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| Combustion of fuels Complete combustion Fuels are substances that react with oxygen to release useful energy. Most of the energy is released as heat, but light energy is also released. About 21 per cent of air is oxygen. When a fuel burns in plenty of air, it receives enough oxygen for complete combustion. Complete combustion needs a plentiful supply of air so that the elements in the fuel react fully with oxygen. Fuels such as natural gas and petrol contain hydrocarbons. These are compounds of hydrogen and carbon only. When they burn completely: the carbon oxidises to carbon dioxide the hydrogen oxidises to water (remember that water, H2O, is an oxide of hydrogen) | In general, for complete combustion: hydrocarbon + oxygen → carbon dioxide + water Here are the equations for the complete combustion of propane, used in bottled gas: propane + oxygen → carbon dioxide + water C3H8 + 5O2 → 3CO2 + 4H2O- |
| Incomplete combustion Incomplete combustion occurs when the supply of air or oxygen is poor. Water is still produced, but carbon monoxide and carbon are produced instead of carbon dioxide. | -In general for incomplete combustion: hydrocarbon + oxygen → carbon monoxide + carbon + water The carbon is released as soot. Carbon monoxide is a poisonous gas, which is one reason why complete combustion is preferred to incomplete combustion. Gas fires and boilers must be serviced regularly to ensure they do not produce carbon monoxide. Carbon monoxide is absorbed in the lungs and binds with the haemoglobin in our red blood cells. This reduces the capacity of the blood to carry oxygen. Here are the equations for the incomplete combustion of propane, where carbon is produced rather than carbon monoxide: propane + oxygen → carbon + water C3H8 + 2O2 → 3C + 4H2O |
| Nitrogen oxides When fuels are burned in vehicle engines, high temperatures are reached. At these high temperatures, nitrogen and oxygen from the air combine to produce nitrogen monoxide. One nitrogen molecule (N2) reacts with one oxygen molecule (O2) to make two nitrogen monoxide molecules (NO). nitrogen + oxygen → nitrogen monoxide N2(g) + O2(g) → 2NO(g) | - |
| When this nitrogen monoxide is released from vehicle exhaust systems, it combines with oxygen in the air to form nitrogen dioxide. One nitrogen molecule (N2) reacts with two oxygen molecules (O2) to make two nitrogen dioxide molecules (NO2) nitrogen monoxide + oxygen → nitrogen dioxide 2NO(g) + O2(g) → 2NO2(g) Nitrogen dioxide is a cause of acid rain. Nitrogen monoxide and nitrogen dioxide are jointly referred to as NOx. | - |
| Sulfur dioxide and acid rain Many fossil fuels contain sulfur impurities. When these fuels are burned, the sulfur is oxidised to form sulfur dioxide. S(s) + O2(g) → SO2(g) This sulfur dioxide then dissolves in droplets of rainwater to form sulfurous acid. SO2(g) + H2O(l) → H2SO3(aq) | - |
| Effects of acid rain Acid rain reacts with metals and rocks such as limestone. Buildings and statues are damaged as a result. Acid rain damages the waxy layer on the leaves of trees and makes it more difficult for trees to absorb the minerals they need for healthy growth. They may die as a result. Acid rain also makes rivers and lakes too acidic for some aquatic life to survive. | - |
| The carbon cycle Most of the chemicals that make up living tissue contain carbon. When organisms die, the carbon is recycled so that it can be used by other organisms. The model that describes the processes involved is called the carbon cycle. | - |
| Stages in the carbon cycle | - |
| Carbon enters the atmosphere as carbon dioxide from respiration and combustion. | |
| Carbon dioxide is absorbed by producers to make carbohydrates during the process of photosynthesis. | |
| Animals feed on the plant passing the carbon compounds along the food chain. Most of the carbon they consume is exhaled as carbon dioxide (formed during respiration). The animals and plants eventually die. | |
| The dead organisms are eaten by decomposers and the carbon is returned to the atmosphere as carbon dioxide. In some conditions decomposition is blocked. The plant and animal material may then be available as fossil fuel in the future for combustion. | |
| Methane Methane, CH4, is a gas that can be produced by: decomposition of vegetation waste gases from digestion in animals Methane is a powerful greenhouse gas and therefore contributes to global warming. | - |
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