AQA GCSE Chemistry

Revision Notes

7.2.2 Combustion of Alkenes


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

Exam Tip

Smoky flames indicates a higher carbon to hydrogen ratio in organic molecules, such as alkenes.

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.

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