9.16C - Complete combustion of alkanes and alkenes
Explain why complete combustion oxidises both alkanes and alkenes to carbon dioxide and water. Then practise balancing the equations by tracking carbon, hydrogen and oxygen atoms. This is a separate Chemistry point.
Products and oxidation
Complete combustion occurs when a fuel burns in a sufficient supply of oxygen. An alkane or alkene contains only carbon and hydrogen, so its complete-combustion products are carbon dioxide and water:
hydrocarbon + oxygen → carbon dioxide + water
The hydrocarbon is oxidised: it combines with oxygen to form oxygen-containing products. Its carbon atoms become part of CO2 and its hydrogen atoms become part of H2O. Here oxidation is described as gain of oxygen.
Both alkanes and alkenes follow this pattern. The C=C bond in an alkene does not change the names of the complete-combustion products. Different molecular formulae do change the amounts of oxygen needed and products made.
Carbon monoxide or solid carbon can form in incomplete combustion when oxygen is insufficient. They are not products of complete combustion.
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