Fossil Fuels
Fossil fuels were formed from the remains of plants and animals that became buried in sediments millions of years ago. Heat and pressure (in the absence of oxygen) converted these remains into three major types of fossil fuel – coal, oil and natural gas.
Coal consists mostly of carbon, but also contains a complex mixture of other chemicals. The burning of coal in power stations provides ~ 20 % of the world’s energy.
C(s) + O2(g) → CO2(g)
Coal can be refined by heating it strongly in the absence of air. This produces coke, which is nearly pure carbon, and a cleaner fuel. It is used in the blast furnace to extract iron from iron ore.
Solid fuels like coal are easy to transport and store. However, they can be hard to ignite, and (in the case of coal), release pollutants on burning.
Many fuels are derived from crude oil, including petrol, diesel oil, kerosine and paraffin wax. All of these fuels are hydrocarbons, so burn to form carbon dioxide and water, as discussed above. Most of these fuels are liquids at room temperature.
Liquid fuels can present storage problems and other hazards, especially petrol, which contains very volatile hydrocarbons. A fuel vapour/air mixture can ignite very easily so a small spark several metres from a petrol spill can cause a fire.
The main constituent of natural gas is methane (CH4). Traces of ethane, propane, nitrogen and other gases are also present.
Gaseous fuels are easy to ignite, and can be burned very efficiently. They are harder to store and transport, and gas leaks can be very dangerous.
Other Fuels
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∙ Ethanol (methylated spirits) (Refer to ‘Chemicals from Oil’ notes.)
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∙ Wood
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∙ Hydrogen – has a high ‘energy density’ and is a very clean burning fuel; the only combustion product is water:
2H2(g) + O2(g → 2H2O(l)
Air Pollution
One of the problems with burning fuels is the production of pollutants. Most of these pollutants are gases, so they diffuse into the atmosphere, where they can have various effects on the environment.
Pollutants from burning fuels include:
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∙ Carbon dioxide, CO2
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∙ Carbon monoxide, CO
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∙ Sulphur dioxide, SO2
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∙ Oxides of nitrogen, NOx
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∙ Unburned hydrocarbons
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∙ Lead compounds
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∙ Smoke
Effects of these pollutants include:
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∙ Acid rain
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∙ The Greenhouse effect
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∙ Smog
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∙ Health problems
Acid Rain
The burning of fossil fuels in industry and in motor vehicles releases sulphur dioxide and oxides of nitrogen.
These oxides react with water and oxygen in the atmosphere to form sulphuric and nitric acids respectively:
SO2(g) + H2O(l) + ½O2(g) → H2SO4(aq)
2NO2(g) + H2O(l) + ½O2(g) → 2HNO3(aq)
Acid rain can react with certain minerals in the soil, forming soluble salts like aluminium sulphate, which is harmful to tree roots, ultimately causing the death of trees. Fish and other water life cannot survive in acidic lakes. Acid rain also accelerates the corrosion of metals and stonework on buildings.
Often wind blows acidic rain clouds hundreds of miles away from the source of the pollution before acid rain actually falls.
Photochemical Smog
In the presence of sunlight oxides of nitrogen and unburned hydrocarbons react with water vapour and oxygen in the air to form complex mixtures of chemicals including ozone (O3) which can be very irritating to our eyes and lungs.
The Greenhouse Effect
The earth’s atmosphere traps some of the sun’s radiation that would otherwise escape. This helps to keep the earth warm. Some gases in the atmosphere are better than others at absorbing this heat, and carbon dioxide is a particularly good ‘greenhouse gas’.
Over the last 300 years, the amount of carbon dioxide in the atmosphere has risen sharply, due to human activities - mainly the increased burning of fossil fuels, and deforestation. This extra carbon dioxide may cause the average temperature of the earth to increase by 2-3 oC, which could have serious consequences, like flooding due to a rise in sea level.
This problem is called ‘global warming’.
The Hole in the Ozone Layer
The only pollutants in the table above which are not connected with the burning of fuels are the chlorofluorocarbons (CFCs).
These compounds have been used extensively as refrigerants, propellants and cleaning solvents in the twentieth century.
The ozone layer protects us from the sun’s harmful UV radiation, which can cause sunburn and skin cancer. In the 1980’s it was discovered that a large hole was developing in the ozone layer, and it was thought that CFCs were responsible.
These compounds are very unreactive and so gradually diffuse into the upper atmosphere. There they are broken down by ultra-violet rays from the sun giving chlorine atoms (‘chlorine radicals’) which are highly reactive. These chlorine radicals catalyse the breakdown of ozone into normal oxygen:
2O3(g) → 3O2(g)
(For further details about these pollutants, and their environmental impact, see handout.)
