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Complete combustion

Fuels are substances that react with oxygen to release useful energy (exothermic). 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 is the equation for the complete combustion of methane, used in Bunsen burners:

methane + oxygen → carbon dioxide + water

CH₄ + 2O₂ → CO₂ + 2H₂O

Here is the equation for the complete combustion of propane, used in bottled gas:

propane + oxygen → carbon dioxide + water

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

Ethanol, a simple alcohol, also combusts to form carbon dioxide and water:

ethanol + oxygen → carbon dioxide + water

C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O