C9.3 - Burning hydrocarbon fuels

• The lighter fractions from crude oil are very useful as fuels. When hydrocarbons burn in plenty of air, they transfer lots of energy to the surroundings.
• Complete combustion occurs when there is plenty of oxygen. The complete combustion of any hydrocarbon in oxygen releases lots of energy. The only waste products are carbon dioxide and water vapour.
• During complete combustion, both carbon and hydrogen from the hydrocarbon are oxidised. Hydrocarbons are used as fuels due to the amount of energy released when they combust completely.
• hydrogen + water → carbon dioxide + water (+energy)
• When burning hydrocarbons, we want them to be fully burnt up. If they aren't then we get less energy out per unit mass than we would otherwise. We also produce more pollutants such as CO and smoke.
• Incomplete combustion occurs if not enough oxygen is available. This can happen because there isn't enough oxygen or too much fuel. In this case, some of the carbon doesn’t react with oxygen so it remains unchanged (as solid carbon or soot). Some of the hydrogen reacts with oxygen but other molecules don’t have time to do so. These remain as hydrocarbons which are gases at room temperature. Carbon monoxide is also formed, which is a toxic gas. Your red blood cells pick up this gas and carry it around in your blood instead of oxygen.
• Equation of complete combustion of methane:  CH4 + 2 O 2 → CO2 + 2H2O
• Equation of incomplete combustion of methane: CH4+O2⟶C(s)+2H2O(l)