Combustion of Alkenes (AQA GCSE Chemistry)

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

• Incomplete combustion occurs when there is insufficient oxygen to burn so a mixture of products can form, e.g:

C₄H₈     +          4O₂          →        4CO           +         4H₂O

butene      +      oxygen    →  carbon monoxide   +  water

• In addition to carbon monoxide, carbon in the form of soot can be produced:

C₄H₈     +          2O₂          →        4C           +         4H₂O

butene      +      oxygen    →  carbon   +     water

• This is more likely to occur in higher alkenes with larger number of carbons

• This is seen as smoky yellow flames when the alkenes burn