Chemistry – Module 4 – Energy



Chemistry – Module 4 – Energy

1. Living organisms make compounds which are important sources of energy.

• Outline the role of photosynthesis in transforming light energy to chemical energy and recall the raw materials for this purpose.

Photosynthesis is the process by light energy is trapped by the chlorophyll in the plants leaves and is used to transform the raw materials of carbon dioxide and water into glucose and oxygen. Photosynthesis occurs in all green plants and is the source of life of all organisms. Without photosynthesis, no organism would be able to use the sun’s energy, thus everything would perish.

Light

Carbon Dioxide + Water Glucose + Oxygen

Chlorophyll

Photosynthesis is an endothermic reaction as it absorbs energy from sunlight

It is the process by which light energy is converted into chemical energy which is stored as glucose. It occurs only in green plants.

When the above chemical reaction occurs, 2830kJ of energy is absorbed per mole of glucose formed. Thus the overall role of photosynthesis is to capture the light energy of the sun and transform it into chemical energy which is stored in plants.

• outline the role of the production of high energy carbohydrates from carbon dioxide as the important step in the stabilisation of the sun‘s energy in a form that can be used by animals as well as plants.

Carbohydrates are compound of hydrogen, carbon and oxygen. Common carbohydrates include glucose, starch and cellulose.

Photosynthesis is a complex multi-step reaction brought about by the chlorophyll in the leaves plants. Thus the energy that is captured by the plant is chemically stored in glucose.

Carbohydrates in plants are the energy source of animals. In cellular respiration, the stored chemical energy is made available to the organism through the following equation:

Glucose + Oxygen Carbon Dioxide + Water + Energy

The amount of energy released during respiration is the same as was absorbed during photosynthesis, namely 2830kJ per mole of glucose. Some of this energy is used for daily activities whereas the majority is dissipated as heat. A small portion of this energy is transformed into protein and fat (or lipids).

Carbohydrates are considered to be high energy compounds because when they react chemically as in respiration they release large amount of energy.

Note: Virtually all the solar energy trapped by plants ends up as heat in the environment to be re-radiated into space.

Note: The longer the food chain, the more inefficient it is.

All forms of life on Earth are dependant upon sunlight for their supply of energy; without the sun there would no life as we know it. Production of carbohydrates by photosynthesis is the main way in which solar energy is collected for use by plant.

• Identify the photosynthetic origins of the chemical energy in coal, petroleum and gas.

Plants harvest solar energy to live and grow, and animals live eating plants. Normally, when plants and animals die, they are decomposed and they are converted back into carbon dioxide, water and nutrients which are released back into the environment, so completing the materials and energy cycles that are part of the living world.

However in certain locations, instead of being fully decomposed to carbon dioxide and water, some plant and animal material was only partially decomposed and remained stored in the Earth as energy-rich compounds. These are known as fossil fuels. They were mainly formed due to intense pressure and extremely high temperatures for millions of years.

Energy rich compounds are those that release large amounts of energy when they undergo chemical reactions. The stored energy is known as chemical energy. By burning fossil fuels we are able to release the stored chemical energy within them. The common fossil fuels are coal, crude oil and natural gas.

Living matter is mainly made up of compounds of carbon. Therefore fossil fuels were naturally synthesized by geological processes acting upon compounds of carbon. Thus it is not surprising when we see that fossil fuels themselves are compounds of carbon.

Dead Plants and Animals

Underground Temperature and Pressure for millions of years

Without Bacterial Action With Bacterial Action

Coal (Solid) Crude Oil (liquid)

Natural Gas (Gas)

The origin of chemical energy in fossil fuels in form the sun. Living organisms obtain energy directly or indirectly from the sun via photosynthesis.

• process and present information from secondary sources on the range of compounds found in either coal, petroleum or natural gas and on the location of deposits of the selected fossil fuel in Australia

|Fuel |Major Australian Resources |

|Black coal |Bowen Basin in Qld There is very little coal in the |

| |Sydney Basin in NSW western half of the continent. |

|Brown coal |Latrobe Valley in Victoria |

|Natural Gas |Gippsland Basin – Victoria |

| |Cooper Basin – South Australia |

|Crude Oil |Bass Strait in Victoria (will be exhausted in a few decades). |

Coal is a particularly important fossil fuel NSW as we use it to generate most of our electricity. Burning coal produced steam which drives the turbines to make electricity. Coal is found in every state in Australia, but Nsw, Vic and Qld are our main producers.

Coal is a rot consisting mainly of carbon (50-98%), some hydrogen (3-13%) and oxygen. Coal can also contain small amount of other elements such as sulfur and nitrogen. Australian coal generally has low sulfur content and most of Australia’s coal is found in 5 basins – Sydney, Bowen, Clarence-Morton, Surat ad Gippsland.

2. There is a wide variety of carbon compounds.

• Identify the position of carbon in the periodic table and describe its electron configuration.

The element Carbon is located in group 4 of the periodic table. It is also located in period 2 and has atomic number of 6. Carbon’s electron configuration is 2,4 indicating the presence of four valence electrons. Carbon is a non-metal but is able to conduct electricity when in the form of graphite.

• Describe the structure of the diamond and graphite allotropes and account for their physical properties in terms of bonding.

When an element exists as more than one crystalline form, those forms are known as allotropes. Allotropes are forms of the one elements (in the same physical state) which have significantly different physical properties (such as density, hardness, electrical conductivity and colour). There are eight main allotropes of carbon:

I. Diamond

II. graphite

III. lonsdaleite

IV. single-walled carbon nanotube (also known as buckytube)

V. Buckminsterfullerene (also known as buckyball)

VI. C540

VII. C70

VIII. Amorphous carbon

Diamond:

Diamond has a three-dimensional crystal structure, which consists of an infinite array of carbon atoms, each of which forms a structure in which each of the bonds makes equal angles with its neighbours. The ends of the bonds are connected, and the structure formed is that of a tetrahedron. Every carbon atom is covalently bonded at the four corners of the tetrahedron to four other carbon atoms (by single bonds).

This arrangement of layers of carbon atoms explains some of diamonds properties. It is extremely difficult to destroy such an arrangement of covalent bonds especially when the covalent bonds are extended throughout the lattice. This means that a extreme amount of energy is required to break the bonds between the carbon atoms in diamond’s three dimensional lattical structure; thus giving diamond extreme hardness, high melting and boiling points, reduced chemical reactivity (since the electrons are tightly bound within the covalent bonds, they are unable to move or be transferred to other elements thus minimizing reactivity), non-conductor of electricity (no free electrons).

Furthermore due to the covalent network bonding present in diamond, it is transparent and high light reflective index. This is simply because the atoms are arranged in an orderly fashion (as can be seen in the image on the previous page), this gives diamond its transparency as light is able to pass between the space between the particles.

Below is a summary of the properties of Diamond:

|Property |Explanation in terms of bonding |

| | |

|Hard |Three-dimensional lattical structure (the tetrahedral arrangement of the carbon atoms arranged systematically in layers that are|

| |not flat) and strong covalent bonds between the carbon atoms that extend across the lattice, give strong intermolecular bonds |

| |which are extremely difficult to break ensuring hardness of diamond. |

|High M.P. and B.P. | |

| |It requires huge amounts of energy to rupture the strong intermolecular bonds between the carbon atoms due to its covalent |

| |network structure in which strong covalent bonds are extended throughout the lattice, the melting points and boiling points must|

| |be high. |

| | |

|Transparency |The carbon atoms are arranged in orderly fashion throughout the entire crystal. This gives the diamond its transparency as light|

| |is able to pass between the atoms giving the diamond its colourless appearance and high light-refractive index making it |

| |extremely attractive. |

|Non-conductor of | |

|electricity |Diamond is a covalent network substance, this means that the bonds between the carbon atoms are covalent bonds. This means that |

| |the valence electrons of the carbon atoms are not free to move and since there are no mobile electron there is no conduction of |

| |electricity. |

|Minimal chemical | |

|reactivity |In order for substances to be reactive they must be able to transfer their electrons easily. In diamond, the electrons are |

| |tightly bound due to the covalent bonds, therefore they are unable to move or be transferred to other elements, giving diamond a|

| |very low reactivity. |

| | |

|High Density |The structure of the diamond shows that the atoms are tightly bound in a strong three dimensional lattical structure. Also the |

| |six-membered rings are stacked on top of one another, giving diamond a high density (3.5g/mL) |

| | |

|Insoluble in all solvents |Diamond is a very hard and extremely unreactive substance. Due to this unreactive state it has because of its covalent network |

| |structure (as there are no free electrons), it is insoluble in all solvents. This simply means that no substance will be able to|

| |chemically react with diamond when the solvent is an aqueous state. |

| | |

|Excellent Thermal |All carbon atoms in diamond are strongly bonded via covalent bonds. The diamond crystal has a symmetric cubic structure. The |

|conductor |atoms in diamond are precisely aligned. Thus diamond is known as an ideal crystal. Atoms in the crystal lattices in solids |

| |vibrate. These are called the atomic vibrations which allow for thermal conduction in solids. In an ideal crystal, the lattices |

| |are aligned so that they don’t interact with each other. Therefore an ideal crystal conducts heat better than a non-ideal |

| |crystal. Diamond being an ideal crystal is a good thermal conductor. |

Graphite:

The structure of graphite is significantly different to the structure of diamond and thus its physical and chemical properties are different as well. Graphite is also a covalent network solid (covalent lattice) like diamond but in this case each carbon atom is only three other carbon atoms (in diamond it was bound to four). This forms a planar structure as shown in the figure above.

Each ring consists of six carbon atoms which is also evident from the diagram. Since each carbon atom only has three other carbon atoms attached to it, it must mean that one electron is not covalently bonded (i.e it is free). These extra valence electrons form a sea of delocalised electrons similar to that in metals. It is the presence of the sea of delocalised electrons which makes graphite an electrical conductor (since the electrons can move when influence by an applied voltage similar to that in metals). However, electricity is only conducted along the plane of layers, graphite does not conduct electricity at 90 degrees to the plane. This is simply because the sea of delocalised electrons are only able to move across the planes and not jump from one plane to another.

It can also be seen that the two-dimensional lattices are packed one upon the other as shown in the figure above. Since, there are only weak intermolecular forces between the layers, they can easily slide across one another, and this explains the slippery-ness of graphite and its good lubricating characteristics.

Another phenomenon when it comes to graphite is that every second layer is stacked identically upon each other. “The crystal structure of graphite amounts to a parallel stacking of layers of carbon atoms. Within each layer the carbon atoms lie in fused hexagonal rings that extend infinitely in two dimensions. The stacking pattern of the layers is ABABA...; that is, each layer separates two identically oriented layers.” [1]

Below is a Summary of the properties of graphite:

|Property |Explanation in terms of Bonding |

| |As previosuly explains, the carbon atoms in graphite are connected in hexagonal rings which connect to form a|

|Slippery |layer. These layers are then piled one on top of the other. The forces that hold these layers together are |

|(Good Lubricant) |known as the van der Waals forces. These forces are extremely weak and the layers are seperated by a large |

| |distance. Due to these two factors the layers can slip over each other easy giving graphite it’s slippery |

| |nature and making it a good lubricant. |

| |The structure of graphite explains why it an extremely soft substance . This is because despite having strong|

|Extremely soft substance |covalent bonds between carbon atoms in each layer, the forces between layers are extremely weak (Van de Waals|

| |forces). This allows layers of carbon to slide over each other in graphite making the substance very soft and|

| |greasy. |

| |Graphite’s density is less than that of diamond. This is due to the structural layout. The layers are |

|Medium Density |seperated by large distances due to the weak van der Waals forces which are unable to tightly bind the layers|

|(2.3 g/mL) |together. Due to this the carbon atoms are more spread out, reducing graphite’s density. |

| |Graphite can be considered as a covalent network substance despite no bonding in the vertical direction. The |

|High M.P. and B.P. |carbon atoms are connected via strong covalent bonds which extend throughout the horizontal lattice. These |

| |intermolecular bonds are hard to break and thus more energy is required to break them. Consequently, the |

| |melting and boiling points of graphite are high. |

| |In graphite, each carbon atom only has three other carbon atoms bound to it via single bonds. Therefore it |

|Good Electrical Conductor |must mean that one electron is not covalently bound (i.e it is free). These extra valence electrons form a |

| |sea of delocalised electrons similar to that in metals. It is the presence of the sea of delocalised |

| |electrons which makes graphite an electrical conductor (since the electrons can move when influence by an |

| |applied voltage similar to that in metals). However, electricity is only conducted along the plane of layers,|

| |graphite does not conduct electricity at 90 degrees to the plane. This is simply because the sea of |

| |delocalised electrons are only able to move across the planes and not jump from one plane to another. |

Note:

“Effect of heat: it is the most stable allotrope of carbon. At a temperature of 2500 degree Celsius, it can be transformed into diamond. At about 700 degree Celsius it burns in pure oxygen forming carbon dioxide.

Chemical activity: it is slightly more reactive than diamond. This is because the reactants are able to penetrate between the hexagonal layers of carbon atoms in graphite. It is unaffected by ordinary solvents, dilute acids, or fused alkalis. However, chromic acid oxidises it to carbon dioxide.” [2]

• Process and present information from secondary sources on the uses of diamond and graphite and relate their uses to their physical properties

Uses of Diamond

|Use |Property(ies) related to use |

|Jewellery |Lustrous, High Light Refractive Index, Scratch resistant, Hard, Transparent |

|Industrial purposes like cutting tools |Hardest known substance on Earth which means it can cut through any substance, high melting point also allows it|

| |to be used in hot environments. |

| |Highest thermal conductivity of any substance which allows it to quickly extract heat from sensitive areas eg. |

|Heat sinks |Computer chips have a diamond layer that is able to quickly remove heat from the area. Also, high melting point.|

|Abrasives |Hard, Scratch resistant |

|Wear resistant parts |Resistant to corrosion, low chemical reactivity |

|Low friction microbearings |These are needed in extremely small mechanical devices. Diamond bearings are used when extreme abrasion |

| |resistance and durability are essential. |

|Diamond windows |transparent, very durable and resistant to heat and abrasion, hard (security) |

|Diamond Speaker Domes |Very stiff material (hard), also rapid vibrations will not cause deformation, therefore it enhances the |

| |performance of high quality speakers. |

Uses of Graphite

|Use |Property(ies) related to use |

|“Lead” Pencils |Soft and slippery nature, layers can easily be separated |

|Refractory crucibles |High melting and boiling points, when mixed with other substances it can become extremely hard. |

|Electrodes |Good electrical conductivity, high melting point |

|Polishes and paints |Soft, slippery nature, metallic luster |

|Lubricant in machines |Slippery nature, high melting points, greasy nature since layers can be easily separated. |

|Electrotypes for printing |Good electrical conductivity, high melting point, soft nature so can be made into a fine powder that is still |

| |able to induce an electrical current. |

|Dry cell batteries |Good electrical conductivity, high melting point |

• Identify that carbon can form single, double or triple covalent bonds with other carbon atoms.

Carbon atoms are able to form single, double or triple covalent bonds with other carbon atoms.

When one pair of electrons is being shared it is known as a single bond.

When two pairs of electrons are being shared it is known as a double bond.

When three pairs of electrons are being shared it is known as a triple bond.

When a hydrocarbon contains only single bonds it is known as an alkane and its molecular formula can be calculated using CnH2n+2 where ‘n’ is the number of carbon atoms.

When a hydrocarbon contains one or more double bonds it is known as an alkene and its molecular formula may be calculated using CnH2n

When a hydrocarbon contains one or more triple bonds it is known as an alkyne and its molecular formula may be calculated using CnH2n-2

Note: If a hydrocarbon is in the form of a ring it is known as a cyclic hydrocarbon. For example pentane in cyclic form would be known as cyclopentane.

• Explain the relationship between carbon’s combining power and ability to form a variety of bonds and the existence of a large number of carbon compounds.

Carbon forms a huge range of compounds. There are more compounds of carbon than of any other element (despite hydrogen because it is almost always present in carbon compounds). There are eight main explanations why the majority of compounds known to chemists are carbon compounds. These are:

- Carbon readily forms carbon-carbon bonds

- These bonds may be either single, double or triple or a combination of them

- Carbon forms cyclic compounds as well as straight and branched chain compounds.

- The intramolecular bonds (covalent bonds) combining the atoms in carbon compounds are strong. These strong intramolecular bonds can help the formation of millions, and even billions of kinds of molecules.

- Carbon is able to form compounds that are stable and durable. For this reason it is able to form a vast array of compounds.

- Due to the formation of isotopes, there may be many carbon compounds with the same molecular formula, yet have different structural formulae. Thus they are regarded as different substances or different compounds.

- There is no limit to the amount of carbon atoms in a chain they bond indefinitely due to the high combining power of carbon.

- Carbon it is able to form four covalent bonds which can arise in different directions allowing for complex organic compounds to be created.

Under all circumstances, carbon always forms four covalent bonds. The fact that it has four valence electrons means that it is able to lose or gain electrons. This means that it can readily combine with both non-metals and metals.

3. A variety of carbon compounds are extracted from organic sources

• Describe the use of fractional distillation to separate the components of petroleum and identify the uses of each fraction obtained.

Crude Oil is a complex mixture of hydrocarbons formed by geological action on decayed aquatic plant and animal matter over millions of years. Oil accumulates under domes of impervious rock hundreds to thousands of meters below the Earth’s surface. It has to be refined before it can be used.

The first step in oil refining is fractional distillation. For separating components of crude oil it is carried out in large steel towers up to 40 meters high. Thus, during this process, the components of oil are separated according to their boiling points. Since boiling point increases as the molecular weight increases, the separation is roughly in order of increasing molecular weights. Crude oil is vaporized and then fed into a fractionating column. The temperature falls as the vapour rises up through the column. Thus the least volatile components condense near the bottom. These liquids are collected at various heights and these are known as the various ‘Fractions’.

|Fraction |B.P. |C atoms/molecule |Uses |

|Gases |350 |>20 |Lubrication |

|Bitumen |Residue |>25 |Road making, roofing. |

The composition of crude oil varies from one oil field to another. The oil product in greatest demand is gasoline, which is the fuel for vehicles. However, the proportion of straight-run gasoline obtained from fractional distillation is not high enough to meet demand.

Thus a process known as cracking is used. Cracking is the process in which heavy fractions (long carbon chains) of crude oil are broken (cracked) into smaller fractions for production of high demand products such as petrol.

There are two main types of cracking: Thermal Cracking

Catalytic Cracking

• Identify and use the IUPAC nomenclature for describing straight chained alkanes and alkenes from C1 – C8.

|No. - Carbon Atom |Alkane |Alkene |Alkyne |

|1 |Methane | | |

|2 |Ethane |Ethene |Ethyne |

|3 |Propane |Propene |Propyne |

|4 |Butane |Butene |Butyne |

|5 |Pentane |Pentene |Pentyne |

|6 |Hexane |Hexene |Hexyne |

|7 |Heptane |Heptene |Heptyne |

|8 |Octane |Octene |Octyne |

Hydrocarbons in which all the bonds are single bonds are called alkanes.

There is a whole family of alkanes made up of different numbers of carbon atoms joined together to from a single chain. They are called straight-chained alkanes meaning that all the continuous string. Straight chain alkanes have all carbon atoms joined together in one string so that no carbon atom is joined to more than two other carbon atoms.

In addition there are branched-chain alkanes with carbon skeletons where one carbon atom is attached to at least three other carbon atoms.

• Compare and contrast the properties of alkanes and alkenes C1 – C8 and use the term homologous series to describe a series with the same functional group.

A family of compounds which can be represented by one general molecular formula is called an homologous series.

Alkanes:

The simplest alkanes (C1- C4) are gases at room temperature. Alkanes with 5-18 carbon atoms per molecule are colourless liquids, while compounds which exceed 20 carbon atoms per molecules are waxy solids.

The melting and boiling points of alkanes increase as the molecular weight increases (i.e. Number of carbon atoms per molecule increases)

The densities of both liquid and solid alkanes are significantly less than that of water. Alkanes are also insoluble in water and do not conduct electricity.

Even though not all alkanes are strictly symmetrical, alkanes are still known as non-polar molecules. This is because C-C bonds are non-polar, C-H bonds are slightly polar but they are mostly cancelled out due to the structure of the alkane.

Thus the only intermolecular forces between alkane molecules are dispersion forces. These are quite weak, thus it is easy to separate the molecules. Thus alkanes have low melting and boiling points.

Note: Dispersion forces increase as molecular weight increases.

The Volatility of a substance is the ease with which it can be converted to a vapour. Volatility increases as boiling point decreases. So for alkanes, volatility decreases as molecular weight increases.

Alkenes:

Hydrocarbons which contain a double bond a pair of carbon atoms are called alkenes.

The straight chain alkenes have similar physical properties to the alkanes. The C2-C4 alkenes are gases while the C5 to C17 ones are liquids with boiling increasing as molecular weight increases. Boiling points of alkenes are slightly lower than those or corresponding alkanes. Densities are similar to those of corresponding alkanes. Like alkanes, they are also insoluble in water and do not conduct electricity.

They are also non polar molecules as C--C bonds are non polar, like C-C bonds. Thus their only intermolecular force is weak dispersion forces, giving them low melting and boiling points.

However, alkenes and alkanes are different in that alkanes are saturated (contain the maximum number of hydrogen atoms that the particular carbon skeleton can accommodate) where as alkenes are unsaturated (it is possible to attach more hydrogen by breaking the double bond and forming single bonds to extra hydrogen atoms i.e converting them to alkanes).

Alkanes are generally more reactive than alkanes.

• explain the relationship between the melting point, boiling point and volatility of the above hydrocarbons, and their non-polar nature and intermolecular forces (dispersion forces)

Look in above dot point.

Recap:

Even though not all alkanes are strictly symmetrical, alkanes are still known as non-polar molecules. This is because C-C bonds are non-polar, C-H bonds are slightly polar but they are mostly cancelled out due to the structure of the alkane. This lack of polarity accounts for alkanes being insoluble in water (a polar solvent).

Thus the only intermolecular forces between alkane molecules are dispersion forces. These are quite weak, thus it is easy to separate the molecules. Thus alkanes have low melting and boiling points.

Note: Dispersion forces increase as molecular weight increases.

The Volatility of a substance is the ease with which it can be converted to a vapour. Volatility increases as boiling point decreases. So for alkanes, volatility decreases as molecular weight increases.

The straight chained alkenes have similar physical properties to the alkanes.

• Assess the safety issues associated with the storage of alkanes C1-C8 in view of their weak intermolecular forces (dispersion forces).

Alkanes, particularly low molecular weight ones such as C1-C8 are extremely flammable. In addition at high concentrations they can be toxic (poisonous). An added hazard is the high volatility (low boiling points) of the liquid ones, which means that if a container is left open to the atmosphere, the liquid quickly evaporates and forms a flammable or explosive mixture in the air.

Safety Precautions include:

- Well maintained cylinders and fittings for gaseous hydrocarbons. Methane and ethane are non-condensable gases ate room temperature and are therefore stored in high pressure cylinders which provide enough pressure to overcome the boiling effect and keep them as a liquid.

- Add odours that are pungent in order to quickly detect if there is a leak.

- Sturdy (preferably metal) containers for liquids + stored in a well ventilated area in case of a leakage.

- Minimize the quantity in use. This will ensure that there is minimal risk of a hazardous situation arising. Also, if large amounts are being used, they should be stored away from populated areas and in a located that is well maintained and ventilated

- Keep alkanes away from naked flames or sparks. The alkanes must be stored in areas where there is no instance of a naked flame or hot filaments. This is to ensure that no accidental combustion occurs as it could be potentially dangerous. Also, ensure that all electrical equipment being used does not produce sparks.

- Always handle in well ventilated areas. Due to the poisonous nature of these substances it is dangerous to use them in confined areas. They should be handled outdoors when ever possible.

Transportation:

- The fuel tank is located at the end remote from the hot engine and is outside the main shell of the vehicle.

- The fuel tank has narrow inlet and outlet pipes, which are both at the top of the tank to minimize chances of leakage during accidents; fuel has to be pumped from the tank by the engine so that in most cars even a fuel line rupture will not cause rapid leakage of petrol.

- When the petrol is transported by road or rail, heavy steel tanks are used. These are well sealed and are designed to withstand most collisions or overturnings without rupture of the tank.

- Features are placed in vehicles to dissipate or prevent any build up of static electricity.

- The fuels are also dyed for easy identification in case of leaks. Eg. Unleaded petrol is colored blue.

4. Combustion provides another opportunity to examine the conditions under which chemical reactions occur.

• Describe the indicators of chemical reactions

Chemical changes are also called chemical reactions. Common indicators that a chemical reaction has occurred are:

- If a gas is produced. For example when copper carbonate is decomposed under the influence of heat it produces carbon dioxide whose presence can be detected using the limewater test.

- If a precipitate is formed. For example when two solutions are mixed, such as sodium chloride and silver nitrate, silver chloride, which is a white solid forms.

- If there is a permanent colour change. For example when potassium permanganate solution (which is originally purple) is combined with hydrogen peroxide, the mixture produced is colourless, indicating a chemical reaction.

- Temperature change in the mixture is quite significant. For example when magnesium ribbon is burnt in air, the metal becomes extremely hot

- Disappearance of a solid. This is not just the dissolution of one solid in a particular solvent but rather a complete re-arrangement of elements in order to produce new substances. For example when magnesium hydroxide powder is combined with hydrochloric acid, a clear solution is produced.

- New substances are created. For example the electrolysis of water. Where water (H20) is decomposed into hydrogen gas (H2) and Oxygen gas (O2)

- Heat or light is given off. For example, when a piece of magnesium ribbon is burnt in air, significant amounts of light and heat energy are emitted, leaving a white powder behind.

- Difficult to reverse the process. For example when wood is burnt, it turns into ash and gases evolve. After the wood has completely burnt burned, it cannot be restored to its original form.

- An odour is produced. For example when sodium hydroxide is added to a solution of ammonium chloride the pungent odour of ammonia is clear.

• Identify combustion as an exothermic chemical reaction

“Combustion is a process in which a self sustaining chemical reaction occurs at temperatures above those of the surroundings. More simply, combustion is burning. Explosions are also a form of combustion. All combustion reactions liberate large amounts of heat. They are called exothermic reactions.” [3]

Combusting (burning) is a process in which a self-sustaining chemical reaction occurs at temperatures above those of the surroundings. It is a chemical reaction because we can detect, by using simple tests, the formation of water vapour and carbon dioxide gas during the burning. Combustion cannot easily be reversed. It is an exothermic reaction because it releases much heat into the surroundings.

Note: An endothermic reaction is one that absorbs heat making the reaction much cooler than the surroundings.

• Outline the changes in molecules during chemical reactions in terms of bond breaking and bond making

In chemical reactions, some bonds in reactant molecules are broken and new bonds are formed to make the product molecules. These two activities occur simultaneously in the mixture. Atoms are not created or destroyed during a chemical reaction, they are just simply rearranged.

Eg 1) Fe(s) + 2HCl(aq) FeCl2 (aq) + H2 (g)

Eg 2) AgNO3 (aq) + NaCl(aq) AgCl (s) + NaNO3 (aq)

In both examples we can see that the actual atoms are not destroyed. Whatever is on the reactant side of the chemical equation must be present on the products side. This is known as the law of conservation of matter, which states that matter cannot be created nor destroyed, but just simply transformed from one form to another. Therefore, the intramolecular bonds between the compounds are broken and new bonds are made depending on the valencies and attractions of the particular elements within the chemical reaction.

Energy must be inputted in order to break chemical bonds. Forming chemical bonds releases energy.

Common Reactions

a) Metal + Acid [pic] Salt + Hydrogen gas

b) Metal + Water [pic] Metal hydroxide + hydrogen

c) Metal + Salt [pic] New Salt + Metal

d) Acid + Base [pic] Salt + Water

e) Acid + Carbonate [pic] Salt + Carbon dioxide + water

f) Salt + salt [pic] New salts

g) Metal Oxide + water [pic] Acid

h) Combustion

i. Element + Oxygen [pic] Element Oxide

ii. Hydrocarbon + Oxygen [pic] Carbon dioxide + Water

i) Heat + Carbonate [pic] Metal Oxide + Carbon Dioxide

j) Decomposition reaction: [pic]

• Heating

• Visible light or UV

• Electrolysis

k) Single displacement reaction: [pic]

l) Combination reaction: [pic]

m) Double displacement reaction: [pic]

• Explain that energy is required to break bonds and energy is released when bonds are formed

Energy must be inputted in order to break chemical bonds. Forming chemical bonds releases energy. In chemical terms, this means that an exothermic reaction occurs when making bonds, whereas an endothermic reaction occurs when breaking bonds.

In any substance, energy is stored in the intermolecular bonds that hold the molecules together in a substance, and also in the intramolecular bonds that hold the individual atoms together in a molecule.

Every chemical reaction involves changes that result in certain bonds being broken and others being formed. When a chemical equation is written that includes energy change, this equation shows the net difference in energy change

Exothermic reactions result in a net release of energy. I.e. More energy is given off rather than absorbed.

One example of this is when hydrogen and fluorine gas combine in order to result in hydrogen fluoride.

H2(g) + F2(g) → 2 HF(g) + 546kJ

In the above chemical reaction, the H-H as well as the F-F bonds have to be broken whereas H-F bonds must be formed. As can be seen the products side has more energy than the reactants side, indicating that more energy is given off rather than absorbed. Here the energy produced when forming H-F bonds is greater than the total energy absorbed when breaking H-H / F-F bonds.

On the other hand, endothermic reactions are those that require a net input of energy. I.e. More energy is needed to break the bonds than create new ones.

One example of this is when sulfur trioxide decomposes to produce sulfur dioxide and oxygen gas.

2 SO3(g) + 198kJ → 2 SO2(g) + O2(g)

It can be clearly seen from the above equation that there is more energy on the left hand side of the equation (reactants) than the right hand side (products). This indicates that more energy is required to break bonds, when compared with energy require to make the new ones.

H = {energy required to break bonds in reactants} – {energy required to make bonds for products}

• Describe the energy needed to begin a chemical reaction as activation energy

The reason why many reactions do not occur spontaneously is that there is often an energy barrier between reactants and products.

The activation energy (EA) of a reaction is the minimum amount of energy reactant molecules must possess in order to form products. A substantial amount of energy is often necessary for a chemical reaction to occur, this is mainly due to the intramolecular bonds that have to broken. Thus an energy barrier has to be surpassed in order for the reaction to occur. The energy required to surpass the energy barrier is known as the activation energy.

Thus activation energy can be defined as the energy needed to begin a chemical reaction. The activation energy barrier is essential because it prevents most reactions from taking place which prevents the decomposition of highly complex natural molecules, thus ensuring a stable environment for all organisms.

Note: Activation energy is expressed in kJ / mol. Also for exothermic reactions, once they begin, they are self sustaining. Endothermic reactions however, need a continuous energy supply in order to operate.

• Describe the energy profile diagram for both endothermic and exothermic reactions

The below figures show graphically the relation between enthalpies of products and reactants, and H for endothermic (right) and exothermic (left) reactions.

For endothermic reactions:

- The products have more energy contents than the reactants.

- The energy level has increased so the change in enthalpy is said to be

positive

For exothermic reactions:

- The products have less energy contents than the reactants

- The energy level has dropped so the change in enthalpy is said to be

negative

Note: The difference between the energy of the reactants and the peak is known as the activation energy. Also, the difference between the energy of reactants and that of the products is known as H.

• Explain the relationship between ignition temperature and activation energy

The ignition temperature of a fuel air mixture is the minimum temperature to which the mixture (or part of it) must be heated in order for combustion to occur spontaneously.

The greater the activation energy, the higher the ignition temperature.

Note: “It is not necessary to heat all of the fuel air-mixture to the ignition temperature; it is often sufficient to heat just a small portion to the required temperature. This is because the combustion reaction, being exothermic, once started at one sport, soon spreads throughout the whole mixture.” [4]

Petrol has a high ignition temperature, this is why it is suitable as fuel for vehicles, because it can be placed in open air, without the risk of a spontaneous reaction occurring.

• Identify the sources of pollution which accompany the combustion of organic compounds and explain how these can be avoided

The combustion of fossil fuels in factories, homes, vehicles and so on are the main sources of pollution on Earth.

There are four main types of pollution that result from the combustion of fossil fuels:

1) Carbon pollution:

Carbon monoxide is a colourless, odourless gas. It is toxic because it combines with the haemoglobin in red blood cells in preference to oxygen, reducing the ability of blood to transport oxygen. It is produced by incomplete combustion when the oxygen supply is limited.

Eg: 2C8H18(l) + 17O2(g) 16CO(g) + 18H2O(l)

If there is insufficient air for the complete combustion of fuel, then some soot (solid carbon) may be formed.

Eg: C5H12(g) + 4O2(g) 3C(s) + 2CO(g) + 6H2O(g)

Carbon monoxide production is prominent in petrol engines where the air to fuel ration is very minimal. Diesel engines and electricity generating stations have a high air to fuel ratio, and thus produced very little carbon monoxide, however, if badly designed they can produced a lot of soot.

However, these production of carbon monoxide and soot can be minimised and this is by allowing the incorporation of excess air into the reaction. I.e. To ensure that the air to fuel ratio is high. In some engines this is not possible (such as petrol where ignition then becomes too difficult). Therefore, the minimization of these substances can be done using a catalyst in the exhaust pipe with is able to convert and carbon monoxide into carbon dioxide. Thus ensuring complete combustion.

2) Sulfur pollutants

Sulfur dioxide is formed by the combustion of sulfur in fossil fuels. This is mainly due to the impurities in the fuel – mostly from coal. When the combustion of coal occurs, the sulfur combines with the oxygen to produce sulfur dioxide which is a pungent gas that can cause breathing difficulties at low concentrations.

S(s) + O2(g) SO2(g)

Sulfur dioxide in the atmosphere then forms acid rain:

2SO2(s) + O2(g) 2SO3(g)

SO3(s) + H2O(l) H2SO4(aq)

The way to reduce the emission of sulfur dioxide into the atmosphere is to use low sulfur coals whenever possible. Also, sulfur dioxide can be removed from the exhaust gas at factories / power stations, but this is generally very expensive to do.

3) Particulates

Particulates are very small droplets of liquids or small solid particles that result from the incomplete combustion of fuels. Vehicles produce limited amounts of particulates, but the main contributors are power generators and industrial factories. From oil and coal, the particulates rise from the incomplete combustion of the fuel.

However, these particulates emitted from power stations and industrial factories can be minimised through the use of electrostatic precipitators. These devices can generate a high voltage, causing the small particulates to combine with one another to produce large amounts of substance, which are then easily filtered out of the exhaust gas.

4) Oxides of Nitrogen

Oxygen – nitrogen reactions only occur at extremely high temperatures (above 1000 degrees Celsius), in order to produce nitric oxide:

N2(g) + O2(g) 2NO(g)

The next step occurs, when nitric oxide reacts with oxygen to produce nitrogen dioxide:

2NO(g) + O2(g) 2NO2(g)

Petrol and diesel engines along with power stations and industrial factories are the main contributors. The main concern with the production of nitrogen dioxide is that under the influence of sunlight it can lead to the production of ozone, which is a very dangerous substance – it is known as a photochemical smog. Nitrogen oxides can cause respiratory problems and also contribute to the formation of acid rain.

Laws are in place to minimise production of nitrogen oxides from petrol / diesel engines. Also relocation of power stations from population centres. Also using catalysts to remove oxides of nitrogen from exhaust gas of power stations. Finally lowering combustion temperatures to prevent the formation of those oxides.

Extra: Pollution due to Carbon Dioxide – The Greenhouse Effect

Carbon dioxide is not considered as a pollutant – this is mainly because it has no damaging affect on humans or any other living organism, and does not spoil any aspect of the environment. Carbon dioxide is a necessary substance on Earth and without it there would be no life.

However, the excessive release of carbon dioxide into the atmosphere contributes to what is known as the greenhouse effect. This is when a layer (which is constantly increasing) of carbon dioxide and other gases surround the earth, causing the Earth to heat up since they reflect heat back to Earth. This is believed to cause significant climate changes such as rising levels of oceans.

Combustion of fossil fuels is the most significant contributor to global warming and the only way to reduce the emission of carbon dioxide into the atmosphere is to reduce the need for fossil fuels, by creating more efficient vehicles / industries.

• Describe chemical reactions by using full balanced chemical equations to summarise examples of complete and incomplete combustion

In complete combustion, the only products formed are that of water and carbon dioxide. For example:

1) C5H12(g) + 8O2(g) → 5CO2(g) + 6H2O(g)

2) 2C8H18(l) + 25O2(g) → 16CO2(g) + 18H2O(l)

3) CH4(g) + 2O2(g) → CO2(g) + 2H2O(l)

4) C2H4(g) + 3O2(g) → 2CO2(g) + 2H2O(l)

5) 2C4H10(g) + 13O2(g) → 8CO2(g) + 10H2O(l)

In all of the above examples, it is clear that the products formed will always be carbon dioxide and water – thus obeying the meaning of complete combustion.

In incomplete combustion (due to lack of oxygen), carbon monoxide and/or carbon (soot) may from instead of or in addition to carbon dioxide.

1) C5H12(g) + 4O2(g) → 3C(s) + 2CO(g) + 6H2O(g)

2) 2C8H18(l) + 17O2(g) → 16CO(g) + 18H2O(l)

3) C5H12(g) + 6O2(g) → CO2(g) + 4CO(g) + 6H2O(g)

4) 2C2H2(g) + 3O2(g) → 4CO(g) + 2H2O(g)

5) C3H8(g) + 2O2(g) → 3C(s) +4H2O(g)

In all of the above examples, it is clear that the products formed will have other substances than carbon dioxide and water – thus obeying the meaning of incomplete combustion.

• identify the changes of state involved in combustion of a burning candle

A solid fuel, such as the large chain hydrocarbons that make up candle wax, must be melted and vaporized (two changes of state) in order to burn.

The wax melts, the molten wax moves up the wick and then this molten wax vaporizes. The wax vapour around the wick is what actually burns.

A solid wax cannot be burned directly by a small flame due to its high activation energy (and thus high ignition temperature). The burning wick melts the wax and the liquid wax is fed up the wick via capillary action.

The burning wick provides energy to heat this small liquid and vaporizes it. The vapour wax due to sufficient energy starts a combustion. The heat energy released from the combustion of the vapour wax hasten further combustion reactions. The reaction is therefore self-sustaining.

5. The rate of energy release is affected by factors such as types of reactant

• Describe combustion in terms of slow, spontaneous and explosive reactions and explain the conditions under which these occur.

The combustion reactions we use in everyday life proceed at very different rates.

- Slow Combustion – This refers to situations such as stove tops where large lumps of wood may take many hours to burn.

- Fast Combustion – This may be the burning of methane in heating appliances

- Explosive combustion – This is in the cylinders of petrol engines in cars.

All of the chemical reactions involved in these combustion processes are known as spontaneous reactions. This means that once the reaction has started they will continue to operate without any further assistance (energy input), and will remain continuous until all the fuel is used up.

Slow combustion occurs when we use big lumps of fuel and limit the supply of air. This means that burning only occurs on the surface of the lump and its speed is controlled by the limited supply of air.

Fast combustion occurs when fuel (eg coal) is ground into very small particles that are sprayed into a plentiful supply of air. There is a large surface area of fuel exposed to an excess of oxygen and there is good mixing to stop oxygen concentrations becoming depleted near the surface of the particles.

An explosion is just an extremely rapid reaction – one that goes to completion within a few microseconds. Explosions occur when there are high concentrations of gases or finely divided solid particles of materials that can undergo combustion

• Explain the importance of collisions between reacting particles as a criterion for determining reaction rates.

The Rate of Reaction is the rater of change of concentration with time.

Alternatively, the average rate of reaction over a small time interval is the change in concentration divided by the time taken for the change to occur.

In terms of a concentration versus time graph, the rate of reaction at any particular time is the magnitude of the slope (gradient) of the curve at that time.

Reaction rates decrease as the reaction proceeds.

Increasing the concentration of a reactant generally increases the rate of reaction.

The rate of reaction increases as the temperature is increased.

Substances that increase the rate of a reaction without undergoing permanent chemical change in the reaction are called Catalysts.

Reactions that occur uniformly throughout a solution are called homogeneous reactions along with reactions that occur uniformly throughout the whole gaseous mixture.

There are many reactions that occur at the interface between two phases; such reactions are called heterogeneous reactions.

Heterogeneous reactions are dependant upon a further two factors:

- The state of division of the solid

- The rate of stirring which is used.

For some reactions, the rate depends upon the intensity (brightness) of visible or ultraviolet light shining upon the reactants.

For a reaction to occur the reactant particles (atoms, molecules or ions), must collide.

Anything that increases the rate at which collisions occur will increase the rate of reaction. Increasing the concentration of reactants, state of division of a solid reactant, or rate of stirring increases the rate of collision and so the rate of reaction.

Concentration: This measures the number of particles of a particular substance per unit volume. Increasing the concentration puts more particles in unit volume and so increases the chance of collision between particles of one reactant and those of another reactant, which increases the reaction rate.

State of Division of a solid: Breaking big lumps of solid into smaller pieces increases the surface area of the solid. The greater the area of the solid, the more collisions that can occur in a given time, so the reaction rate increases.

Stirring: This has two effects:

- 1) It keeps the solid suspended in the solution so the maximum surface is exposed to the solute / gas.

- 2) For reactions in solution, stirring quickly replaces solution in which the reactant has been used up with fresh solution, so ensuring that there is always plenty of solute for the solid to react with.

• Explain the relationship between temperature and the kinetic energy of particles.

As the temperature increases, the average kinetic energy (and so the speed) of particles increases.

This means that the rate of collisions will increase, which will cause an increase in reaction rate.

In order for a reaction to proceed, it is necessary not only for the reactant molecules to collide, but also for the colliding molecules to possess a certain minimum amount of kinetic energy so that they can reach the top of the energy barrier.

If the colliding molecules have insufficient , they just bounce apart and stay as reactants. Kinetic energy is energy of motion: the faster the particles are moving, the higher is their kinetic energy.

If the temperature is increased, not only is the average kinetic energy of the molecules increased, but also the fraction of the molecules having more than enough kinetic energy to scale the energy barrier is dramatically increased, thus the reaction rate increases.

Reactions with greater activation energy will have a smaller reaction rate. But reactions with the higher activation energy will have more rapid reaction rate increases as the temperature is increases.

Many reactions have quite small activation energies so proceed quite rapidly at room temperature.

• Describe the role of catalysts in chemical reactions, using a named industrial catalyst as an example.

Catalysts increase the reaction rate. Catalysts may be homogeneous or heterogeneous. A catalyst is a substance that changes the rate of a chemical reaction without being used up by the reaction. A catalyst only changes the rate at which a reaction occurs. It can speed up a reaction by providing an alternate pathway for the reaction that needs lower activation energy.

Homogeneous Catalysts work throughout the bulk of the reaction mixture (gas or solution). Nitrogen dioxide is a homogeneous catalyst for the reaction between sulfur dioxide and oxygen.

Heterogeneous Catalysts provide a surface on which the reaction occurs more rapidly than it does in the bulk of the reaction mixture. Finely divided nickel catalyses the reaction between alkenes and hydrogen to form alkanes. The nickel is a heterogeneous catalyst. The reaction occurs between gaseous hydrogen and the liquid or gaseous alkene on the surface of the solid nickel particles.

Haber’s Process – Magnetite – Heterogeneous Catalyst – Fe3O4

N2(g) + 3H2(g) Oxides of Iron 2NH3(g)

How do heterogeneous catalysts work?

The solid heterogeneous catalyst provides the surface for the reactants to be absorbed. As the reacting particles are absorbed on the surface of the catalysts, their chemical bonds are weakened, resulting in more chance for successful collision to occur. Furthermore, the surface of the catalyst provides a direct route of contact between the particles – more than what would occur naturally.

• Explain the role of catalysts in changing the activation energy and hence the rate of chemical reaction

Catalysts are particularly useful when the uncatalysed reaction has a very high activation energy (and is therefore very slow). The catalyst usually provides a pathway of lower activation energy.

Although catalysts decrease the activation energy of reactions, they have absolutely no effect upon H, the enthalpy change for the reaction.

The reason for this is that the reactants and the products are exactly the same for both the catalysed and uncatalysed reactions.

• analyse information and use the available evidence to relate the conditions under which explosions occur to the need for safety in work environments where fine particles mix with air

Explosions occur when the reactions become extremely rapid. This usually happens when there is good contact between reactant particles and when the reaction is highly exothermic with high activation energy.

Once the reaction is initiated, it liberates energy, which heats up the reaction mixture. This makes the reaction go faster, releasing energy more quickly, so there is an extremely rapid escalation in temperature and reaction rate, causing an explosion. In order for the rate to increase this way there must be a good supply of oxygen available to fuel, otherwise a limiting amount of oxygen will slow down the reaction.

Large lumps of fuel such as coal rarely explode because they rapidly use up the oxygen available ate their surfaces. However, very small particles of flammable material dispersed through a volume of air have great potential for causing explosions. The total surface area of the particles is large and each particle has a ready supply of oxygen.

Consequently, one aspect of providing safe working conditions is ensuring that there can be no build up of concentrations of flammable substances. Formation of flammable dust should be minimised, and what does form must be efficiently removed from the air.

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[1] "carbon." Encyclopædia Britannica. 2009. Encyclopædia Britannica Online. 07 Jun. 2009

URL:

[2] “Allotropes of carbon” Wikipedia 1st June 2009 – Accessed 10 June 2009

URL:

[3] Direct Quote: Smith, Ronald. Conquering Chemistry - Preliminary Course Australia: McGraw-Hill, 2004

[4] Direct Quote: Smith, Ronald. Conquering Chemistry - Preliminary Course Australia: McGraw-Hill, 2004

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