Describe two differences between complete combustion and incomplete combustion of a hydrocarbon fuel. For each difference, state what is produced and explain how availability of oxygen causes that difference.

1. In complete combustion, the hydrocarbon reacts with a plentiful supply of oxygen to give carbon dioxide (CO₂) and water (H₂O). In incomplete combustion, limited oxygen causes the fuel to give carbon monoxide (CO) and/or elemental carbon (soot) along with water.
2. Complete combustion releases more energy because the fuel is fully oxidised; incomplete combustion releases less energy because the oxidation is incomplete.
Both processes require oxygen, but the amount determines whether CO₂ or CO/soot is formed and how much energy is released.