Combustion of Hydrocarbons (Edexcel GCSE Chemistry)

Combustion of alkenes

• Alkanes and alkenes undergo combustion in the presence of air
• Complete combustion occurs to form water and carbon dioxide gas
• For example, the simplest alkane, methane burns as follows:

CH₄ +  2O₂ → CO₂ +  2H₂O

• Gasoline is largely composed of isomers of octane, C₈H₁₈ , which requires large amounts of oxygen to combust fully

2C₈H₁₈ + 25O₂ → 16CO₂ + 18H₂O

• The efficiency of car engines does not usually enable all the gasoline to burn, so car exhaust will contain small amounts of unburnt hydrocarbons as well as other products such as carbon monoxide and soot which lead to environmental problems
• The carbon dioxide produced is a major contributor to global warming and the replacement of combustion engines with electric vehicles is a major on-going challenge for all countries
• Methane, the main component of natural gas, is the fuel used in Bunsen burners which can control the degree of combustion and hotness of the flames:

Combustion of Methane in a Bunsen Burner

Combustion of alkenes

• These compounds undergo complete and incomplete combustion, but because of the higher carbon to hydrogen ratio they tend to undergo incomplete combustion, producing a smoky flame in air
• Complete combustion occurs when there is excess oxygen so water and carbon dioxide form e.g:

C₄H₈           +          6O₂       →      4CO₂       +         4H₂O

butene      +      oxygen    →  carbon dioxide   +  water