What are the products of complete combustion of a hydrocarbon?

• A. Carbon monoxide and water.
• B. Carbon dioxide and water.
• C. Oxygen and water.
• D. Carbon dioxide and oxygen.

Answer: (B)

Complete combustion occurs when a hydrocarbon burns with plenty of oxygen, so the carbon atoms become carbon dioxide and the hydrogen atoms form water. For example, methane burns as CH4 + 2 O2 → CO2 + 2 H2O. More generally, a hydrocarbon CxHy reacts with O2 to give x CO2 and (y/2) H2O. Oxygen is used up, not produced, so you don’t get oxygen as a product. If the air supply is limited, incomplete combustion can occur, producing carbon monoxide or soot instead. So the typical products of complete combustion are carbon dioxide and water.