCDD shown on page 335 (Objective 7). Please note that OCDD is the major product. There are minor side reactions that occur that will generate small amounts of other dioxin congeners. However, this is beyond the scope of this course. Unlike most of the organochlorines studied in this section PCP is a wood preservative (Objective 8).
Exercises
Do Problems 6–8, 6–9, 6–10, and 6–11 within the chapter.
Do Additional Problems 3, 4, and 5 at the end of the chapter on page 374.
PCBs—The Chemical Structure of PCBs
Objectives
After completing this section, you should be able to
1. draw the structure of a PCB given its name.
2. name a PCB given its structure.
Key Term
PCB (polychlorinated biphenyl)
Reading Assignment
Read pages 337–339 in the textbook.
Study Notes
Use the ring numbering scheme shown on page 338 to assist you with PCB nomenclature (objectives 1 and 2).
Exercises
Do Problems 6–12 and 6–13 within the chapter.
PCBs—The Properties and Uses of PCBs
Objectives
After completing this section, you should be able to
1. state the major chemical and physical properties of PCBs.
2. list at least three past commercial uses for PCBs.
3. explain what is meant by an “open use.”
4. describe where PCBs occur in the environment.
Key Terms
hydrophobic
chemically inert
electrical insulator
open use
electrical transformer
incineration
mass balance
persistence
biomagnification
Reading Assignment
Read pages 339–342 in the textbook.
Study Notes
PCBs are virtually chemically inert, resist combustion, have high thermal stability, do not conduct electricity, and are hydrophobic (Objective 1). Because of these properties they were used for a wide variety of purposes described at the top of page 340 (Objective 2). The most extensive use was in electrical transformers and capacitors for electrical insulation and thermal cooling. Many of these are still in use, but are gradually being replaced as old transformers are decommissioned.
You should understand and be able to use the term “open use” (Objective 3). As we began to understand the potential harm of PCBs open uses were initially banned. We are seeing this now with CFCs. You cannot go into a hardware store anymore and just buy a can of CFCs to use as a solvent. However, CFCs (and their replacement compounds) might be used in closed systems (refrigerators and air conditioners) where their eventual disposal is regulated and controlled by law.
The persistence and lipophilicity of PCBs have allowed them readily establish themselves in the food chain and undergo bioaccumulation and biomagnification as shown schematically in Figure 6–7 (Objective 4).
Exercises
Do Problem 6–14 within the chapter.
Corrections: In Problem 6–14 replace “. . . averaged 0.47 ppt in 1991, . . .” with “. . . averaged 0.047 ppt in 1994, . . .”; also replace “. . . fall to 0.10 ppt?” with “. . . fall to 0.010 ppt?”
Do Additional Problem 8 at the end of the chapter on page 375.
PCBs—Furan Contamination of PCBs
Objectives
After completing this section, you should be able to
1. draw the structure of a PCDF given its name.
2. name a PCDF given its structure.
3. predict the PCDF that would be generated from a given PCB.
Key Terms
furan
dibenzofuran
PCDF (polychlorinated dibenzofuran)
HCl elimination
Reading Assignment
Read pages 343–344 and Box 6–3 (pages 346–347) in the textbook.
Study Notes
Use the ring numbering scheme shown on page 344 to assist you with PCDF nomenclature (objectives 1 and 2). Have a close look at the mechanism of PCDF formation at the top of page 344. You should be able to predict what PCDFs would form given a PCB and, conversely, given PCDF products predict what PCB produced them (Objective 3). You will find that Box 6–3 will help you immensely in achieving this last objective.
Note: As we will discuss in more detail later in the unit, PCBs themselves are not all that toxic. It is their trace byproducts, the dioxins and furans, that can cause extreme biological damage and are primarily responsible for the health risks associated with PCBs.
Exercises
Do Problems 6–15, 6–16, and 6–17 within the chapter.
Do Additional Problem 9 at the end of the chapter on page 375.
PCBs—Other Sources of Dioxins and Furans
Objectives
After completing this section, you should be able to
1. list sources of furans and dioxins in the environment.
2. identify chlorine pulp bleaching as a major dioxin and furan source.
3. state the bleaching agents used in a totally chlorine–free pulp mill.
4. identify incinerators as the largest source of dioxins and furans.
5. describe how dioxins and furans form in an incinerator and the conditions required.
6. explain how dioxins and furans are transported within the environment.
Key Terms
bleached pulp
chlorine dioxide (ClO2)
totally chlorine–free pulp mill
adsorbable organohalogen content (AOX)
PVC (polyvinylchloride, a polymeric plastic)
Reading Assignment
Read pages 345–348 in the textbook.
Study Notes
Natural sources of dioxins and furans in the environment include forest fires and volcanoes. However, anthropogenic sources include byproducts of PCBs and chlorophenols, bleaching pulp, incineration of garbage, recycling metals, petroleum refining, and solvent production (Objective 1). Chlorine bleaching of pulp is identified as a major source (Objective 2). However, some pulp mills use less chlorination or are sometimes totally chlorine–free because they use alternative bleaching agents such as ozone and hydrogen peroxide (Objective 3).
Incineration is the largest environmental source of dioxins and furans (Objective 4). This not only includes the incineration of hazardous wastes such as PCBs, chlorophenols, or other organochlorines, but would mostly include basic industrial and domestic garbage. The key is that temperatures exist that are high enough for dioxin and furan production, but too low for complete destruction. Any time there is a low temperature (less than 500°C) and an organic chlorine source, one has the potential to generate furans and dioxins (Objective 5). Once in the environment, the volatile nature of these compounds leads to transport via the atmosphere (Objective 6).
Note: In North America one of the only facilities licensed to burn PCBs is the hazardous waste treatment centre in Swan Hills, Alberta, Canada. They use a two–stage burning system so that the exhaust from the primary burn is incinerated again. The final exhaust is processed through a series of scrubbers, which remove solids and noxious gases. Natural gas is used as the fuel and a destruction level of better than 99.9999% (US EPA standard) is maintained. In addition a new system is being developed at Swan Hills that would also use discarded paint, toluene and xylenes as a fuel for the PCB burn. Test runs of the new system indicate destruction levels of better than “eight nines” or 99.999999%.
In October 1996 the Swan Hills facility had an explosion in one of their units and vented some of this material to the air for an eight–hour period. This accident raised safety and health concerns, forced a temporary closure of the facility’s PCB transformer furnace, and triggered a private lawsuit from the local First Nations band. The lesson here is that there is always a possibility of accidents. In any of these methods, especially centralized systems, the mere act of handling and transporting PCBs always opens the possibility of accidental spillage into the environment.
Exercises
No exercises have been assigned for this section.
The Health Effects of Dioxins, Furans, and PCBs—
Toxicology of PCBs, Dioxins, and Furans
Objectives
After completing this section, you should be able to
1. explain what component of PCBs make them a health concern.
2. state that different congeners of dioxins and furans have different toxicities.
3. state the pattern of chlorine substitution in dioxins that lead to greater toxicity.
4. explain in general terms the mechanism of toxicity for TCDD and TCDD–like molecules emphasizing molecular size and shape.
5. explain what a coplanar PCB is and what structural feature can destroy that coplanar conformation.
Key Terms
chloracne
in utero
coplanar geometry
ortho position
meta position
para position
specific biological receptor
Reading Assignment
Read pages 348–353 in the textbook.
Study Notes
As mentioned earlier, although PCBs are usually only mildly toxic on their own it is small components of dioxins (PCDDs) and furans (PCDFs) that is of primary health concern (Objective 1). These components are so toxic they often determine the overall toxicity of the bulk PCB. You should realize that these dioxins and furans vary greatly in toxicity as shown in Table 6–6 (Objective 2). Keep in mind a few of the more toxic examples. This should help you to remember the pattern of chlorine substitution that leads to greater toxicity (Objective 3).
From your reading you should realize by now that 2,3,7,8–TCDD is the most toxic organochlorine and is treated as the “gold standard” against which all other furans, dioxins, and PCBs are compared. The mechanism of action of 2,3,7,8–TCDD is that it binds to the Ah receptor protein very strongly. The Ah receptor travels into the nucleus of the cell where it associates with DNA binding sites to initiate production of messenger RNA and subsequent toxic responses. The strong binding to the Ah receptor is dependent on lipophilicity, size, and shape of the ligand. It turns out that 2,3,7,8–TCDD is ideal and any organochlorine that is isoteric (i.e., similar shape and size) also has similar toxic effects (Objective 4).
Of particular importance is the planarity of the ligand to enhance receptor affinity. Similarly substituted PCDDs and PCDFs are prime candidates because they are rigidly planar. PCBs are not rigidly planar, but they can rotate about the central carbon–carbon bond and can therefore adopt a coplanar geometry. So a molecule like 3,3__,4,4__,5__–pentachlorobiphenyl that has a shape, size and planarity reminiscent of 2,3,7,8–TCDD, will show some degree of toxicity (see Table 6–6). As the textbook illustrates on page 351, chlorines in the ortho positions to the biphenyl’s carbon–carbon bond can prevent rotation to the fully coplanar orientation. If this (rotation?) occurs, binding to the Ah receptor is very weak or does not occur and that particular mechanism of toxicity is lost (Objective 5).
Exercise
Do Problem 6–18 within the chapter.
The Health Effects of Dioxins,
Furans, and PCBs—Health Effects in
Humans and Summary of Organochlorines
Objectives
After completing this section, you should be able to
1. perform calculations using TEQs to determine the overall toxicity of a mixture of organochlorines.
2. calculate average residence time (Tavg) of a toxin given total amount (C) and input rate (R).
3. list at least three sub–lethal effects of 2,3,7,8–TCDD.
Key Terms
toxicity equivalency factor (TEQ)
relative acute toxicity
absolute risk
immune system
Great Lakes IJC (International Joint Commission)
persistent organic pollutant (POP)
Reading Assignment
Read pages 353–358 in the textbook.
Study Notes
One can use TEQs mathematically to do a weighted average as shown in the example towards the bottom of page 353. The more toxic components are given more numerical weighting, so that an overall toxicity can be calculated (Objective 1). Some students have difficulty with the concept of weighted averages. Try out the exercise at the end of this section in the Study Guide. It uses an example that will be very familiar to you.
Use the equation shown on page 354 to calculate average residence time (Objective 2). Note this formula is similar to the one on page 190 of the textbook.
You should be able to keep in mind a few sub–lethal effects for 2,3,7,8–TCDD including carcinogenicity, teratogenicity (i.e., birth defects), reproductive complications, skin lesions (e.g., chloracne), and suppression of the immune system (Objective 3). The other TCDD–like organochlorines will have similar symptoms, but to a lesser degree. Several of these effects are based on animal studies and reflect acute responses. Curiously, the Ah receptor is present in a more labile form in humans (i.e., does not bind as strongly) than in other vertebrates, which might account for the lesser sensitivity of humans towards TCDD toxicity.
Exercises
Question 6–A
{What the hell is this doing here?]
This environmental chemistry course has of a 20% midterm examination, a 40% final examination, a 20% term paper, and four assignments worth 5% each. You obtained the following grades listed below, what is your overall mark in the course?
TMA 1 83%
TMA 2 56%
TMA 3 92%
TMA 4 75%
Term Paper 87%
Midterm 51%
Final 88%
Do Problems 6–19 and 6–20 within the chapter.
Do Additional Problem 10 at the end of the chapter on page 375.
Polynuclear Aromatic Hydrocarbons (PAHs)—
The Structure of PAHs
Objectives
After completing this section, you should be able to
1. identify and name naphthalene, anthracene, and phenanthrene given their structures.
2. draw all PAH isomers given a specific empirical chemical formula.
Key Terms
fused ring
naphthalene
anthracene
phenanthrene
aromatic
polynuclear aromatic hydrocarbon (PAH)
Reading Assignment
Read pages 358–359 in the textbook.
Study Notes
The structures of the three most basic PAHs are given on page 358 (Objective 1). The exercises within the chapter will give you practice in drawing PAH isomers (Objective 2).
Exercises
Do Problems 6–21, 6–22, and 6–23 within the chapter.
Polynuclear Aromatic Hydrocarbons (PAHs)—
PAHs as Pollutants
Objectives
After completing this section, you should be able to
1. explain how PAHs are formed during incomplete combustion.
2. list at least three sources of PAHs in the environment.
3. identify the bay region of a PAH.
4. explain the bay region theory.
Key Terms
respirable size
soot
crystallite
graphite
creosote
hydrocarbon cracking
pyrene
benzo[a]pyrene (BaP)
benz[a]anthracene
scrotal cancer
bay region
nitropyrene
dinitropyrene
Reading Assignment
Read pages 359–365 in the textbook.
Correction: The 1–nitropyrene and 1,8–dinitropyrene structures at the bottom of page 364 are wrong. The correct structures are shown below. Note that they do not contain a bay region, but are still suspected carcinogens in diesel exhaust.
Study Notes
You should be able to explain in your own words how C2 radical fragments combine to form stable C6 rings during incomplete combustion (Objective 1). A detailed description is offered in the middle of page 361. Sources of PAHs include creosote, gasoline and diesel exhaust, cigarette smoke, wood and coal smoke, charred food, and any other form of incomplete combustion (Objective 2).
The bay region of a PAH is identified at the top of page 364 (Objective 3). The presence of the bay region means that the metabolites of these PAHs in the body will potentially be carcinogenic (Objective 4). A detailed explanation of the mechanism is given in Box 6–4.
Exercise
Do Problem 6–24 within the chapter.
Long–Range Transport of Atmospheric Pollutants
Objectives
After completing this section, you should be able to
1. describe the three physical properties used to predict the ultimate deposition zone of volatile compounds.
2. explain the grasshopper effect.
Key Terms
long–range transport of atmospheric pollutants (LRTAP)
global fractionation process
vapor pressure
condensation temperature
Koa (1–octanol/air partition coefficient)
grasshopper effect
Reading Assignment
Read pages 365–368 in the textbook.
Study Notes
You should be able to state the three physical properties (listed in Table 6–7) that essentially determine the mobility of a compound (Objective 1). You should be able to use these properties qualitatively to predict where a substance may eventually deposit.
It is important to realize that unless a compound is incredibly mobile, it will undergo a hopping effect as it migrates towards the poles. You should be able to describe this, as well as predict the qualitative nature of the hopping effect based on the three physical properties listed above (Objective 2).
Exercises
Do Problem 6–25 within the chapter.
Environmental Estrogens
Objectives
After completing this section, you should be able to
1. list two examples of organochlorine–based environmental estrogens.
2. list two examples of non–organochlorine environmental estrogens.
3. explain the health concern surrounding estrogen mimics.
4. describe the unusual feature of the dose–response curves for estrogen and estrogen mimics compared with most toxic substances.
Key Terms
hormone
estrogen
estradiol
environmental estrogen
endocrine system
DES (diethylstilbestrol)
bisphenol–A
nonylphenol
phthalate ester
phytoestrogen
Reading Assignment
Read pages 368–372 in the textbook.
Study Notes
This section provides many examples of chlorine and non–chlorine containing environmental estrogens. The organochlorine examples should seem quite familiar from previous sections and several non–chlorine examples are introduced. Make a list for both and commit at least two from each to memory (Objectives 1 and 2).
You should be able to describe in general terms the role of hormones (in particular estrogen) in the body and how estrogen–mimics can interfere with that (Objective 3). Have a close look at the dose–response curve on page 372. You need to be able to contrast this with what is usually seen in dose–response curves for most other toxic substances (Objective 4).
Exercises
No exercises have been assigned to this section.
Extra Exercise Answer
The following are answers to extra questions posed within this Study Guide. Short answers are available to in–chapter problems can be found at the end of the textbook. In addition, detailed solutions are available in the accompanying Solutions Manual for Environmental Chemistry by Colin Baird for all problems found in the textbook.)
Answer 6–A
|Component | |Grade | |Weight | |
|TMA 1 | |83% | | 5% | |
|TMA 2 | |56% | | 5% | |
|TMA 3 | |92% | | | |
|TMA 4 | |75% | | 5% | |
|Term Paper | |87% | |20% | |
|Midterm | |51% | |20% | |
|Final | |88% | |40% | |
Review Procedure
1. Review the unit objectives and make sure that you can define, and use in context, the key terms introduced in this unit.
2. Go over the Review Questions (pp. 372–374 in the textbook) to test your factual knowledge of the material covered this unit.
3. Do some of the Supplementary Exercises for Chapter 6 for additional practice or examination preparation. Supplementary Exercises (with answers) can be found on the resource Web pages that accompany the textbook.
4. Proceed to Unit 7.
Unit 7
Toxic Heavy Metals
Overview
In ancient Rome wine was stored in lead vessels, which would make the
wine somewhat sweet. The lead would leach into the wine and react to form lead acetate, which added sweetness, but unbeknownst to the Romans also made the wine poisonous. Metals—especially heavy metals—pose a unique environmental pollution problem. Heavy metals are especially toxic because their ions are water–soluble and readily taken up by the body. Once in the body they can incorporate and combine with vital enzymes to interfere with their proper function. They bioaccumulate and surprisingly small amounts can cause substantial physiological and neurological damage. In addition, unlike many other pollutants, metals cannot be destroyed or rendered completely harmless. Although there are several “problem” metals, this Unit 7 deals with the more notorious metals: mercury, lead, cadmium and arsenic.
Introduction and Common Features
Objectives
After completing this section, you should be able to
1. explain the difference between a heavy metal and a light metal.
2. describe how most heavy metals are transported from place to place.
Key Terms
heavy metal
mercury (Hg)
lead (Pb)
cadmium (Cd)
arsenic (Ar)
Reading Assignment
Read pages 381–382 in the textbook.
Study Notes
There are no study notes for this section.
Exercises
No exercises have been assigned for this section.
Common Features—Toxicity of the Heavy Metals
Objectives
After completing this section, you should be able to
1. explain the biochemical toxicology of heavy metals and their interaction with sulfhydryl groups
2. describe the medical treatment for acute heavy metal poisoning
3. explain the role of speciation and chemical form on the level of toxicity of a metal
Key Terms
sulfhydryl groups (–SH)
British Anti–Lewisite (BAL)
ethylenediaminetetraacetic acid (EDTA)
speciation
blood–brain barrier
Reading Assignment
Read pages 382–383 in the textbook.
Study Notes
You should be able to describe how heavy metal cations interfere with metabolic reactions within the body by reacting with sulfhydryl groups found in enzymes to form metal–sulfur bonds (Objective 1). Over a longer period
of time heavy metals can incorporate themselves into various structures
in the body. For example, Pb2) is similar in size and charge to Ca2) so it incorporates well into bones. However, acute heavy metal poisoning assumes that most of the metal is still in the blood. In this case, chelation therapy is the preferred medical treatment using compounds like BAL or EDTA to complex the metal (Objective 2).
Warning: Chelation therapy carries a real risk. Organic ligands in the blood can potentially chelate and effectively remove other non–harmful or more importantly necessary metal ions. The treatment is usually closely monitored and often blood serum is supplemented to offset the removal of essential ions.
It is important for you to realize that all metals are a natural part of the environment. Heavy metals are widely distributed, mostly in forms and amounts that do no harm (i.e. rocks). These heavy metals only become a
health and environmental hazard when they are found in high concentrations in or near biological systems. The chemical form (speciation) of a metal is also important in determining its toxicity (Objective 3). For example, C2H5HgCl
is approximately 250 times more toxic than its inorganic analog HgCl2.
The type of exposure also determines the amount of harm the metal can inflict on an organism. Large amounts of elemental liquid mercury can be digested without detrimental effects, because the digestive tract cannot absorb mercury well in its elemental form. In fact, ancient Romans used to drink liquid mercury as a cure for constipation. It is the vapours from liquid mercury that are dangerous. When breathed in they can work their way into the blood stream past the blood–brain barrier directly into the central nervous system. A more modern example is the use of the so–called “barium meal,” which is a suspension of barium sulfate used as a contrasting agent in diagnostic x–ray work. Although barium is toxic, the barium sulfate has a solubility product in the order of 10*10, which means it releases virtually no Ba2) ions. The barium sulfate itself cannot be absorbed in the digestive tract so, as with the elemental mercury example, it too shall pass.
Exercise
Do Problem 7–1 within the chapter.
Common Features—
Bioaccumulation of the Heavy Metals
Objectives
After completing this section, you should be able to
1. state which heavy metals undergo bioconcentration and/or biomagnification
2. relate (mathematically) average lifetime and half–life of a substance
3. calculate the steady–state concentration of a substance (Css) in an organism given the rate of intake (R) and rate constant for elimination (k) for that substance
Key Terms
biomagnification
steady–state concentration (Css)
half–life (t1/2)
average lifetime (Tavg)
Reading Assignment
Read pages 384–386 in the textbook.
Study Notes
By now, you should be able to make a clear distinction between terms like bioconcentration, biomagnification, and bioaccumulation. Many aquatic organisms show evidence of bioconcentrating heavy metals. However, it is only mercury that really exhibits biomagnification (Objective 1).
To perform your calculations remember to use the formulae Css + and
Tavg + 1.44t1/2 (Objectives 2 and 3). You should also recall the concept of steady state, which we discussed in the Study Notes of the section entitled “The Chemistry of the Ozone Layer—Catalytic Processes of Ozone Destruction” in Unit 2. The two schematics below illustrate the similarity between a steady–state system in a chemical reaction where there is an intermediate species and a steady–state system in an organism exposed to a substance.
Exercises
Do Problems 7–2 and 7–3 within the chapter.
Do Additional Problems 1 and 2 found at the end of the chapter on page 416.
Correction: Additional Problem 1 answer replace 138 and 276 with 69 and 138, respectively (AN–5 textbook).
Mercury—The Free Element
Objectives
After completing this section, you should be able to
1. list at least three commercial uses of elemental mercury
2. differentiate between the toxicity of mercury liquid and gas
3. state three sources of mercury vapor in the environment
Key Terms
fluorescent light
blood–brain barrier
mercury vapor (gas)
Reading Assignment
Read pages 386–387 in the textbook.
Study Notes
The toxicity of mercury coupled with its general use has made it a serious environmental concern. Even though many of the health problems have long been known, they have often been ignored because mercury is a useful and versatile substance. Some of the more major commercial uses of elemental mercury include fluorescent light tubes and arc lamps, electrical contacts and switches, and mercury batteries (Objective 1).
We already alluded to the difference in toxicity between gaseous and liquid mercury (Objective 2). The mechanism of entry into the blood and eventually the brain is increased by breathing in vapors as opposed to ingesting the element. You should note that in very young children the blood–brain barrier has not yet fully developed and so they are more susceptible to poisoning by lead and mercury.
The textbook mentions input of mercury into the atmosphere from volcanic eruptions. However, anthropogenic sources such as coal and fuel oil combustion, as well as discarded batteries (Objective 3).
Exercises
No exercises have been assigned for this section.
Mercury—Mercury Amalgams
Objective
After completing this section, you should be able to explain what a mercury amalgam is and give two examples.
Key Terms
amalgam
dental amalgam
porcelain filling
extraction
Amazon River
Reading Assignment
Read pages 387–388 in the textbook.
Study Notes
There are no study notes for this section.
Exercises
No exercises have been assigned for this section.
Mercury—Mercury and the Chloralkali Process
Objectives
After completing this section, you should be able to
1. describe the traditional chloralkali process.
2. explain improvements made to this process using fluorocarbon–based membranes
3. explain the risks and environmental damage associated with the choralkali process.
Key Terms
sodium–mercury amalgam
chloralkali plant
electrolysis
fluorocarbon membrane
Reading Assignment
Read pages 388–389 in the textbook.
Study Notes
In describing each of the processes involved, you should be able to describe which electrochemical reaction occurs at both the anode and cathode (Objectives 1 and 2). You are not responsible for the concept of overvoltage.
A. Chloralkali process with mercury
Cl*(aq) → 1/2 Cl2(g) ) e* (anode)
Na)(aq) ) e* → Na/Hg (cathode)
The Na)(aq) rather than H)(aq) is reduced due to large overvoltage for reduction of H)(aq) at the mercury cathode, because of the lowering of free energy for the formation of the sodium amalgam. After electrolysis, the mercury is recovered and sodium hydroxide and hydrogen gas are generated.
2 Na/Hg ) H2O(l) → 2 NaOH(aq) ) H2(g) ) 2 Hg(l)
B. Chloralkali process with a fluorocarbon membrane
Cl*(aq) → 1/2 Cl2(g) ) e* (anode)
H)(aq) ) e* → 1/2 H2(g) (cathode)
Note the Na)(aq) does not react at the cathode in this case and merely forms an aqueous NaOH solution with the OH*(aq).
The fluorocarbon membrane process must take care not to let the chlorine and hydrogen gases mix as they will violently explode in the presence of light. In the case of the mercury cathode process, the hydrogen gas is generated in a completely different part of the process during treatment of the sodium amalgam after electrolysis. This eliminates the need for keeping the gases in the electrolytic cell completely separate. However, in the recycling of the mercury from the amalgam some mercury is lost to the environment through cooling water or even escapes to an extent in gaseous form. Once in the environment, the mercury can oxidize and be taken up by fish in natural water systems. This would eventually poison any food chain dependent on those fish (Objective 3).
Exercises
No exercises have been assigned for this section.
Mercury—Ionic Mercury
Objectives
After completing this section, you should be able to
1. state two uses for mercury in batteries
2. list at least three symptoms of mercury poisoning
Key Terms
mercury (II) ion (mercuric ion or Hg2))
nervous disorder
mercury cell battery
Reading Assignment
Read pages 389–390 in the textbook.
Study Notes
Mercury cell batteries (developed by Ruben–Mallory in the 1930s) has the advantage that it can maintain a constant voltage (1.3 V) for up to 95% of the battery’s total capacity. The half reactions involved are given below showing the involvement of the mercuric paste in the cathode.
Zn(s) ) 2OH*(aq) → ZnO(s) ) H2O(l) ) 2e* (anode)
HgO(s) ) H2O(l) ) 2e* → Hg(l) ) 2OH*(aq) (cathode)
However, in an ordinary flashlight battery (a dry cell) the anode is zinc and the cathode is a carbon rod. Mercury had been used on the zinc electrode to prevent corrosion. You are not responsible to remember the half reactions in detail, but you should know in general terms how the mercury is being used in each of these two different batteries (Objective 1).
Some mercury poisoning symptoms were mentioned earlier in the section entitled “Mercury—The Free Element.” The symptoms including loss of eyesight, muscle tremors, depression, memory loss, paralysis, and insanity are all essentially nervous disorders (Objective 2).
Aside: Lewis Carroll did not invent the phrase “mad as a hatter.” By the time Alice in Wonderland (1865) was published, it was already well known. One of the first written examples of its use is from a work by Thomas Chandler Haliburton (aka Judge Haliburton), of Nova Scotia, who was well known in the 1830s for his comic writings about the character Sam Slick. In the The Clockmaker (1836) he wrote: “Father he larfed out like any thing; I thought he would never stop—and sister Sall got right up and walked out of the room, as mad as a hatter.” It would seem that since Thomas Haliburton found no need to explain this phrase, it was already in use in his part of the world at that time.
Exercise
Do Problem 7–4 within the chapter.
Mercury—Methylmercury Formation
Objectives
After completing this section, you should be able to
1. describe, in general terms, how and where dimethylmercury and methylmercury are formed
2. state the major source of methylmercury exposure in humans
3. explain why mercury vapor and methylmercury compounds are more toxic to humans than other mercury species
Key Terms
dimethylmercury (Hg(CH3)2)
methylmercury ion (CH3Hg))
blood–brain barrier
human placental barrier
Reading Assignment
Read pages 390–392 in the textbook.
Study Notes
Keep Figure 7–3 in mind when you describe the formation of dimethylmercury and methylmercury in the mud of natural water systems (Objective 1). Please note that these formation processes are necessarily anaerobic in nature. It is not surprising then to know that the consumption of fish constitutes the main source of methylmercury for humans (Objective 2). You should appreciate the small concentrations and the toxicity levels involved. It does not take much of these organomercury compounds to pose a health concern.
Aside: To illustrate this point, Dr. Karen E. Wetterhahn, an experienced chemist at Dartmouth College (USA), died from mercury poisoning in June 1997. In August 1996, during transfer of dimethylmercury in a fume hood, one to several drops spilled on her disposable latex gloves. Approximately three months later, Dr. Wetterhahn began experiencing nausea and vomiting episodes. She then began to lose her balance, then her hearing and eyesight, went into a coma and died 10 months after the initial exposure. This recent incident was well publicized and is mentioned briefly in the next section of the textbook.
The concept of speciation is key to the accessibility of mercury. The fact that ionic species (Hg2) and Hg22)) are not readily transported across biological membranes, greatly reduces their toxicity compared with non–ionic elemental mercury and methylmercury compounds (Objective 3).
Exercises
No exercises have been assigned for this section.
Mercury—Methylmercury Toxicity
Objectives
After completing this section, you should be able to
1. describe Minimata disease and the history surrounding it
2. state at least three of the symptoms of methylmercury poisoning
3. state at least two symptoms of offspring of the mothers of methylmercury poisoning
Key Terms
catalyst
polyvinyl chloride
Minimata disease
cerebral palsy
mental retardation
Reading Assignment
Read pages 392–393 in the textbook.
Study Notes
You should be able to give a general description of the disaster that occurred at Minamata Bay and the “disease” that was named after this small Japanese fishing village of 1300 people (Objective 1). About 200 people died and blood mercury levels of clinical patients were in the 70–900 ppm range. Although Minamata Bay was one of the worst examples, similar poisonings occurred to a lesser degree in other parts of the world. Take for example, the Reed Paper controversy in Dryden, Ontario. A chloralkali plant used to generate chemicals to bleach pulp lost approximately 10 tonnes of mercury to the environment between 1962 and 1970. Two local native bands fished extensively in the water system associated with the paper and chloralkali plants. Their diet included a substantial amount of fish from these waters and some members of these bands had mercury tissue levels of 600 ppb (just on the lower cusp of clinical mercury poisoning range). After 1970 mercury use was limited and after 1975 it was discontinued.
Methylmercury poisoning is very similar to mercury poisoning. Note the symptoms listed on page 393 (Objective 2). Notice that since methylmercury readily passes through the human placental barrier that fetus of an afflicted mother is also subject to methylmercury poisoning. In the case of these infants exposed during development, the symptoms are even more extreme (Objective 3). You should be able to list these.
Exercises
Do Problems 7–5 and 7–6 within the chapter.
Mercury—Other Sources of Methylmercury
and Other Forms of Mercury
Objective
After completing this section, you should be able to list three commercial uses of organomercury compounds.
Key Terms
fungicide
World Health Organization’s “safe limit”
phenylmercury ion (C6H5Hg))
slimicide
Reading Assignment
Read pages 393–395 in the textbook.
Study Notes
The toxicity of mercury coupled with its general use has made it a serious environmental concern. Eventhough many of the health problems have long been known, they have often been ignored because mercury is a useful and versatile substance. Despite the introduction of mercury to the environment by a variety of sources, there is little evidence of worldwide contamination on the same scale as has occurred with lead. It should also be noted that introduction of mercury to the environment by all routes has dramatically decreased in the last 30 years. Stringent control of mercury emissions can be attributed to public awareness of the problem brought about by much publicised events such as the Reed Paper controversy or the Minamata Bay disaster.
Exercises
Do Problem 7–7 within the chapter.
Lead—The Free Element
Objectives
After completing this section, you should be able to
1. describe the two physical features of lead that make it a functional material
2. state at least two uses of elemental lead
Key Terms
Roman
Renaissance
solder (low–melting tin/lead alloy)
tin
lead shot
Reading Assignment
Read pages 395–396 in the textbook.
Study Notes
Lead is both malleable and has a relatively low melting point (327°C) making it easy to work with and shape (Objective 1). In addition, it also has a relatively high density (11.35 g mL*1), which is why it is used in ammunition. Other uses for lead have included water ducts, piping, cooking vessels, solder, roofing, flashing, and soundproofing (Objective 2).
Exercises
No exercises have been assigned for this section.
Lead—Ionic 2+ Lead
Objectives
After completing this section, you should be able to
1. state the two common ionic forms of lead
2. explain how lead can dissolve in water stating the required conditions and the relevant chemical equation
3. explain why lead contamination is less common in hard water areas than in soft water areas
4. list three common commercial past uses for lead(II) salts
Key Terms
galena
lead storage battery
lead solder
lead carbonate (PbCO3)
lead (II) oxide (PbO)
lead–glazed pottery
lead pigment
lead chromate (PbCrO4)
red lead (Pb3O4)
white lead (Pb3(CO3)2(OH)2)
titanium dioxide (TiO2)
PVC stabilizer
lead arsenate (Pb3(AsO4)2)
Reading Assignment
Read pages 396–401 in the textbook.
Omit “Environmental Instrumental Analysis 7–1” (pp. 398–400 in the textbook).
Study Notes
Lead is commonly found in Pb(II) and Pb(IV) oxidation states (Objective 1). We will deal with the lead(IV) species in a later section in this unit. Similar to mercury, many of these lead species are aqueous inorganics. Elemental lead can oxidize in the presence of air (oxygen) if it is in an acidic aqueous medium as suggested by the chemical equation shown on page 396 (Objective 2). The oxygen is necessary for the oxidation to occur, lead will not oxidize to lead(II) with dilute acids on its own. Also, keep in mind that hard water containing dissolved carbonates can form a protective insoluble PbCO3 layer (in the presence of oxygen) on the surface of lead (Objective 3). Although the formation of PbCO3 drastically reduces availability of lead(II) ions in the water, we will see in the next section that this “protective” layer is not completely insoluble.
Applications that involve lead(II) have included lead–acid batteries found commonly in automobiles, lead arsenate pesticides, and polymeric stabilizers in some PVC mini–blinds to name a few. However, exploitation of colours formed from lead(II) salts has been one of the oldest and more universal applications that includes pottery glazes, as well as an array of paint pigments (Objective 4).
Exercise
Do Problem 7–8 within the chapter.
Lead—The Solubilization of “Insoluble” Lead Salts
Objectives
After completing this section, you should be able to
1. explain why PbS and PbCO3 are more soluble in water of low pH.
2. perform basic solubility and equilibrium calculations
Key Terms
lead sulfide (PbS)
lead carbonate (PbCO3)
solubility
Reading Assignment
Read pages 401–403 in the textbook.
Study Notes
Read through the equilibrium equations on page 402 carefully and make sure you understand how the relationship [Pb2)] + 2.5 10*4 [H)] was derived. Another qualitative way of looking at this process is to think of Le Châtelier’s Principle. Consider the two solubility equilibria at the top of page 402.
PbS(s) ↔ Pb2)(aq) ) S2)(aq)
PbCO3(s) ↔ Pb2)(aq) ) CO32*(aq)
As H) is added it will react with S2* (or CO32*) to go on and produce other products (i.e. H2S or H2CO3). To compensate this loss on the products side, the equilibria move to the right, so more PbS and PbCO3 dissolves and re–establishes the S2* and CO32* concentrations (Objective 1).
You may wish to review both equilibrium and acid–base topics in your first–year chemistry course to help refresh your memory to do basic Ksp calculations (Objective 2).
Exercises
Do Problem 7–9 within the chapter.
Do Additional Problem 3 found at the end of the chapter on page 416.
Lead—Ionic 4+ Lead
Objectives
After completing this section, you should be able to
1. recognize lead(II), lead(IV) and mixed lead(II/IV) species given the chemical formula
2. explain the operation of a lead storage battery
Key Terms
highly oxidizing environment
mixed oxide
red lead (Pb3O4)
lead storage battery
Reading Assignment
Page 403 in the textbook.
Study Notes
Lead has )2 and )4 as common valence states and you should be able to recognize this within any chemical formula of a given lead compound (Objective 1). In some cases you may also find mixed oxides (see Problem 7–A below).
The lead storage battery or lead–acid battery is a secondary cell (a chemical cell that can be recharged) and is commonly found in automobiles. The first two chemical equations on page 403 represent the half reactions in a lead storage battery at the anode and cathode, respectively (Objective 2).
Exercise
Question 7–A
Identify how many lead(II) and lead(IV) are in the paint pigment red lead (Pb3O4).
Lead—Tetravalent Organic Lead
Objectives
After completing this section, you should be able to
1. identify the two common anti–knock agents added to leaded gasoline
2. explain the purpose of adding organohalide compounds to leaded gasoline
Key Terms
tetravalent
tetramethyllead (TML, Pb(CH3)4)
tetraethyllead (TEL, Pb(CH2CH3)4)
neurotoxin
Reading Assignment
Page 404 in the textbook.
Study Notes
Tetramethyl– and tetraethyllead were common antiknock agents for gasoline (Objective 1). They are now banned in most developed countries to curb contamination of the environment by lead. To prevent lead build up in the engines organohalides were also added to the gasoline to produce various lead halides that are removed in the exhaust of the automobile (Objective 2).
Exercise
Do Additional Problem 6 at the end of the chapter on page 417.
Lead—Lead in the Environment and
Its Health Effects
Objectives
After completing this section, you should be able to
1. state at least three symptoms of lead poisoning
2. explain why children are more susceptible to lead poisoning
3. describe the medical treatment for acute lead poisoning
Key Terms
catalytic converter
blood–brain barrier
safe level
threshold level
Reading Assignment
Read pages 404–408 in the textbook.
Study Notes
Lead has many similarities to mercury. It is a toxic heavy metal that in high enough concentrations attacks the central nervous system. It is a cumulative poison and because it is a useful substance there has been widespread use and subsequently also widespread pollution of the environment. (In fact, much more environmental contamination than mercury.) However, unlike mercury it does not undergo biomagnification.
Lead is a cumulative poison with a whole body half–life of about six years. Because Pb2) and Ca2) are similar in ionic radius, lead can replace calcium in calcium containing tissue (skeleton, teeth, hair) of the body. Symptoms and effects of lead poisoning include mental retardation of young children, lower birthweights for newborns, higher risk of premature birth, blindness, cerebral palsy, interference with hemoglobin synthesis, growth inhibition and hypertension (Objective 1). The most important biochemical effect of lead is interference in the synthesis of heme. It inhibits key enzymes involved in heme production and results in the accumulation of metabolic intermediates, which the body eventually eliminates. The net result is impairment of synthesis of hemoglobin and other respiratory pigments, such as cytochromes, that require heme. Because of the blood–brain barrier in young children has not fully developed they are especially susceptible to lead poisoning (Objective 2).
In cases of acute lead poisoning a similar technique to mercury poisoning is used. That is, the free ionic species in the blood is chelated before it can pass the blood–brain barrier. For lead ethylenediaminetetraacetic acid (EDTA) is used to complex Pb2) ions in the blood (Objective 3).
Exercises
Do Problem 7–10 within the chapter.
Do Additional Problem 4 found at the end of the chapter on page 416.
Cadmium—The Free Element
Objectives
After completing this section, you should be able to
1. list three sources of cadmium release into the environment
2. explain how a nicad battery works
Key Terms
zinc smelting
nicad (nickel–cadmium) battery
cadmium hydroxide (Cd(OH)2)
Reading Assignment
Read pages 408–409 in the textbook.
Study Notes
Sources of environmental cadmium include release during zinc, copper, or lead smelting, burning coal, and incineration of waste materials (Objective 1). You should know that nicad batteries are secondary cells (i.e. rechargeable) and commonly used calculators, video cameras, and other portable electronic devices. You do not have to memorize the half reactions, but you should know that cadmium metal is oxidized (anode) as shown at the top of page 409 and that nickel(III) is reduced (cathode) to nickel(II) to produce a charge (Objective 2). The reverse reactions occur during recharging.
Aside: Nicad batteries have so–called memory effect. The chemical reactions are reversible, but the physical changes in the electrodes may not be. If a recharge is started before the battery is completely discharged, the next discharge may stop at that point and not provide further energy.
Exercises
No exercises have been assigned for this section.
Cadmium—Environmental Cadmium
Objectives
After completing this section, you should be able to
1. list at least three commercial uses for cadmium compounds
2. state the main source of cadmium exposure for humans
3. describe itai–itai disease
4. explain what metallothionein is and its function in the human body
Key Terms
cadmium pigment
photovoltaic device
photoelectric cell
itai–itai disease
metallothionein
cumulative poison
Reading Assignment
Read pages 409–411 in the textbook.
Study Notes
If you are a painter, you will immediately recognize cadmium yellow as being a unique brilliant yellow paint. Similar to lead, cadmium is important in pigments for paints and plastics. Cadmium is also used in electronic devices like photovoltaic devices, nicad batteries, specialty alloys, and as phosphors in TV screens (Objective 1). We are exposed to cadmium from various sources including those already mentioned in the previous section (i.e. zinc, copper, or lead smelting, burning coal, and incineration of waste materials), but also through cigarette smoke. If you are not a smoker or live near a smelter your exposure through breathing air or drinking water is minimal. The major source of cadmium exposure for most people is seafood and organ meats (Objective 2).
Itai–itai disease is described in detail on page 410 and rests on the principle that Cd2) ions are similar in size and charge to Ca2). Replacement of calcium by cadmium in bone material destroys some of the structural integrity of the bone (Objective 3).
Metallothionein protein is used normally to control zinc levels in the body, but is also the body’s first defense against cadmium poisoning (Objective 4).
Exercise
Do Additional Problem 8 found at the end of the chapter on page 417.
Arsenic
Objectives
After completing this section, you should be able to
1. state the two common valencies of arsenic
2. list three sources of arsenic release into the environment
3. state the major source for human arsenic exposure
4. explain why exposure to organic arsenic in food is not a serious health hazard
5. describe the most toxic species of arsenic and the mechanism of the toxicity
6. describe the symptoms of acute arsenic poisoning
Key Terms
valence shell
trivalent arsenic (As(III))
pentavalent arsenic (As(V))
leachate
lead arsenate (Pb3(AsO4)2)
calcium arsenate (Ca3(AsO4)2)
sodium arsenite (Na3AsO3)
Paris Green (Cu3(AsO3)2)
synergistic
death by wallpaper
arsine gas (AsH3)
trimethyl arsine (As(CH3)3)
Reading Assignment
Read pages 411–414 in the textbook.
Omit “Box 7–1 Organotin Compounds” (p. 413 in the textbook)
Study Notes
Arsenic is found in either the )3 or )5 valence state (Objective 1). There are many minor uses of arsenic like GaAs electronics (see Unit 5 on solar cells) or producing glass. In the past it was heavily used in pesticides, before organic products were available (see Unit 6). Arsenic was even present in early medications like Salvarsan_® (arsphenamine) used to cure syphilis. Its present release into the environment includes use of arsenic pesticides, wood preservatives, mining (lead, gold, copper, and nickel), producing iron and steel, and combustion of coal (Objective 2). Most people are exposed to arsenic through drinking water (Objective 3) and through foods they eat. However, foods will most contain organic arsenic sources that are water–soluble and excreted (i.e. not readily taken up by the body). In addition, any absorbed organic arsenic can be readily methylated and excreted by the body (Objective 4).
Arsines (e.g. AsH3 and As(CH3)3) are the most toxic arsenic compounds, because of their tendency to bond strongly to sulfhydryl groups of enzymes in the body (Objective 5). Acute poisoning can result in damage to the digestive tract and produces symptoms like vomiting and diarrhea (Objective 6). Similar to acute mercury poisoning, the treatment for acute arsenic(III) poisoning is chelation therapy with BAL.
Aside: It had long been rumoured that Napoleon died in 1821 of deliberate arsenic poisoning during his exile on St. Helena. However, it is a more likely scenario that trimethyl arsine gas produced by moulds on wallpaper containing the arsenic pigment Scheel’s green (copper arsenite) which was not uncommon in Napoleon’s time, lead to eventual “death by wallpaper.” Hair samples from Napoleon show a slow buildup of arsenic (arsenic accumulates in the keratin of hair and fingernails), which is consistent with chronic exposure and bioaccululation rather than an acute poisoning.
Exercises
Do Problem 7–11 within the chapter.
Do Additional Problems 5 and 7 found at the end of the chapter on pages 416 to 417.
Extra Exercise Answer
(Note that the following is an answer to an extra question posed within this Study Guide. Short answers are available to in–chapter problems can be found at the end of the textbook. In addition, detailed solutions are available in the accompanying Solutions Manual for Environmental Chemistry by Colin Baird for all problems found in the textbook.)
Answer 7–A
Red lead (Pb3O4) is a mixed oxide with three lead atoms and four oxygen atoms. Assume all the oxygen atoms are *2 in charge, for a total of *8 charge. The lead atoms must balance this to obtain a neutral Pb3O4 compound. This can only be achieved by two lead(II) atoms plus one lead(IV) atom within the compound.
2Pb(II) ) 1Pb(IV) ) 8O(II) + Total Charge
2(2)) ) 1(4)) ) 8(2*) + 0
Review Procedure
1. Review the unit objectives and make sure that you can define, and use in context, the key terms introduced in this unit.
2. Go over the Review Questions (pp. 415–416 textbook) to test your factual knowledge of the material covered this unit.
3. Do some of the Supplementary Exercises for Chapter 7 for additional practice or examination preparation. Supplementary Exercises (with answers) can be found on the resource Web pages that accompany the textbook.
4. Do the tutor–marked assignment for Units 6 and 7 (TMA 3), make a photocopy for yourself and send the original to your tutor. Then proceed to Unit 8.
Unit 8
The Chemistry of Natural Waters
Overview
Water is life. About three–quarters of the Earth’s surface is covered in water. Living cells consist of 70–95% water. A human can survive 60 days without food, but only three days without water. We use large quantities of it in agriculture, energy conversions, mining and industry; water systems are used for transportation. Chemists consider water to be the universal solvent. Although it is often ignored, water is an integral and necessary part of our lives. Unit 8 deals with the chemistry of various natural waters (i.e., ground water, oceans, lakes, and rivers) and the interaction of the water with both the atmosphere and underlying rock. Emphasis will be given to describing the quality of natural water systems based on dissolved gases and solids in that water.
Introduction and Groundwater
Objectives
After completing this section, you should be able to
1. describe various regions in the soil in relation to groundwater.
2. explain why groundwater has been traditionally considered a pure form of water.
3. describe what is meant by an aquifer and an artesian aquifer.
Key Terms
natural water
acid–base reaction
oxidaton–reduction (redox) reaction
aeration (unsaturated) zone
saturated zone
groundwater
water table
aquifer
artesian (confined) aquifer
Reading Assignment
Read pages 421–423 in the textbook.
Study Notes
Figure 8–1 clearly illustrates the various regions in soil (Objective 1). You should be able to characterize and distinguish between the saturated and unsaturated zones. Surface water slowly seeps through soil filtering out organic matter. The water usually contains fewer microorganisms and so it is considered potable (Objective 2). All aquifers are permanent reservoirs of water, but artesian (confined) aquifers are actually sandwiched between layers of impermeable rock and are under pressure from gravity. Depending on the height of the surrounding water table, water can literally “spring” from a well or natural opening up to the surface.
Figure 8.1: Artesian Aquifer
(1) Water enters aquifer through porous soil.
(2) Water percolates and seeps laterally.
(3) Water escapes through well under pressure.
Exercises
No exercises have been assigned for this section.
Oxidation–Reduction Chemistry in
Natural Waters—Dissolved Oxygen
Objectives
After completing this section, you should be able to
1. perform calculations based on Henry’s Law.
2. explain how thermal pollution can lead to fish kills.
Key Terms
molecular oxygen (O2)
Henry’s Law constant (KH)
thermal pollution
Reading Assignment
Read pages 423–424 in the textbook.
Study Notes
You should have been introducted to the concept of Henry’s Law and its use in first–year general chemistry (Objective 1). We did Henry’s Law calculations in Unit 3 in the section entitled “Detailed Chemistry of the Troposphere—Aqueous–Phase Oxidation of Sulfur Dioxide.”
Fish need oxygen in the water to breathe. Fish kills from reduced dissolved oxygen (t 5 ppm) can occur because of presence of oxidizable materials (e.g., sewage) or thermal pollution. You should be able to connect the concept of dissolved molecular oxygen, Henry’s Law, and resulting fish kills from elevated water temperatures (Objective 2).
Exercises
Do Problems 8–1 and 8–2 within the chapter.
Oxidation–Reduction Chemistry in
Natural Waters—Oxygen Demand
Objectives
After completing this section, you should be able to
1. state two major causes of fish kills by dissolved oxygen depletion.
2. explain the two analytical methods to determine the concentration of dissolved oxygen in water.
3. differentiate between TOC and DOC.
4. explain how values of BOC, COD, TOC, and DOC relate to amount of organic substances present in a sample of water.
5. perform basic calculations related to BOD, COD, TOC, and DOC.
Key Terms
polymerized carbohydrate (CH2O)
aerated water
stagnant water
biochemical oxygen demand (BOD)
chemical oxygen demand (COD)
total organic carbon (TOC)
dissolved organic carbon (DOC)
Reading Assignment
Read pages 424–427 in the textbook.
Study Notes
Dissolved oxygen can be depleted through oxidation reactions with organic matter such as agricultural runoff, sewage, decomposing algae blooms, and factory effluents. In addition to this chemical reduction, the dissolved oxygen can be reduced through thermal pollution. In either case, if oxygen levels fall below 5 ppm fish will suffocate (Objective 1). You should be able to describe BOD and COD tests in general terms (Objective 2). In addition, you should know specifically what TOC and DOC measure and the key difference between these two values (Objective 3).
For all the tests mentioned, you should realize that they are all direct or indirect ways of measuring the amount of organic matter in a water sample (Objective 4). In the case of TOC and DOC, the amount of carbon, and therefore organic matter, is reported directly. However, the demand for oxygen (BOD and COD) reflects the amount of organic substances indirectly. The in–chapter exercises will give you an indication of the type of calculations you are expected to perform (Objective 5).
Exercises
Do Problems 8–3, 8–4, 8–5, and 8–6 within the chapter.
Corrections: Problem 8–5 answer change units from mL to mg L*1 and, conversely,
Problem 8–6 answer change units from mg L*1 to mL (p. AN–6 textbook).
Problem 8–6 answer on second line change 3.0 to 30 (p. 94 in the Solutions Manual).
Do Additional Problem 1 at the end of the chapter on page 458.
Oxidation–Reduction Chemistry in
Natural Waters—Anaerobic Decomposition
of Organic Matter in Natural Water
Objectives
After completing this section, you should be able to
1. draw a stratified lake in the summer labeling reducing (anaerobic) and oxidizing (aerobic) areas.
2. identify stable forms of carbon, nitrogen, sulfur, and iron that would be found in aerobic and anaerobic strata in your drawing for Objective 1.
Key Terms
anaerobic (oxygen–free)
fermentation reaction
marsh or swamp gas (methane)
stable stratification
Reading Assignment
Read pages 427–431 in the textbook.
Omit “Environmental Instrumental Analysis 8–1” (pp. 428–430 in the textbook).
Study Notes
Fermentation is essentially any oxygen–free reaction that extracts energy from an organic compound. There are various types of fermentation reactions. For example, most of us are familiar with lactate fermentation. When you exercise vigorously you accumulate and “oxygen debt” in your muscles by producing lactic acid from glucose. The lower pH reduces the capacity of the muscle fibres to contract, which produces a sensation of muscle fatigue. Eventually, the lactate diffuses into the blood and is subsequently metabolized by the liver. Another fermentation reaction you might be familiar with is alcoholic fermentation. Yeast can take energy anaerobically from sugars in malt (for beer) or grape juice (for wine) by converting them to carbon dioxide and ethanol.
You should memorize Figure 8–2 and keep straight in your mind what occurs chemically in each stratum (Objectives 1 and 2).
Exercises
No exercises have been assigned for this section.
Oxidation–Reduction Chemistry in
Natural Waters—The pE Scale
Objectives
After completing this section, you should be able to
1. describe the concept of pE.
2. state the characteristic values of pE one would expect in the upper and lower strata of a lake.
3. calculate pE from a standard electrode potential and concentration of species in the half reaction.
4. determine the ratio of species in a half reaction given the pE and the standard electrode potential.
Key Terms
pE (effective concentration (activity) of electrons)
E (electrode potential)
E0 (standard electrode potential)
Reading Assignment
Read pages 431–434 in the textbook.
Correction: Fourth line, last paragraph replace “divided by RT/F” with “divided by 2.30 RT/F” on page 432. (The 2.30 factor corrects for the ln x to log x conversion.)
Study Notes
At this time, you would do well to review your acid–base and electro–chemistry with special emphasis on the concepts of pH and the Nernst equation. Much of the mathematical manipulations involved in pE calculations parallels these two concepts (Objective 1).
Have a look again at Figure 8–2 (p. 431 textbook). The top oxidizing (aerobic) layer is expected to have larger pE values (pE [ 13.7) than the lower reducing (anaerobic) layer (pE [ *4.1). Although you should know these approximate values, they can vary substantially depending on specific circumstances (Objective 2).
Your calculations will be based on two important sets of equations as follows (Objectives 3 and 4):
pE + pE0 * logQ
where Q is the reaction quotient, which is the concentration of products over reactants in the reaction.
Exercises
Do Problems 8–7, 8–8, and 8–9 within the chapter.
Do Additional Problem 2 at the end of the chapter on page 458.
Oxidation–Reduction Chemistry in Natural Waters—Sulfur Compounds in Natural Waters
Objectives
After completing this section, you should be able to
1. explain what gives swamp gas its unpleasant odor.
2. identify the oxidation state of sulfur in a given compound.
3. give an example of a highly reduced and highly oxidized sulfur in environmentally important compounds.
4. write the balanced equation by which sulfate can oxidize organic matter.
Key Terms
hydrogen sulfide (H2S)
sulfuric acid (H2SO4)
anaerobic bacteria
Reading Assignment
Read pages 434–435 in the textbook.
Study Notes
In low concentrations, hydrogen sulfide gas smells distinctly of rotten eggs. The gas is incredibly toxic and is insidious, because at higher concentrations it cannot be smelled. Other related sulfur–containing organics released from swamps, such as methanethiol and dimethyl sulfide, also have an unpleasant odour (Objective 1).
Note: Compounds containing the mercapto group (*SH) are called thiols. Skunk scent is caused by simple thiols like 2–butene–1–thiol and 3–methyl–1–butanethiol. It is ironic that although toxic hydrogen sulfide is removed from natural gas, the gas is later spiked with small amounts of volatile thiols (non–toxic) so that leaks can be easily detected.
You should be familiar with Table 8–1 so that you can identify the oxidation state of sulfur given any of those species (Objective 2). In addition, you should be able to offer at least one example each of a sulfur species having a *2 and a )6 oxidation state (Objective 3). Finally, you should memorize the equation at the bottom of page 435 (Objective 4).
Exercises
Do Problem 8–10 within the chapter.
Do Additional Problem 3 at the end of the chapter on page 458.
Oxidation–Reduction Chemistry in
Natural Waters—Acid Mine Drainage
Objectives
After completing this section, you should be able to
1. explain the process of acid mine drainage.
2. write the balanced net equation representing processes in acid mine drainage.
Key Terms
iron pyrites (fool’s gold, FeS2)
disulfide ion (S22*)
iron(III)sulfate (Fe2(SO4)3)
Reading Assignment
Read pages 436–437 in the textbook.
Study Notes
You are expected to be able to give both a qualitative description of acid mine drainage and the chemical processes involved (Objective 1), as well as to write out the overall reaction in this phenomenon (Objective 2).
Exercise
Do Problem 8–11 within the chapter.
Oxidation–Reduction Chemistry in Natural Waters—Nitrogen Compounds in Natural Waters
Objectives
After completing this section, you should be able to
1. identify the oxidation state of nitrogen in a given compound.
2. give an example of a highly reduced and highly oxidized nitrogen in environmentally important compounds.
3. differentiate between nitrification and denitrification processes.
Key Terms
ammonia (NH3)
nitrate ion (NO3*)
nitrite ion (NO2*)
nitrification
denitrification
Reading Assignment
Read pages 437–438 in the textbook.
Study Notes
You should be familiar with Table 8–2 so that you can identify the oxidation state of nitrogen given any of those species (Objective 1). In addition, you should be able to offer at least one example each of a nitrogen species having a *3 and a )5 oxidation state (Objective 2).
Be aware that there is a complex web of nitrogen–containing compounds (often called the “nitrogen cycle”) that occurs in the natural world. You may have heard the term “nitrogen fixation” when this nitrogen cycle is discussed. As molecular nitrogen is inert it must be combined with other elements
(i.e., fixed) to participate in biological reactions. The conversion (fixation) of molecular nitrogen to ammonium or nitrates can occur through natural processes or synthetically (production of fertilizers).
Some plants do use ammonium directly from the soil, but the majority need the nitrate form so the nitrification of ammonia is important. You will recall our discussion of N2O generation in Unit 4. Have another careful look at Figure 4–14 (p. 201 in the textbook), which summarizes nitrification and denitrification in the soil (Objective 3).
Exercises
No exercises have been assigned for this section.
Oxidation–Reduction Chemistry in Natural Waters—Nitrates and Nitrites in Food and Water
Objectives
After completing this section, you should be able to
1. state the main source of nitrates in drinking water.
2. explain the health concern surrounding the presence of nitrates in food and water.
Key Terms
ecosystem
algal bloom
limiting nutrient
methemoglobinemia (blue baby syndrome)
hemoglobin
non–Hodgkin’s lymphoma
epidemiological investigation
Reading Assignment
Read pages 438–439 in the textbook.
Study Notes
Agriculture is usually the main source of nitrates in drinking water (Objective 1). The nitrates that we ingest can be converted to nitrites through naturally occurring intestinal bacteria (e.g., Escherichia coli). The nitrites are toxic because they bind strongly with hemoglobin to form the complex “methemoglobin.” It is important to remember how hemoglobin works in the body. Hemoglobin in the blood transports oxygen from the lungs to body tissues, and moves carbon dioxide to the lungs for expiration. Various compounds such as oxygen and carbon dioxide bind to the iron centre of hemoglobin. The nitrite ion binds more strongly to the iron than oxygen does. If this happens to a large extent, and enough hemoglobin is “tied up” with nitrite ions, the body tissues essentially suffocate. The brain is the most susceptible to damage by low oxygen levels (Objective 2). We have already discussed a dramatic example of this sort of poisoning with another substance, carbon monoxide, in Unit 3 in the section entitled “Indoor Air Pollution—Nitrogen Dioxide and Carbon Monoxide.”
Exercise
Do Problem 8–12 within the chapter.
Oxidation–Reduction Chemistry in
Natural Waters—Nitrosamines in Food and Water
Objectives
After completing this section, you should be able to
1. draw the chemical structure for NDMA.
2. state the precursors of nitrosamines and name two places where they can be generated.
Key Terms
nitrosamine
carcinogen
NDMA (N–nitrosodimethyl amine)
DNA base
botulism
hemoprotein
Reading Assignment
Read pages 440–441 in the textbook.
Study Notes
The chemical structure of NDMA is given on page 440 (Objective 1). Although NDMA can be found directly in drinking water near industrial point sources, usually our exposure to nitrites is closer to home. The textbook mentions the reaction of nitrites with amines to produce nitrosamines either in cooking cured meats or simply by digesting cured meats and cheeses in the stomach (Objective 2).
Exercise
Do Problem 8–13 within the chapter.
Correction: Problem 8–13 answer in part (ii) only, change 8e* to 6e* (p. 97 in the Solutions Manual).
Acid–Base Chemistry in Natural Waters:
The Carbonate System—The CO2/Carbonate System
Objectives
After completing this section, you should be able to
1. write the balanced chemical equations associated with the dissolution of carbon dioxide in water.
2. name the acid and base that dominate the chemistry of most natural water systems.
3. state the major source for carbonate ions in natural waters.
Key Terms
carbonic acid (H2CO3)
carbonate ion (CO32*)
hydrogen carbonate ion (bicarbonate ion, HCO3*)
limestone (CaCO3)
calcareous water
three–phase system
Reading Assignment
Read pages 441–442 in the textbook.
Study Notes
The important reactions of carbon dioxide and water are (a) the dissolution of carbon dioxide gas into water and the formation of carbonic acid, (b) the first ionization of carbonic acid to form the bicarbonate ion, and (c) the second ionization of carbonic acid to form the carbonate ion. These reactions can be summarized as a series of equilibria:
CO2(g) ) H2O(l) ↔ H2CO3(aq) ↔ H)(aq) ) HCO3*(aq) ↔ H)(aq)
) CO32*(aq)
You should know these three reactions and be able to reproduce them (Objective 1).
The dominant acid and base are carbonic acid and the carbonate anion (Objective 2). Equations 2 and 4 on page 442 describe their aqueous acidic and basic character, respectively. Calcareous waters are quite common and so limestone would be the major source of carbonate ion in natural waters (Objective 3). In some regions the underlying rock is dolomitic limestone (1/2 CaCO3 @ MgCO3), which is slightly more soluble than calcium carbonate. Please remember that even distilled water posses carbonate ions from exposure to atmospheric carbon dioxide and generation of carbonic acid. Inexperienced chemistry students are often quite surprised that distilled water is acidic (pH [ 5.7) rather than neutral (pH + 7.0).
Exercises
No exercises have been assigned for this section.
Acid–Base Chemistry in Natural Waters:
The Carbonate System—Water in Equilibrium
with Solid Calcium Carbonate
Objectives
After completing this section, you should be able to
1. explain that when two equilibrium reactions are added together, the equilibrium constant of the overall reaction is a product of the equilibrium constant of the two individual reactions.
2. write the approximate net reaction of calcium carbonate in water (exclude atmospheric CO2).
3. explain why water over limestone is alkaline (exclude atmospheric CO2).
Key Terms
solubility (S)
iterative procedure
Reading Assignment
Read pages 443–446 in the textbook.
Study Notes
One feature of equilibrium reactions and their equilibrium constants that you should know is that when reactions add the constants are multiplied together (Objective 1). The following generic example will illustrate this point for you. Consider two equilibrium reactions and their respective equilibrium constants:
Reaction 1: A ) 2B ↔ C ) 3D K1 +
Reaction 2: E ) D ↔ B ) 1/2 F K2 +
If we add these reactions together, we get the following overall reaction and equilibrium constant:
A ) B ) E ↔ C ) 2D ) 1/2 F K3 +
First convince yourself that K3 is in fact products over reactants of the overall reaction. Now notice that K3 + K1K2, that is the overall equilibrium constant is the product of the equilibrium constants of the first two reactions.
Most of this section develops simple equilibrium equations from first principles to help you understand a complicated system. You may need to read this section several times to understand what is happening. The central net reaction to this section is Equation 5 on page 443 (Objective 2). Careful observation of the OH* generated on the product side will underscore the fact that water over solid calcium carbonate (limestone) is expected to be alkaline (Objective 3). Make sure you are comfortable with this section before going on.
Exercises
Do Problems 8–14 and 8–15 within the chapter.
Do Additional Problem 4 found at the end of the chapter on page 458.
Acid–Base Chemistry in Natural Waters:
The Carbonate System—Water in Equilibrium
with Both CaCO3 and Atmospheric CO2
Objectives
After completing this section, you should be able to
1. write the approximate net reaction of calcium carbonate in water exposed to atmospheric CO2
2. explain why water over limestone is exposed to atmospheric CO2 is essentially neutral
3. explain the synergistic effect the presence of limestone and atmospheric CO2 has on getting the other to dissolve in water to a greater extent
Key Terms
Le Châtelier’s Principle
titration
supersaturated
Reading Assignment
Read pages 446–449 in the textbook.
Correction: In the second line of the [OH*] equation, change 9.9 to 9.0 (middle of p. 448 in the textbook).
Study Notes
Do not let this section overwhelm you! You will already have the basic background for the reactions and the mathematics. However, there is just a lot of material packed together. As a first step, have a close look at Equation 7 (Objective 1). Try to come up with Equation 7 as an overall equation by adding together the chemical equations for the solubility of CaCO3 in water, solubility of CO2 gas in water, first and second acid dissociation of carbonic acid, and the ionization of water. Does this make sense given the total equilibrium constant (K7 + ) on page 447? (Hint: It should.) You will notice that in Equation 7 there is no H) or OH* generated on the products side (Objective 2).
The textbook describes the interaction within this three–phase system as a gigantic titration. The presence of acid (CO2 dissolving in water to form carbonic acid) allows for more base (CaCO3 dissolving in water to form the basic carbonate ion) to come into the system. Conversely, the more base we have the more acid can be accommodated (Objective 3).
Exercises
Do Problems 8–16, 8–17, and 8–18 within the chapter.
Do Additional Problems 5 and 6 at the end of the chapter on page 458.
Correction: Additional Problem 5 answer change to 1.5 10*3 M,
1.5 10*3 M, 6.2 10*3 M, and 3.4 10*4 M (p. AN–6 textbook).
Additional Problem 5 answer is incomplete and only correct till the line
K + . Delete K + 1.1 10*10 (p. 106 in the Solutions Manual). The solution should continue as follows:
Substituting in the following values
Ksp + 5.1 10*7
Kb + 2.1 10*4
KH + 3.4 10*2
Ka + 4.5 10*7
Kw + 1 10*14
gives
N [Ca2)] + 1/2 S + 1.5 10*3 M
N [Mg2)] + 1/2 S + 1.5 10*3 M
N [HCO3*] + 2 S + 6.2 10*3 M
Solubility constant of the dolomitic limestone is:
Ksp + [Ca2)]1/2 [Mg2)]1/2 [CO32*]
Rearrange to solve for carbonate ion concentration
Acid–Base Chemistry in Natural Waters: The Carbonate System—Measured Ion Concentrations in Natural Waters and Drinking Water and Seawater
Objectives
After completing this section, you should be able to
1. state the natural source for fluoride in water
2. explain why some places artificially increase their fluoride level to 1 ppm
3. describe the health concern for high levels of sodium or sulfate ions in drinking water
Key Terms
aluminosilicate
potassium feldspar (KAlSi3O8)
fluorapatite (Ca5(PO4)3F)
sea salt
Reading Assignment
Read pages 449–452 in the textbook.
Study Notes
Fluorapatite (Ca5(PO4)3F) is the main natural source of fluoride ions in natural water systems (Objective 1). Ironically, fluorides are added to drinking water and toothpaste to strengthen tooth enamel and minimize caries. Tooth enamel is composed of the mineral hydroxylapatite (Ca5(PO4)3OH), which is somewhat soluble in acidic environments. However, if the hydroxide ion is replaced by fluoride ion the resulting fluorapatite formed is inherently less soluble and makes the tooth less susceptible to decay (Objective 2).
Various dissolved solids have health implications. Too many sodium (and chloride) ions can lead to an increase in blood pressure. These ions along with potassium are also involved in the body’s electrolytic system and should not be ingested to any great excess. In addition, despite the benefits of low–level fluoride ions in protecting our teeth, their concentration must be kept quite low. Excess fluorides (uu 2 ppm) are not only very toxic, but can also paradoxically cause fluorosis of teeth.
Exercises
No exercises have been assigned for this section.
Acid–Base Chemistry in Natural Waters:
The Carbonate System—Alkalinity Indices
for Natural Waters
Objectives
After completing this section, you should be able to
1. differentiate between total alkalinity and phenolphthalein alkalinity.
2. perform calculations to determine the pH of a water sample.
3. perform titration calculations to determine endpoint, concentration, and pH.
Key Terms
total alkalinity
phenolphthalein alkalinity
algae
potential fertility
Reading Assignment
Read pages 452–454 in the textbook.
Study Notes
This section essentially deals with the buffering capacity of water. This refers to the ability to add relatively large amounts of acid or base to a solution without causing much change in its pH.
As we have already seen, carbon dioxide from the atmosphere (also from microbial oxidation) produces aqueous species such as HCO3* and CO32*, which act as a buffering system. A water sample may contain additional components such as phosphates, silicates, borates and other aqueous species, which also act as buffers. The resulting number of equilibrium reactions to be considered is enormous and make calculations of buffering capacity very difficult. To remedy this, the concept of alkalinity is used.
The capacity of water to neutralize acid is called alkalinity. This is determined by titrating a water sample with a standard acid solution. The titration curve in Figure 8–2 below shows two inflections at pH 8 and pH 4 because the alkalinity is based on the conjugate bases of carbonic acid (H2CO3), which is diprotic.
Figure 8.2: Bicarbonate buffering zone
The first inflection (pH 8) is the phenolphthalein endpoint where all CO32* has been converted to HCO3*. At the next inflection (pH 4), we reach the methyl orange endpoint which represents total alkalinity and both CO32* and HCO3* have been converted to H2CO3 (Objective 1). Section AB on the above figure shows the bicarbonate (HCO3*) buffering zone. The carbonate buffering system determines the ability of natural water body to withstand large additions of acid without changing the pH considerable. The longer the AB section is, the more stable the water.
You will find that this unit contains several problems involving solubility and acid/base calculations. It is recommended that you review equilibrium, solubility and titration chapters of your first–year chemistry course. The problem questions that you will encounter in this unit are at the same level, but will have a more environmental slant in their applications.
To achieve part of Objectives 2 and 3, recall that in a titration:
naCaVa + nbCbVb
where
na + number of acid (H)) equivalents
nb + number of base (OH*) equivalents
Ca + concentration of acid
Cb + concentration of base
Va + volume of acid
Vb + volume of base
Note that if both acid and base react in a one–to–one ratio (e.g., NaOH ) HCl), then the above equation simplifies to CaVa + CbVb.
Exercises
Do Problems 8–19, 8–20, and 8–21 within the chapter.
Correction: Problem 8–20 answer change 2.4 to2.5, 3.5 to 3.7, 1.04 to 1.02, and 9.6 to 8.9 (p. AN–6 textbook).
Do Additional Problem 8 found at the end of the chapter on page 459.
Acid–Base Chemistry in Natural Waters:
The Carbonate System—Hardness Index
for Natural Waters
Objectives
After completing this section, you should be able to
1. define hardness index
2. explain the factors that determine the hardness of water
3. perform calculations involving equilibrium constants, solubility products, and hardness index
Key Terms
hardness index
soft water
hard water
EDTA (ethylenediaminetetraacetic acid)
dolomitic limestone (1/2 CaCO3 @ MgCO3)
calcareous water
Reading Assignment
Read pages 454–455 in the textbook.
Study Notes
At this point, you will have gathered that hard water does not necessarily refer to conditions for a good ice hockey game. The hardness index is defined well at the beginning of this section on page 454 (Objective 1). Hardness is dependent on both exposure of the water to minerals containing magnesium and calcium, as well as the temperature of the water (Objective 2). Hardness can actually be reduced by heating the water. Essentially the reverse of Equation 7 occurs:
Ca2) ) 2HCO3* ↔ CaCO3(s) ) CO2(g) ) H2O(l)
The increased heat reduces the solubility of CO2 (Henry’s Law). To compensate, the equilibrium shown above shifts to the right (Le Châtelier’s Principle) and precipitates CaCO3 further driving the reaction to the right. The dissolved calcium ions and hence the hardness are reduced. The precipitated calcium carbonate usually appears as a scale and is found in many hot water systems. You may have noticed this scale in kettles or on your showerhead. Scaling can occur in hot water pipes and lead to blockage. Where large amounts of heated water are used (e.g., industrial boilers), an initial softening of the water is often required.
The exercises will assist you with Objective 3.
Exercises
Do Problems 8–22 and 8–23 within the chapter.
Do Additional Problem 7 at the end of the chapter on page 459.
Acid–Base Chemistry in Natural Waters: The Carbonate System—Aluminum in Natural Waters
Objectives
After completing this section, you should be able to
1. identify the most abundant ion(s) in water above and below pH 4.5
2. explain why aluminum concentration in acidic waters are greater than neutral water
3. explain the potential danger of aluminum to fish in water
Key Terms
aluminum ions
Alzheimer’s disease
neurological damage
Reading Assignment
Read pages 455–456 in the textbook.
Study Notes
While calcium and magnesium ions are dominant at pH u 4.5, more acidic conditions favor aluminum ions (Objective 1). Have another look at the equilibrium of aluminum hydroxide on page 455. Again, we can invoke Le Châtelier’s Principle and notice that as pH increases (i.e., more OH* ions) the equilibrium is pushed towards the reactant (aluminum hydroxide) side. Conversely, if we have more acidic conditions and the pH drops, any OH* ions generated will react with the extra H) to be neutralized. The result will be a shift in equilibrium to the product side (Al3) and OH*). Consequently, acidic water will have more aluminum ions (Objective 2).
Aluminum hydroxide precipitates as a colourless gel. Now, imagine acidic water saturated with aluminum ions. If we add base to this solution, aluminum hydroxide will precipitate. This may not seem significant unless you are a fish. As a fish, your gills are more basic than the surrounding waters and if it contains aluminum ions a slime will form on your gills and basically suffocate you (Objective 3).
Exercises
Do Problems 8–24 and 8–25 within the chapter.
Correction: Problem 8–24 answer change to 8.2 10*7 g L*1 (p. AN–6 in the textbook).
Review Procedure
1. Review the unit objectives and make sure that you can define, and use in context, the key terms introduced in this unit.
2. Go over the Review Questions (pp. 457–458 in the textbook) to test your factual knowledge of the material covered this unit.
3. Do some of the Supplementary Exercises for Chapter 8 for additional practice or examination preparation. Supplementary Exercises (with answers) can be found on the resource Web pages that accompanies the textbook.
4. Select an essay topic and start your literature search as outlined in the Assignment Manual.
5. Proceed to Unit 9.
Unit 9
The Purification of Polluted Water
Overview
Next to the air we breathe, the quality of the water we drink is the most important health issue. However, water pollution goes beyond immediate public health concerns surrounding the water we draw on; it also encompasses ecological concerns about the quality of used water returned to the environment. Although there are several definitions of pollution that can be applied to water, we can consider that in general any chemical, physical, or biological substance that has an adverse effect on desirable living organisms is essentially a pollutant. Unit 8, The Chemistry of Natural Waters, introduced some of the chemistry involved with water. This unit will continue from there and concentrate on how water is treated to make it suitable for drinking, as well as how wastewater and sewage is handled before being safely introduced into natural water systems.
The Contamination of Groundwater
Objectives
After completing this section, you should be able to
1. differentiate between surface water and groundwater.
2. explain why groundwater contamination was ignored for a long time.
3. state two types of organic contaminants found in groundwater and give two examples of each.
4. predict the vertical location of organic contaminants in an aquifer based on their densities.
5. explain plume formation in an aquifer.
6. explain why BTX and MTBE components of gasoline are commonly found in groundwater.
7. state the three sources for nitrates in groundwater.
Key Terms
surface water
groundwater
leachate
BTX (benzene–toluene–xylene)
water table
plume of polluted water
US Superfund
pump–and–treat
MTBE (methyl tert–butyl ether)
nitrate ion (NO3*)
Reading Assignment
Read pages 461–466 in the textbook.
Study Notes
Groundwater is essentially the water below the Earth’s surface that is found in the saturated zone (see Figure 8–1 p. 422 in the textbook), while surface waters include rivers, ponds, and lakes (Objective 1). The slowness to recognize groundwater contamination as a serious environmental problem is attributed to both the “out–of–sight, out–of–mind” attitude, as well as the long–standing belief that groundwater is always safe (Objective 2).
Apart from pesticides, the two major types of organic contaminants commonly found in groundwater are and chlorinated solvents and petroleum products. You should be able to give a couple examples of each from those listed in Table 9–1 (Objective 3). Note that this table also lists the densities of each substance. Water nominally has a density of 1.0 g mL*1 and so substances with a higher density (e.g., chlorinated solvents) will sink, while those having a lower density (e.g., BTX) will float within an aquifer (Objective 4). Figure 9–1 (p. 464 in the textbook) nicely illustrates this vertical separation of organic contaminants within an aquifer. Figure 9–1 also shows that, with lateral movement of water within the aquifer, a plume of organic contaminants can be generated downstream of the water flow (Objective 5). This plume formation in water is analogous to a plume of smoke in air that forms downwind from a smokestack.
The solubility of substances in water makes them prime candidates for groundwater contamination. This is certainly true for many substances, including nitrates, BTX, and MTBE (Objective 6). The textbook lists application of nitrogen fertilizers, atmospheric deposition, and human sewage as sources for groundwater nitrates (Objective 7).
Exercise
Do Problem 9–1 within the chapter.
Correction: Problem 9–1 answer change 4.8 to 5.5 (p. AN–6 textbook).
The Purification of Drinking Water—
The Stages of Purification
Objectives
After completing this section, you should be able to
1. list the four major stages of water treatment
2. explain the process and role of the first three major stages
3. explain what activated carbon is and how it is used in water purification
4. explain the process of hardness removal
Key Terms
raw water (untreated water)
aeration
activated carbon (activated charcoal)
physical absorption
colloidal particle
gelatinous hydroxide
aluminum hydroxide (Al(OH)3)
ferric hydroxide (Fe(OH)3)
Reading Assignment
Read pages 466–469 in the textbook.
Correction: On 4th line after Problem 9–2 change “page 476” to “page 478” (page 468 in the textbook).
Study Notes
Read the following notes carefully, because they vary somewhat with what is discussed in the textbook. There are four major stages of drinking water purification—namely primary settling, aeration, coagulation and disinfection (Objective 1). The textbook does not mention primary settling, which is used mainly for surface waters. Raw water enters a holding tank through some sort of screen to remove larger objects (e.g., sticks and leaves). Particulate matter is then allowed to settle and in some cases the pH is adjusted at this stage. Not all water purification plants have a primary settling stage. The major stages of aeration and coagulation are described well in the textbook. Together with primary settling, you should be able to describe and explain the purpose of each of these stages (Objective 2). Note that the fourth major stage of disinfection will be discussed in more detail in the following sections.
You should remember that water various greatly from place to place and not surprisingly the corresponding treatment it receives also varies. There are other water treatment steps that are sometimes integrated within the four major ones we have already identified. These steps include filtration, adsorption (activated carbon), softening (or hardness removal), fluoridation, membrane processes, and corrosion control. You should be aware that these steps may exist at any given water purification plant. With the exception of adsorption and softening, you are not responsible to know these in any detail (Objectives 3 and 4).
Aside: Canadians use three times as much water for flushing the toilet than they use for drinking and cooking combined. Water use inside Canadian homes is as follows:
Bathing and showering 35%
Toilet 30%
Laundry 20%
Drinking and cooking 10%
Cleaning 5%
Exercises
Do Problems 9–2 and 9–3 within the chapter.
The Purification of Drinking Water—Water Disinfection by Methods Other Than Chlorination
Objectives
After completing this section, you should be able to
1. describe three methods of disinfecting drinking water other than chlorination
2. state one advantage and one disadvantage of each of the methods listed in Objective 1
3. explain the purpose of nanofiltration (or ultrafiltration)
Key Terms
bacteria
virus
fecal matter
ozone (O3)
residual protection
bromate ion (BrO3*)
chlorine dioxide gas (ClO2)
chlorite ion (ClO2*)
chlorate ion (ClO3*)
ultraviolet light
germicidal action
humic substance
membrane system
nanofilter
Reading Assignment
Read pages 469–471 in the textbook.
Study Notes
The three major disinfection methods discussed in this section are ozonolysis, treatment with chlorine dioxide, and ultraviolet radiation (Objective 1).
The major advantage of using ozone is that it disinfects without leaving a harmful chemical residue. The product is oxygen. However, it must be produced on site and is quite expensive, especially when used on a small scale. The use of chlorine dioxide can eliminate taste and odour problems due to chlorinated phenols. It also does not produce toxic trihalomethanes. In both cases this is because ClO2 acts as an oxidating agent and not a chlorinating agent like Cl2. However, there are several disadvantages to using chlorine dioxide: it is toxic (causing hemolysis); it is expensive; and it cannot be stored and therefore must be produced on location. Finally, ultraviolet radiation produces no harmful residues and can be run efficiently even on a small scale. However, the presence of substances in the water can limit the effectiveness of this method (Objective 2).
Aside: The precursor to ClO2 is sodium chlorite (NaClO2). Finished water should contain no more than 1.0 ppm of ClO2 and therefore no more than 1.0 ppm of the chlorite ion. European and World Health Organization maximums for chlorite are set at 0.2 ppm, and Health Canada is currently considering this as a Canadian limit. This severely restricts the use of chlorine dioxide. A limit may also be set for chlorate ions in the near future.
The use of membrane systems or other types of ultrafiltration methods is mentioned only briefly at the end of this section, as well as the next section entitled “Water Disinfection by Chlorination.” Ultrafiltration is becoming one of the more important steps water treatment in the fight against certain small pathogens (Objective 3). Some small organisms are becoming more resistant to common disinfection methods, including chlorination. Most surface waters are filtered and the US EPA has called for filtration of all surface supplies for protection against protozoans like Giardia and Cryptosporidium after a major outbreak of the pathogen in Milwaukee (1993) that killed over 100 people and made hundreds of thousands of others sick. Other related incidences have occurred including the recent (May 2000) tragedy in Walkerton, Ontario where Escherichia coli in the drinking water caused over two thousand cases of bleeding diarrhea and resulted in seven deaths.
Exercises
No exercises have been assigned to this section.
The Purification of Drinking Water—
Water Disinfection by Chlorination
Objectives
After completing this section, you should be able to
1. write the balanced chemical equations associated with chlorine and chlorite ion dissolved in water
2. list the practical sources of HOCl used for disinfecting water
3. explain the advantage of HOCl versus ClO* as a microbial disinfecting agent
4. explain why pH control of water in a swimming pool is important
5. describe the formation of chloramines and its role in residual disinfection
6. explain why the chlorine use in outdoor swimming pools is greater than indoor pools
7. explain the problems of chlorinated phenol and trihalomethane formation associated with using chlorine as a disinfectant
8. compare the advantages and disadvantages of using chlorination to disinfect water
Key Terms
hypochlorous acid (HOCl)
chlorination
residual disinfection power
chlorinated phenol
trihalomethane (THM)
mutagenic
typhoid
cholera
chloramine
combined chlorine
Reading Assignment
Read pages 471–475 in the textbook.
Study Notes
You should be able to write out the balanced chemical reactions of Cl2 and ClO* in water shown on page 471 and 472, respectively (Objective 1). Obviously Cl2 injected in water is a direct and practical source for HOCl. However, for smaller operations or for safety reasons NaOCl or Ca(OCl)2 are practical indirect sources that can be added to water. Available acid in the water then converts the ClO* ion formed to the needed HOCl (Objective 2).
It is important to note that the toxicity of a substance is partially dependent on its ability to enter a living cell. Cell membranes consist of a double layer of lipids with proteins imbedded in it. The membrane is only 7.5 to 10 nm thick. The rate of diffusion of a substance through this membrane is dependent on its size and solubility in lipids. Small molecules pass through the membrane more readily than large molecules. Non–polar lipid–soluble molecules pass through the membrane more readily than polar (ionic) molecules. This is why a neutral molecule like HOCl penetrates the cell membranes of microorganisms more effectively than an ionic molecule like ClO* (Objective 3). This same concept was seen previously in Unit 7 Toxic Heavy Metals and explains the increased toxicity of neutral organometals compared to their aqueous soluble ionic counterparts.
If you keep the chemical equilibrium shown at the top of page 472 in mind, it will help you realize that pH (i.e., OH* concentration) can control the balance between OCl* and HOCl (Objective 4). The chemical equation right below on the same page shows the formation of nitrogen trichloride. However, depending on the amount of HOCl available the other chloramines NHCl2 and NH2Cl can also form. Note that chloramine (NCl3), is sometimes produced in situ on purpose by adding together chlorine and ammonia to provide a stable residual disinfectant in distribution systems. This is particularly important if there is a long time period or distance between source and tap (Objective 5).
The last equation on page 472 shows that the chlorite ion is susceptible to destruction by UV–B radiation. Outdoor swimming pools would therefore lose ClO* (and HOCl through equilibrium) in sunlight and would consequently use more chlorine (Objective 6).
The last part of this section discusses the formation of chlorinated phenols and THMs, which are toxic. You are not responsible to learn the detailed mechanism of formation of these by–products of chlorination (Objective 7). However, it should be emphasized that the benefits of disinfection by chlorination strongly outweigh any risk due to chlorinated phenols and THMs (Objective 8).
Exercises
Do Problem 9–4 within the chapter.
Do Additional Problems 1, 2, and 4 found at the end of the chapter on pages 499 to 500.
The Contamination of
Surface Waters by Phosphates
Objectives
After completing this section, you should be able to
1. state the two major sources of phosphates polluting our water systems
2. state the effects of phosphate nutrients on algae blooms and BOD values of a body of water
3. list at least two builders found in detergents
4. explain the roles of a builder in a detergent
5. differentiate between a point and nonpoint source of pollution
6. describe a method of phosphate removal from waste water
Key Terms
phosphate ion (PO43*)
polyphosphate
detergent
phosphate fertilizer
algal growth
Great Lakes Water Quality Agreement (1972)
builder
sodium tripolyphosphate (STP)
chelating agent
sodium nitrilotriacetate (NTA)
zeolite
point source
nonpoint source
Reading Assignment
Read pages 476–479 in the textbook.
Study Notes
Two major sources of phosphates polluting water systems are detergents and agricultural fertilizers (Objective 1). The uptake of nutrients into biomass occurs in the approximate ratio 100:15:1 for C:N:P. Phosphorus is generally the limiting nutrient in most systems. It occurs in very low concentrations in natural waters, while C and N nutrients are in excess. This implies that usually P nutrients have an effect on algae growth. The algae bloom in turn reduces the amount of oxygen (i.e., BOD) in the water and can result in fish kills (Objective 2).
Both STP and NTA are cited as builders in laundry detergent. However, sodium citrate, sodium carbonate (washing soda), sodium silicate, and zeolites can also be used as builders (Objective 3). Builders chelate calcium and magnesium ions to enhance a detergent’s cleansing potential. It also increases the pH somewhat to help remove dirt from some fabrics (Objective 4).
In explaining the difference between point sources and nonpoint sources you should be able to offer examples of each as indicated in the last paragraph of this section (Objective 5). Phosphate removal is done by addition of Ca(OH)2 to precipitate insoluble calcium phosphates (Objective 6). Note that this is the same process we discussed earlier in the section entitled “The Purification of Drinking Water—The Stages of Purification,” in which water hardness is reduced by deliberate addition of phosphates to remove magnesium and calcium ions.
Exercise
Does Additional Problem 3 found at the end of the chapter on page 499.
The Treatment of Wastewater and Sewage
Objectives
After completing this section, you should be able to
1. list and describe the three stages of treatment of wastewater and sewage
2. list three disposal routes for sludge obtained from primary and secondary treatment
3. state the concern in using sludge as a fertilizer
4. describe the use of trickling filters and activated sludge reactors in the secondary treatment of sewage
5. state the main disadvantage of chlorinating finished sewage
6. list at least three types of tertiary treatments
7. describe the three methods of desalinating water
8. list two methods of further removal of nitrogen compounds from wastewaters
Key Terms
sanitary sewer
storm sewer
primary treatment (mechanical treatment)
sludge
secondary treatment (biological treatment)
trickling filter
activated sludge process
tertiary treatment (advanced or chemical treatment)
desalination
reverse osmosis
semipermeable membrane
electrodialysis
ion exchange
air strip
nitrifying bacteria
artificial marsh (constructed wetland)
septic tank
Reading Assignment
Read pages 479–486 in the textbook.
Study Notes
Figure 9–5 (p. 480 textbook) provides a good visual summary of the three stages of treatment of wastewater and sewage (Objective 1). Various methods exist for handling sludge. It can be anerobically digested to some extent, but most commonly it undergoes a water reduction process followed by landfilling, ocean dumping, incineration, or use as fertilizer (Objective 2). There is some concern over the concentration of heavy metals in sludge when it is used as a fertilizer (Objective 3).
You should be able to describe the role of trickling filters and activated sludge reactors in the reduction of BOD in secondary treatment (Objective 4). Although chlorination is an effective method of disinfection, it does generate organochlorine compounds. This is especially true for wastewater and sewage where the concentration of organics is greater than in the chlorination stage of drinking water (Objective 5).
The exact tertiary treatment used at any particular wastewater plant is determined by local conditions and amount of money available to finish the wastewater. You should be able to give a few examples of tertiary treatment (Objective 6). Note that you should already be familiar with a few of these methods from the previous discussion on purifying drinking water. The desalination of water is also considered a tertiary treatment and you should be able to discuss the three methods of desalination in detail (Objective 7). You should also be able to describe how nitrogen compounds (like ammonia) are removed (Objective 8).
Exercises
Do Problems 9–5, 9–6, 9–7, and 9–8 within the chapter.
The Treatment of Cyanides
and Metals in Wastewater
Objectives
After completing this section, you should be able to
1. describe two methods of cyanide removal from wastewater.
2. describe how transition metal pollutants can be removed from wastewater.
Key Terms
cyanide ion (CN*)
hydrogen cyanide or cyanic acid (HCN)
redox chemistry
electrolytic reduction
Reading Assignment
Read pages 486–488 in the textbook.
Study Notes
Removal of cyanide includes oxidation of the cyanide using (a) oxygen under forcing conditions or a strong oxidzing agent and (b) electrochemical processes (Objective 1). Removal of transition metal pollutants is achieved through precipitation or reduction (Objective 2).
Exercises
Do Problems 9–9, 9–10, and 9–11 within the chapter.
Do Additional Problems 5 and 9 (a and b only) found at the end of the chapter on page 500.
Modern Wastewater and
Air Purification Techniques—
Destruction of Volatile Organic Compounds
Objective
After completing this section, you should be able to describe how VOCs in wastewater are removed and destroyed.
Key Terms
VOC (volatile organic compound)
air stripping
catalytic oxidation
adsorption
activated carbon
synthetic carbonaceous adsorbent
pore size
high surface area
Reading Assignment
Read pages 488–489 in the textbook.
Study Notes
There are no study notes for this section.
Exercises
No exercises have been assigned for this section.
Modern Wastewater and
Air Purification Techniques—
Advanced Oxidation Methods for Water Purification
Objectives
After completing this section, you should be able to
1. describe the principle behind AOMs.
2. identify the key reactive species in AOMs and three methods in which it can be generated.
3. state the biggest liability associated with AOMs.
Key Terms
trichloroethene (TCE)
perchloroethene (PCE)
Advanced Oxidation Methods (AOMs)
mineralize
hydroxyl free radical (OH)
ultraviolet (UV) light
ozone/H2O2 method
trichloroacetic acid (Cl3C(+O)OH)
dichloroacetic acid (Cl2HC(+O)OH)
Reading Assignment
Read pages 489–492 in the textbook.
Study Notes
The strategy behind AOMs is to generate OH to react with organics to convert them entirely to CO2, water, and mineral acids (Objective 1). In describing the three methods to generate the reactive hydroxyl radical species, you should be able to reproduce all the chemical equations shown on page 491 (Objective 2). The formation of toxic byproducts is cited as the most serious liability of AOMs (Objective 3).
Exercise
Do Problem 9–12 within the chapter.
Modern Wastewater and Air Purification Techniques—Photocatalytic Processes
Objective
After completing this section, you should be able to describe two photocatalytic methods that can destroy organic wastes.
Key Terms
solid semiconductor photocatalyst
titanium dioxide (TiO2)
Reading Assignment
Read pages 492–493 in the textbook.
Study Notes
There are no study notes for this section.
Exercises
No exercises have been assigned for this section.
Modern Wastewater and
Air Purification Techniques—
Chlorine–Compound Reductive Degradation
Objectives
After completing this section, you should be able to explain how a compound can be reductively dechlorinated.
Key Terms
reductive degradation
electrochemical dechlorination
Reading Assignment
Read pages 493–494 in the textbook.
Study Notes
To help you with this section, keep the following general two–step chemical equation representing reductive dechlorination in mind.
E*@ ) RCl → E ) [RCl* ⋅] → E ) R@ ) Cl*
Exercises
Do Additional Problems 7 and 10 found at the end of the chapter on page 500.
Modern Wastewater and Air Purification Techniques—Other Advanced Oxidation Methods
Objectives
After completing this section, you should be able to
1. describe direct chemical oxidation.
2. list two oxidants used in direct chemical oxidation.
Key Terms
direct chemical oxidation
peroxydisulfate (S2O82*)
peroxymonosulfate (HSO5*)
ferrate ion (FeO42*)
cold plasma (nonthermal plasma)
exhaust gas
catalytic ozone oxidation
Reading Assignment
Read pages 494–495 in the textbook.
Study Notes
Direct chemical oxidation is an AOM not requiring UV light (Objective 1). Examples of oxidants are listed on page 494 in the first paragraph of this section (Objective 2).
Exercises
Do Problem 9–13 within the chapter.
Do Additional Problem 8 at the end of the chapter on page 500.
In Situ Remediation of
Groundwater Containing Chloroorganics
Objective
After completing this section, you should be able to describe an in situ technique for treating groundwater contaminated by chloroorganics.
Key Terms
underground permeable wall
soil remediation
highly oxidized heavy metal
bioremediation
anaerobic condition
Reading Assignment
Read pages 495–498 in the textbook.
Study Notes
You are not required to memorize the chemical equations in this section. However, in your description of the in situ method used you should explain that the iron is being oxidizied while the chloroorganics are being reductively dechlorinated.
Exercises
Do Problems 9–14, 9–15, 9–16, 9–17, and 9–18 within the chapter.
Review Procedure
1. Review the unit objectives and make sure that you can define, and use in context, the key terms introduced in this unit.
2. Go over the Review Questions (pp. 498–499 textbook) to test your factual knowledge of the material covered this unit.
3. Do some of the Supplementary Exercises for Chapter 9 for additional practice or examination preparation. Supplementary Exercises (with answers) can be found on the resource Web pages that accompany the textbook.
4. Arrange a time and place for your final examination through the Office of the Registrar.
5. Complete your essay assignment, make a photocopy for yourself and send the original to your tutor.
6. Proceed to Unit 10.
Unit 10
Wastes, Soils, and Sediments
Overview
Throughout history human society has generated solid waste as a function of normal activities and living. Our landfills not only dwarf the great pyramids in Egypt but serve as a lasting testimonial to our consumer–driven urban lifestyle. Indeed, “garbage archeology” has established itself as a new branch of scientific research. However, domestic rubbish is only part of the problem. Waste generated from agricultural and industrial activities is nothing short of staggering. Unit 10 reviews the pathways and the effect of contamination of our lithosphere by hazardous wastes as well as so–called bulk wastes. In this final unit we will look at the disposal of solid wastes, the containment and treatment of hazardous wastes, and the remediation of contaminated soils and sediments.
The Nature of Hazardous Wastes
Objectives
After completing this section, you should be able to
1. describe the function of the Superfund Program in the United States.
2. explain the status of toxic waste site remediation in Canada.
3. define and use the term hazardous waste.
4. list five common types of hazardous waste.
Key Terms
Superfund Program (USA)
National Priorities List (USA)
Canadian Environmental Protection Act (CEPA)
Clean Canada Fund
hazardous waste
toxic
ignitable
corrosive
reactive
radioactive
Reading Assignment
Read pages 505–507 in the textbook.
Study Notes
Details of the USEPA Superfund program is given in Box 10–1 on page 506 (Objective 1). As of 2002, Canada has no equivalent federal program. In 1999 the Canadian Environmental Protection Act (CEPA) was passed. Although focussed mostly on pollution prevention, the legislation also covers the remediation of toxic sites. In addition, small targeted funds such as the Great Lakes Cleanup Fund ($52 million) or the Action 21 Program ($680,000) have been available in Canada for specific project themes. In July 2001, a coalition of 28 Canadian environmental groups (headed by the Sierra Club) formally urged the government in Ottawa to set up a special $2 billion “Clean Canada Fund” to start clearing up around 10,000 toxic waste sites across the country. The proposed Clean Canada Fund would run along the same lines as the Superfund program in the United States (Objective 2).
In simplest terms hazardous wastes are discarded substances that pose a danger (Objective 3). Hazardous waste can be grouped into four categories: chemical, radioactive, biohazardous, and material that is sharp. This section deal with only the first two groups—chemicals, which can be toxic, ignitable, corrosive, and reactive, as well as radioactive wastes (Objective 4).
Exercises
No exercises have been assigned to this section.
The Nature of Hazardous Wastes—Ignitable Wastes
Objectives
After completing this section, you should be able to
1. describe how flashpoint is used to define flammable and combustible liquids.
2. define and differentiate between upper and lower flammability limits.
3. list at least three common oxidizers found in hazardous wastes.
Key Terms
flammable (inflammable)
flash point
pyrophoric
combustible
lower flammability limit (LFL)
upper flammability limit (UFL)
Reading Assignment
Read pages 507–509 in the textbook.
Study Notes
The definitions of flammability (t60.5°C) and combustibility (60.5–93.3°C) determined by flashpoint are set by Transport Canada. You should be aware that these limits (definitions) can vary in other countries. You should also be aware that liquids having a flashpoint greater than 93.3°C (200°F) are still considered combustible even though they are not required by law to be handled as a combustible hazardous material. They are by no means “non–combustible” (Objective 1).
You should know what LFL and UFL mean and how to use these values to determine if ignition and combustion of a vapour will occur (Objective 2). Finally, the bulleted list on page 508 of the textbook gives a good summary of common oxidizers found in hazardous wastes (Objective 3). It is helpful to remember that for any combustion you need both a fuel and an oxidizer. The oxidizer need not be atmospheric molecular oxygen.
Exercises
Do Problems 10–1, 10–2, and 10–3 within the chapter.
The Nature of Hazardous Wastes—
Reactive Substances
Objectives
After completing this section, you should be able to
1. explain the bonding characteristics that would make a molecule reactive.
2. list two examples of this bond type.
Key Terms
reactive substance
weak covalent bond
alkali metal
Reading Assignment
Read pages 509–510 in the textbook.
Study Notes
Relatively weak covalent bonds make a molecule more reactive (Objective 1). Examples are listed at the top of page 510 in the textbook. You should remember at least two of these (Objective 2).
Some alkali earths and especially alkali metals (Groups 1 and 2 of the periodic table) are reactive in water and release hydrogen gas. They must be safely stored and well labelled to alert people (especially firefighters) of the hazard they pose in the presence of water.
Exercises
Do Problems 10–4 and 10–5 within the chapter.
The Nature of Hazardous Wastes—
Corrosive Substances
Objectives
After completing this section, you should be able to
1. define and use the term corrosive substance.
2. list five strong acids and five strong bases.
3. describe the process for disposing of an acid or base.
Key Terms
corrosive substance
strong acid
strong base
dehydrating agent
oxidizing agent
neutralization
Reading Assignment
Read pages 510–511 in the textbook.
Study Notes
Use the definition of corrosive substance given in the first sentence of the section (Objective 1).
There are only a few strong aqueous acids and bases, that is, an acid or base that completely ionizes in water. You should know these already from your first–year general chemistry course. A summary of these are given in Table 10–1 below. All other acids and bases not listed in this table can be considered weak acids and bases (Objective 2).
|Table 10.1: Strong acids and bases in water | | |
|Strong acids | |Strong bases |
|HCl, HBr, HI | |Group 1 Hydroxides |
|H2SO4 | |(e.g., LiOH, NaOH, KOH, RbOH, CsOH) |
|HNO3 | | |
|HClO4 | |Group 2 Hydroxides (e.g., Ca(OH)2, Sr(OH)2, Ba(OH)2) |
|HClO3 | | |
Caution: HF is a weak acid, but is still considered a corrosive substance because of its oxidizing and fluorinating strength.
Most acids and bases are disposed of by careful neutralization by the appropriate weak base or acid to form a salt and water (Objective 3).
Exercises
No exercises have been assigned to this section.
The Nature of Hazardous Wastes—
Toxic and Radioactive Substances
Objectives
No objectives have been assigned to this section.
Key Terms
toxic substance
radioactive substance
Reading Assignment
Read page 511 in the textbook.
Study Notes
We have previously covered both toxic and radioactive substances in other units of this course, which you are invited to review.
Exercises
No exercises have been assigned for this section.
Domestic Garbage and Landfills
Objectives
After completing this section, you should be able to
1. list the five largest categories of solid waste for developed countries.
2. describe the major components of leachate.
3. explain how leachate is controlled and destroyed.
4. explain what is meant by a sanitary landfill.
Key Terms
solid waste
commercial sector
industrial sector
domestic sector
municipal solid waste (MSW)
landfill
leachate
micropollutant
sanitary landfill
anaerobic decomposition
Reading Assignment
Read pages 512–515 in the textbook.
Study Notes
For developed countries, the five largest identified categories of solid waste are paper, vegetable matter, glass, plastics, and metals (Objective 1). Leachate is essentially the liquid waste that seeps from a landfill under anaerobic conditions. Its contents are described at the bottom of page 513 (Objective 2). A sanitary landfill will be lined to isolate the leachate to the landfill. The leachate is usually collected, removed and treated by aerobic oxidation or AOMs (Objective 3).
Use Figure 10–2 to remind you of the key components of a sanitary landfill.
A more detailed description is offered at the bottom of page 514 (Objective 4).
Exercises
Do Problem 10–6 within the chapter.
Do Additional Problem 1 at the end of the chapter on page 553.
The Elimination of Wastes—Incineration
Objectives
After completing this section, you should be able to
1. describe two common types of incinerators for domestic MSW.
2. differentiate between fly ash and bottom ash.
3. describe two methods of reducing emissions from an incinerator.
4. describe two common types of incinerators for hazardous wastes.
5. explain the concern surrounding organic PICs.
Key Terms
incineration
one–stage mass burn unit
two–stage modular unit
bottom ash
fly ash
baghouse filter
gas scrubber
destruction and removal efficiency (DRE)
rotary kiln incinerator
cement kiln
liquid injection incinerator
products of incomplete combustion (PICs)
fugitive emission
molten salt combustion
fluidized bed incinerator
plasma incinerator
Reading Assignment
Read pages 515–519 in the textbook.
Study Notes
One–stage and two–stage incinerators for MSW are described in the second paragraph of this section (Objective 1). In addition to gaseous emissions, incineration does produce solid materials such as bottom ash and fly ash (Objective 2). Most incinerators will attempt to reduce emitted particulates through filtration and gaseous emissions through chemical scrubbing (Objective 3). Note that these emission control methods are similar to other processes, such as smelting ores or burning coal, mentioned earlier in this course.
Hazardous wastes are usually treated separately by incineration and often under more rigorous conditions. The two common methods are rotary kiln and liquid injection incineration (Objective 4). With all incineration techniques (especially in the case of hazardous wastes) the goal is efficient and complete combustion of the material having nominally only CO2 and H2O as products. The risk is that not only might you not destroy hazardous components of the original material, but actually create new organic toxic compounds through incomplete combustion (Objective 5). We have already discussed an example of this in Unit 6 under the section entitled PCBs—
Other Sources of Dioxins and Furans. The incomplete destruction of PCBs can lead to the formation of the more toxic furans and dioxins.
Exercises
No exercises have been assigned for this section.
The Elimination of Wastes—
The Use of Supercritical Fluids
Objectives
After completing this section, you should be able to
1. draw the phase diagram for water and identify on it the critical point and triple point.
2. differentiate between wet air oxidation and SCWO processes.
Key Terms
supercritical fluid
critical point
triple point
Supercritical Water Oxidation (SCWO)
Wet Air Oxidation
Reading Assignment
Read pages 519–522 in the textbook.
Study Notes
You are expected to be able to reproduce a phase diagram of water (see Figure 10–4) and label both the three phases, as well as the triple and critical points (Objective 1). Note the area beyond the critical point is supercritical fluid and the triple point is where all three phases co–exist. For water the triple point is about 0.006 atm and 0.01°C. In reproducing the phase diagram for water you need not give exact temperatures and pressures. However, it is expected that you know that at 1 atm water freezes at 0°C and boils at 100°C.
You should also be able to describe briefly both wet air oxidation and SCWO processes, and note the advantages of each (Objective 2).
Note: Caffeine stimulates cerebral cortex in the brain to keep us alert and also speeds up the metabolic rate. A typical cup of coffee has a 70 mg dose of caffeine, a cup of tea 50 mg, and Jolt™ cola 76 mg. A lethal amount of caffeine is about 10 g. At one time chlorinated hydrocarbons were used to decaffeinate coffee, but now the extraction process uses supercritical CO2 because it is more specific to caffeine and leaves no residue. Interestingly, the caffeine extracted from coffee is used in cola drinks.
Exercises
No exercises have been assigned for this section.
The Elimination of Wastes—
Non–Oxidative Processes
Objectives
After completing this section, you should be able to
1. describe chemical reduction as a process to destroy hazardous waste.
2. state two methods of chemical dechlorination of PCBs.
Key Terms
chemical reduction process
reducing atmosphere
chemical dechlorination
Reading Assignment
Read pages 522–523 in the textbook.
Study Notes
You should also be able to explain the chemical reduction process itself, and state advantages over other reductive and oxidative methods (Objective 1).
Chemical dechlorination can be achieved by either chlorine substitution of the ring using KOR or NaOR or by using sodium metal, which will also polymerize the rings as the chlorine is removed (Objective 2). The latter is also known as the Wurtz reaction.
2 RCl ) 2 Na → R–R ) 2 NaCl where R + [pic]
Exercise
Do Problem 10.7 within the chapter.
Recycling of Household and Commercial Waste
Objectives
After completing this section, you should be able to
1. state what the four R’s represent in waste management.
2. explain how recycling metal is justified by economics and energy conservation.
Key Terms
the four R’s
reduce
reuse
recycle
recover
Reading Assignment
Read pages 523–524 in the textbook.
Study Notes
The four R’s are listed in the first paragraph of this section (Objective 1). The conversion of bauxite (an aluminum oxide) to aluminum metal is a major industry in Canada and is used as an example in the textbook to argue for the recycling of metals (Objective 2). Other arguments need to be made for recycling other materials such as paper and rubber.
Exercise
Do Problem 10–8 within the chapter.
Recycling of Household and Commercial Waste—
Recycling of Paper and Recycling of Tires
Objectives
After completing this section, you should be able to
1. describe the process of recycling paper.
2. explain the limitation in the number of times paper can be recycled.
3. explain the danger of stockpiling rubber tires.
4. describe three uses of recycled tires.
Key Terms
newsprint
deink
virgin fiber
pulp fiber
tire fire
pyrolysis
carbon black
Reading Assignment
Read pages 524–526 in the textbook.
Study Notes
You should be able to describe briefly how paper is mechanically dispersed, cleaned, deinked, and mixed with virgin fiber to recycle it (Objective 1). The ever–shortening fiber length after each recycling means that there is a limit to how often paper can be re–pulped and formed without losing its structure (Objective 2).
In addition to the massive waste of material, the risk of tire fires makes stockpiling rubber tires undesirable (Objective 3). Attempts to tap this potential resource have resulted in using shredded scrap tires to produce liquid fuels, activated carbon, and rubberized asphalt (Objective 4). The first two are described in the textbook, but the latter is essentially a combination of rubber tires with asphalt to produce a so–called rubber modified asphalt (RMA). Recent studies in Ontario and elsewhere have shown that RMA can significantly widen the temperature span of asphalt pavements when compared to conventional asphalt binders. The main RMA advantages include increased resistance to rutting, reflective and thermal cracking. In addition, it has better de–icing properties, it reduces traffic noise (tire hum) and it significantly increases the service life of the road and therefore reduces the life cycle cost.
Exercises
No exercises have been assigned to this section.
Recycling of Household and Commercial Waste—
Recycling of Plastics
Objectives
After completing this section, you should be able to
1. list at least four types of commonly recycled plastics.
2. state the advantages and disadvantages of recycling plastics.
3. describe the four basic strategies for recycling plastic.
Key Terms
plastic
polymeric organic molecule
polyethylene (polyethane)
low–density polyethylene (LDPE)
high–density polyethylene (HDPE)
polyvinyl chloride (PVC)
polypropylene (PP)
polystyrene (PS)
poly(ethylene terephthalate) (PET)
reprocess
depolymerize
transform
burn
reductive process
oxidative process
Reading Assignment
Read pages 526–530 in the textbook.
Study Notes
Recycling is becoming a viable option. Several municipalities are now collecting used plastics. There are several plastics summarized in Table 10–3 that are commonly recycled (Objective 1). Several problems surrounding the recycling of plastics are mentioned in the textbook. You should be able to discuss the arguments for and against recycling plastics (Objective 2). Despite the extensive list shown in Table 10–3, you should realize that recycled products are usually lower grade and that owing to recycling problems, most plastics collected have been stockpiling because there is no good use for them at the present time.
You should be able to list and give explanations of the four recycling strategies noted at the top of page 529 (Objective 3).
Exercises
No exercises have been assigned for this section.
Soils and Sediments—Soil Chemistry
Objectives
After completing this section, you should be able to
1. describe the main inorganic components of soil.
2. name and describe the origin of the main organic components of soil.
3. perform cation exchange capacity calculations for soils.
4. describe methods for soil remediation that is too acidic or alkaline.
Key Terms
silicate mineral
clay mineral
colloid
humus
protein
lignin
humic acid
fulvic acid
cation–exchange capacity (CEC)
microorganism
reverse acidity
liming
sediments
pore water
acid volatile sulfide (AVS)
Reading Assignment
Read pages 530–535 in the textbook.
Study Notes
The inorganic part of soil consists mainly of silicates and aluminum cations followed by several other interstitial cations, such as H), Na), K), Mg2), Ca2), and Fe2) (Objective 1). The textbook mentions clay as defined by particle size. For your information the following classifications are used by the International Society of Soil Science:
|Soil Particle | |Size (mm) | | |
|clay | |t 2 | | |
|silt | |2–20 | | |
|fine sand | |20–200 | | |
|coarse sand | |200–2000 | | |
|gravel* | |u 2000 | | |
|* particle considered non–soil | | | | |
The organic component is mainly decomposed (humic and fulvic acids) and undecomposed (protein and lignin) plant material (Objective 2). Solving in–chapter Problem 10–10 will help you meet Objective 3.
Finally, alkaline soils can be treated with elemental sulfur or the sulfate salt of Fe(III) or Al(III). While acidic soils are usually treated with calcium carbonate and are said to be limed (Objective 4), you should be able to explain the chemical strategy behind these treatments.
Exercises
Do Problems 10–9, 10–10, 10–11, and 10–12 within the chapter.
Soils and Sediments—The Binding of
Heavy Metals to Soils and Sediments
Objectives
After completing this section, you should be able to
1. state the three ways heavy metals bind to sediments and soils.
2. explain how mercury stored in sediments can enter the food chain.
3. describe the concern with using sewage sludge as fertilizer.
Key Terms
heavy metal cation
carboxylic acid group (–COOH)
adsorption
complexation
precipitation
methylmercury poisoning
sewage sludge
Reading Assignment
Read pages 535–538 in the textbook.
Study Notes
You should be able to list adsorption, complexation, and precipitation as routes for heavy metal binding in soils and sediments (Objective 1). You should be able to both explain and give an example of each process. A brief description of conversion of Hg2) to methylmercury and subsequent release into water and therefore the food chain is summarized in a previous unit in Figure 7–3 (Objective 2). Take a moment now to review how mercury gets into the food chain.
The use of sewage sludge as an agricultural fertilizer is somewhat controversial (Objective 3). If the availability of heavy metals is kept low, the use of sewage sludge is a cost–effective and safe fertilizer. However, its use requires rigorous monitoring, because the amount of heavy metals available can vary with both the amount of heavy metals added, and the soil type. Timing can also be crucial. For example, application of sewage sludge to growing crop should be avoided. The sludge needs time to incorporate into the soil and the heavy metals need some time to become immobilized before plants use the soil to grow.
Exercise
Do Problem 10–13 within the chapter.
Do Additional Problem 3 at the end of the chapter on page 553.
Soils and Sediments—
The Remediation of Contaminated Soil
Objectives
After completing this section, you should be able to
1. state the three categories of technologies used to remediate contaminated soil and give an example of each.
2. state two soil remediation technologies that can be used in situ.
Key Terms
PCP (pentachlorophenol)
in situ
ex situ
containment (immobilization)
vitrification
mobilization
destruction
soil vapour extraction
thermal desorption
soil washing
sufactant
biosurfactant
Reading Assignment
Read pages 538–540 in the textbook.
Study Notes
The three main categories of technologies used to remediate contaminated soil are containment (immobilization), mobilization, and destruction (Objective 1). You should be able to explain and give an example of each category. You should also be able to identify in which category each technology listed in Table 10–4 belongs. Finally, you should be able to describe two in situ soil remediation technologies (Objective 2).
Exercise
Do Problem 10–14 within the chapter.
Soils and Sediments—The Analysis and Remediation of Contaminated Sediments
Objectives
After completing this section, you should be able to
1. explain why analysis of total amounts of heavy metals is not a good measure of the extent of heavy metal contamination in soil.
2. describe two methods of decontaminating sediments in situ.
Key Terms
sediment particles
decontaminating sediments
Reading Assignment
Read pages 540–542 in the textbook.
Study Notes
Measuring the extracted amount of heavy metals from soil by water or slightly acidic water is a better reflection of the available heavy metals (Objective 1). Analysis of total amounts of heavy metals would also include metals that were bound to the soil and unavailable to the food chain. The last two paragraphs of the section describe several in situ methods of decontamination (Objective 2). You should be able to describe at least two of these if asked for examples.
Exercises
No exercises have been assigned for this section.
Bioremediation
Objectives
After completing this section, you should be able to
1. state the three conditions that must be fulfilled for bioremediation to operate effectively.
2. describe the mechanism of biodegradation of PCBs both aerobic and anaerobic conditions.
3. state the advantage of H2S production in anaerobic degradation.
Key Terms
bioremediation
microorganism
bacteria
biodegrade
recalcitrant (biorefractory)
aerobic treatment
anaerobic degradation
genetic engineering
white–rot fungi
Reading Assignment
Read pages 542–546 in the textbook.
Study Notes
You can find the three conditions that must be fulfilled for bioremediation to operate effectively listed at the top of page 543 (Objective 1). Bioremediation can be applied to a wide range of organics and is not limited to PCBs. However, PCBs are generally thought of as recalcitrant and so their in situ destruction by microorganisms under the right conditions is worthy of note. You should be able to explain and differentiate PCB biodegradation under aerobic and anaerobic conditions (Objective 2). Finally, in situ production of sulfides (e.g., H2S) binds heavy metal cations and is therefore a bonus of anaerobic degradation (Objective 3).
Exercises
Do Problem 10–15 within the chapter.
Do Additional Problem 4 at the end of the chapter on page 553.
Bioremediation—Phytoremediation
Objectives
After completing this section, you should be able to
1. state the three mechanisms by which plants can remediate pollutants.
2. describe how plants can be used to deal with metal contaminants.
Key Terms
phytoremediation
phytoextraction
fungi
microbes
hyperaccumulator
Reading Assignment
Read pages 546–548 in the textbook.
Study Notes
Use the bulleted list at the beginning of this section showing the three mechanisms by which plants can remediate pollutants (Objective 1). In most cases the targeted pollutants are organic compounds. However, plants that are hyperaccumulators can be an effective method to deal with high metal concentrations in soil (Objective 2).
Exercises
No exercises have been assigned for this section.
The Prevention of Pollution—Green Chemistry
Objectives
After completing this section, you should be able to
1. state the two principal strategies of green chemistry.
2. list two examples each of objectives that green chemistry tries to maximize and to minimize.
3. explain the green advantage in using both supercritical fluids and solid–state catalysts.
Key Terms
green chemistry
zero emissions
closed–cycle facility
atom economy concept
supercritical carbon dioxide
solid–state catalyst
biocatalyst
Reading Assignment
Read pages 548–550 in the textbook.
Study Notes
Green chemistry is becoming an important and recognized area of chemistry. You should be able to state the two basic strategies used in green chemistry listed in the middle of page 549 (Objective 1). In applying these two general strategies green chemistry attempts to maximize or minimize various objectives in any chemical process. It will attempt to maximize process efficiency, use of renewable feedstocks, and the use of catalytical processes. At the same time, it will attempt to minimize hazardous waste, the spread of toxic materials, energy use, and the potential for accidents (Objective 2).
You will notice that the most of these objective examples also make good business sense.
Another example of green chemistry is the careful selection of a catalyst. Normally organic reactions involving Grignard–type reagents (which are magnesium–based) cannot be carried out in water, so organic solvents must be used. Recently it was discovered that in certain reactions one could use Grignard–type reagents of indium that is not sensitive to water. This allows for analogous reactions to be carried out in water as a solvent rather than the expensive and less–environmentally friendly organic solvents.
Finally, you should be able to briefly describe the use and advantages of both supercritical carbon dioxide and solid–state catalysts (Objective 3).
Exercises
No exercises have been assigned for this section.
The Prevention of Pollution—
Life Cycle Assessments
Objective
After completing this section, you should be able to state the two main purposes of a life cycle assessment (LCA).
Key Term
life cycle assessment (LCA)
Reading Assignment
Read pages 550–552 in the textbook.
Study Notes
In the past, assessments done by business and industry would look at only a narrow portion of a product’s life. With LCA the idea is to look at a wider range of inputs and outputs to assess the true cost of a product.
Use the two bulleted points at the bottom of page 550 to meet this section’s learning objective. It is important to keep in mind that the LCA process is rather complex and the answer is often dependent on the information and the importance of various “costs” that one includes. Using LCA to select a product can be controversial. For example, deciding between using disposable diapers for a baby or a diaper service is not as easy as it first appears. The overall energy and water use of a diaper service, coupled with wastewater generation, does have an appreciable “cost” even when compared to disposable diapers.
Exercises
No exercises have been assigned for this section.
Review Procedure
1. Review the unit objectives and make sure that you can define, and use in context, the key terms introduced in this unit.
2. Go over the Review Questions (pp. 552–553 textbook) to test your factual knowledge of the material covered this unit.
3. Do some of the Supplementary Exercises for Chapter 10 for additional practice or examination preparation. Supplementary Exercises (with answers) can be found on the resource Web Read pages that accompany the textbook.
4. Do the tutor–marked assignment for Units 8, 9, and 10 (TMA 4), make a photocopy for yourself and send the original to your tutor.
5. Prepare to write the final examination by reviewing Units 2 to 10 with emphasis on Units 6 to10. A sample practice final examination has been provided for you in the Student Manual.
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Figure 2.1 goes here
insert Stratospheric Circulation [dk where is it?]
Figure 3.1 goes here
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Environmental Chemistry
Chemistry 330 / Study Guide
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