Complete combustion of an alkane produces which products?

• A. Carbon dioxide and water
• B. Carbon monoxide and water
• C. Carbon and water
• D. Hydrogen and carbon dioxide

Answer: (A)

Complete combustion occurs when there is plenty of oxygen, so every carbon in the alkane ends up as carbon dioxide and every hydrogen ends up as water. For an alkane with formula CnH2n+2, burning in excess oxygen gives the balanced idea that produces n molecules of carbon dioxide and (n+1) molecules of water. An easy concrete example is methane: CH4 + 2 O2 → CO2 + 2 H2O. This shows why the products are carbon dioxide and water.

The other possibilities don’t fit this situation. Carbon monoxide and water would come from incomplete combustion when oxygen is limited, not when there is enough oxygen. Carbon and water would imply carbon is released as an unoxidized solid, which isn’t the typical product of burning in air. Hydrogen and carbon dioxide would suggest hydrogen gas is produced, which doesn’t occur during normal combustion of alkanes.