8.7 - Complete combustion of hydrocarbon fuels

Products and energy transfer

During complete combustion, a hydrocarbon reacts with a sufficient supply of oxygen. Carbon atoms in the fuel become carbon dioxide; hydrogen atoms become water.

hydrocarbon + oxygen → carbon dioxide + water

Combustion is exothermic: energy transfers to the surroundings, for example heating water above a burner. Energy is conserved, not created. It is an energy transfer rather than a chemical substance in the equation.

This product rule applies to the hydrocarbon itself. Impurities in real fuels can form additional products.

Balancing combustion equations

Balance carbon first, hydrogen second and oxygen last. Change numbers in front of formulae; never alter a substance's formula to balance an equation.

For methane:

CH₄ + 2O₂ → CO₂ + 2H₂O

There is one C atom, four H atoms and four O atoms on each side.

Worked example: propane, C₃H₈

1. Three C atoms need 3CO₂.
2. Eight H atoms need 4H₂O.
3. The products contain (3 × 2) + 4 = 10 oxygen atoms, supplied by 5O₂.

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

If oxygen needs a fractional coefficient, multiply every coefficient by the same number to obtain whole numbers.