Combustion of Fuels and Incomplete Combustion — AQA GCSE Chemistry
Burning hydrocarbon fuels releases energy, but the products depend on how much oxygen is available.
Complete combustion
When a hydrocarbon burns in plenty of oxygen, complete combustion occurs: hydrocarbon + oxygen → carbon dioxide + water
This is an exothermic reaction that releases a lot of energy, which is why hydrocarbons are used as fuels. During combustion, both the carbon and hydrogen are oxidised.
Example
methane + oxygen → carbon dioxide + water CH₄ + 2O₂ → CO₂ + 2H₂O
Incomplete combustion
If there is not enough oxygen, incomplete combustion occurs, producing:
- Carbon monoxide (CO) — a toxic gas that is colourless and odourless; it combines with haemoglobin and reduces the blood's ability to carry oxygen.
- Soot (carbon particles) — causes breathing problems and blackens surfaces.
- as well as water.
This is why appliances that burn fuels need a good air supply and ventilation.
Exam tips
- Complete combustion → carbon dioxide + water (plenty of oxygen).
- Incomplete combustion → carbon monoxide + soot (+ water) (limited oxygen).
- Carbon monoxide is toxic because it reduces oxygen transport in the blood.
- Combustion is exothermic and oxidises both carbon and hydrogen.