Combustion of Alkanes

This lesson covers: 

  1. Reactivity of alkanes
  2. Complete and incomplete combustion reactions of alkanes

Alkanes are generally unreactive

Alkanes are relatively unreactive due to their non-polar nature and their strong covalent bonds. The electronegativity difference between carbon and hydrogen in alkanes is minimal, leading to an almost equal sharing of electrons and no significant partial charges. This makes it difficult for alkanes to attract nucleophiles or electrophiles.


However, alkanes do undergo certain reactions, including:

  • Combustion - The reaction of alkanes with oxygen to release energy.
  • Substitution - The replacing of H atoms with halogens in the presence of light.

Combustion reactions of alkanes

Alkanes serve as efficient fuels, releasing a significant amount of energy when burnt. They are used in various applications such as power generation, heating, and transportation due to this property.


Alkanes can undergo two types of combustion reactions: complete and incomplete, depending on the oxygen availability.

Complete combustion of alkanes

When there is an ample supply of oxygen, alkanes combust completely, forming carbon dioxide and water vapour as products.


For instance, the complete combustion of methane (CH4) is represented by the following equation:

CH4(g) + 2O2(g) ➔ CO2(g) + 2H2O(g)


Key points:

  • Alkanes in their liquid state need to be turned into gas (vaporised) before they can combust.
  • Smaller alkanes, due to their lower boiling points, vaporise and thus combust more readily.
  • Larger alkanes have more chemical bonds, hence when combusted, they release more energy per mole, making them better fuels.

Incomplete combustion of alkanes

When the oxygen supply is limited, alkanes undergo incomplete combustion, leading to the formation of carbon monoxide and water vapour.


For example, incomplete combustion of methane (CH4) can be represented by the following equation:

CH4(g) + 3⁄2O2(g) ➔ CO(g) + 2H2O(g)


Incomplete combustion may also lead to the production of solid carbon (soot) and the release of unburnt hydrocarbons into the atmosphere.