TWEED RIVER HIGH SCHOOL



TWEED RIVER HIGH SCHOOL

PRELIMINARY CHEMISTRY

Unit 3

Water

Part 5

• Water has a higher heat capacity than many other liquids.

• Explain what is meant by the specific heat capacity of a substance.

• The specific heat capacity, C, is the number of joules of heat energy required to raise the temperature of one gram of the substance by one Celsius degree, (C or one Kelvin, K.

• Specific heat is measured in J g-1 K-1

• Compare the specific heat capacity of water with a range of other solvents.

|Solvent |Specific Heat Capacity |

| |J g-1 K-1 |

|Water |4.18 |

|Octane |2.22 |

|Glycerol |2.38 |

|1,2-ethanediol |2.39 |

|Ethanol |2.44 |

|Methanol |3.53 |

|Benzene |1.05 |

|Carbon tetrachloride |0.54 |

• Explain and use the equation:

(H = -mC(T

• The specific heat capacity may be used to determine the energy absorbed or released when the temperature of a substance increases or decreases. The equation used to calculate the heat energy involved is:

(H = -mC(T

Where:

(H is the heat energy absorbed or released, (joules, J)

m is the mass of substance being heated or cooled, (g)

C is the specific heat capacity of the substance being heated or cooled,( J g-1 K-1).

(T is the temperature change of the substance being heated or cooled, (K). (Note that the symbol (, (delta), means ‘change in’).

H is also called the enthalpy of the substance and represents stored chemical energy of the substance.

Examples 16.1 & 16.2p273

Exercises 1, 2

• Explain how water’s ability to absorb heat is used to measure energy changes in chemical reactions.

Calorimetry

• Calorimetry is the measurement of heat changes in a system.

• A calorimeter is equipment used to measure the heat change. A calorimeter consists of a thermally insulated container containing a known mass of water. Heat is released or absorbed from a change and then absorbed or released by the water. This causes a change in the temperature of the water.

i) Calorimeter

[pic]

|  |

|Fig. 2 Experimental Calorimeter |

Tian-Calvet Microcalorimeter 

The calorimeter consists of two symmetrical thermal fluxmeters, each constructed by a series of 480 thermocouples surrounding a cylindrical hole for the measurement cells. The electric signal delivered by the difference in output voltage of the two fluxmeters is proportional to the thermal effect occuring in the cells. It is calibrated by using electrical heaters inside the cells. The temperature of the calorimeter (-190 to 200°C) can be regulated by using a cold nitrogen gas flow and electric heating.

The calorimeter can be used for the measurement of enthalpies of mixing and solution, enthalpies of fusion and transition, enthalpies of reactions, enthalpies of adsorption and desorption and heat capacities. For each application suitable measurement cells are available.

[pic]

Scheme of the Tian-Calvet Microcalorimeter

Homework: What is a ‘bomb calorimeter’. Include diagram              

• Describe dissolutions which release heat as exothermic and give examples.

• A reaction in which energy is released to the surroundings is called an exothermic reaction.

• The enthalpy of the products is lower than the reactants:

For example: Energy is released to the surroundings when methane is burnt in air.

CH4(g) + 2O2(g)

reactants

This amount

of energy is

released to the

surroundings

CO2(g) + 2H20(g)

products

• If the reaction is exothermic, (Tis positive, so the (H value is negative.

• Describe dissolutions which absorb heat as endothermic and give examples.

• A reaction in which energy is absorbed from the surroundings is called an endothermic reaction.

• The enthalpy of the products is higher than the reactants:

For example: Energy is absorbed from the surroundings the solvent carbon disulfide is manufactured:

4C(s) + 8S(g) + energy ( 4CS2(g)

4CS2(g)

products

This amount

of energy is

absorbed to the

surroundings

4C(s) + 8S(g)

reactants

• If the reaction is endothermic, (Tis negative, so the (H value is positive.

• The change in enthalpy, ((H), during a chemical reaction is known as the heat of reaction.

(H = H(products) - H(reactants)

heat of reaction enthalpy of products enthalpy of reactants

Fig 16.3 p276 text

• Values for the heat of reaction are usually given per mole of one of the reactants or products

• For example the decomposition of water is an endothermic reaction:

H2O(l) ( H2(g) + ½ O2(g)

(H for the reaction is +286Kj. Which means that 286 kJ of energy are absorbed in the decomposition of one mole of water.

Two common methods for indicating (H are:

H2O(l) ( H2(g) + ½ O2(g) (H = +286kJ

H2O(l) + 286kJ ( H2(g) + ½ O2(g)

• Explain why water’s ability to absorb heat is important to aquatic organisms and to life on earth generally.

• Chemically all living things can be pictured as aqueous solutions surrounded by membranes. The chemical reaction in the aqueous solutions and on the surface of the membranes are catalysed by enzyme proteins which function in a narrow temperature range.

It is critical that the narrow temperature range be maintained otherwise the structure of enzymes can change so much that they cannot function.

A temperature rise in a human can be fatal as essential enzymes stop functioning. Normal body temperature is 37(C while at 40.5(C the person can become delirious. Permanent body damage or death can occur at 44(C.

The following properties of water all help to minimise temperature change and act as a thermal regulator. Water has:

1. a high heat capacity

2. low viscosity and relatively high heat conductivity which support movement of heat away from a source

3. large heat of vaporisation

(The latent heat of vaporisation of a substance is the heat absorbed per mole when the substance changes from liquid to gas at constant temperature.)

In an aquatic environment the temperature is held in a narrow range. The high heat capacity of water minimises fluctuations in temperature. Most aquatic animals’ body temperature depends on the temperature of their surroundings. Large fish such as tuna which chase other animals have ways of reducing heat loss. This ensures that chemical reactions releasing energy can proceed at an adequate rate to supply the bursts of energy needed to catch prey.

A higher proportion of terrestrial animals maintain a constant body temperature. Sweating to cool the body and behaviours like seeking shade, hunting at night help minimise fluctuations in temperature. Dogs pant by breathing with their moist mouth open and moist tongue hanging out. Evaporation of this moisture cools the dogs body.

• Explain what is meant by thermal pollution and discuss the implication for life if a body of water is affected by thermal pollution.

• Thermal pollution occurs when heat energy is added to the environment causing unwanted or harmful effects. Human activity producing increased temperature in waterways is the main source of thermal pollution.

• Because of water’s large heat capacity it is often used as a cooling fluid for industry or electrical energy generation in power stations. When hot water is released into the environment such as a river or shallow lake the high heat capacity of water results in release of a large amount of heat energy. This large amount of heat energy may take a long time to disperse.

• The main effect of thermal pollution on lakes and rivers is the reduction in the concentrations of dissolved gases such as oxygen and carbon dioxide in the water. At the higher temperatures associated with thermal pollution the concentration of dissolved oxygen may fall below that necessary to sustain particular life forms that are present.

• Fish are particularly susceptible to changes in water temperature. At higher temperatures fish have a higher metabolic rate, requiring a higher concentration of dissolved oxygen when in fact the levels of dissolved oxygen are lower. Fish such as trout that thrive in cold water will die if the temperature goes up more than a few degrees.

• Other ecological effects can include the displacement of plankton, the food base of aquatic ecosystems, by blue-green algae at higher temperatures.

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