What are the main products of complete combustion of a hydrocarbon?

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

Answer: (A)

When a hydrocarbon burns completely, there is enough oxygen for every atom to be fully oxidized. The carbon in the molecule becomes carbon dioxide and the hydrogen becomes water. That’s why the main products are carbon dioxide and water (often seen as water vapor at the high temperatures of combustion).

A simple example is methane: CH4 plus 2 O2 yields CO2 plus 2 H2O. More generally, a hydrocarbon CxHy reacts with oxygen to give x molecules of CO2 and (y/2) molecules of H2O, following the balanced form CxHy + (x + y/4) O2 → x CO2 + (y/2) H2O.

If oxygen were limited, incomplete combustion could occur, producing carbon monoxide or carbon as well as water, which is why complete combustion is specifically associated with CO2 and H2O.