Burning Fuels - Fuels are substances that release energy when they react with oxygen. This reaction is known as burning, or combustion. Combustion is an exothermic reaction
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Fuels Burning Fuels Fuels are substances that release energy when they react with oxygen. This reaction is known as burning, or combustion. Combustion is an exothermic reaction. The energy which is given out by combustion is used in a variety of ways: > > Energy from burning methane in gas cookers is used for heating. > > Energy from burning candles is used for lighting. > > Energy from burning petrol in car engines is used to create motion. > > Energy from burning coal in power stations is used to generate electricity. The oxidation of fuels gives chemical products, as well as energy. Most of the fuels used today are hydrocarbons. When hydrocarbons burn, carbon dioxide and water are the usual chemical products: e.g. CH4(g) + 2O2(g) ® CO2(g) + 2H2O(l) Incomplete Combustion If the air/oxygen supply for burning is limited, then incomplete combustion may occur. The combustion will not only be less exothermic, but the chemical products will also differ - carbon monoxide is formed instead: e.g. CH4(g) + 11/2O2(g)
* * Natural Gas 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 * * Ethanol (methylated spirits) (Refer to 'Chemicals from Oil' notes.) * * Wood * * 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: * * Carbon dioxide, CO2 * * Carbon monoxide, CO * * Sulphur dioxide, SO2 * * Oxides of nitrogen, NOx * * Unburned hydrocarbons * * Lead compounds * * Smoke Effects of these pollutants include: * * Acid
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.)
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