CIE AS Chemistry (9701) exams from 2022

Revision Notes

3.2.2 Combustion & Free Radical Substitution of Alkanes

Combustion & Free Radical Substitution of Alkanes

  • Alkanes are combusted (burnt) on a large scale for their use as fuels
  • They also react in free-radical substitution reactions to form more reactive halogenoalkanes

Complete combustion

  • When alkanes are burnt in excess (plenty of) oxygen, complete combustion will take place and all carbon and hydrogen will be oxidised to carbon dioxide and water respectively
    • For example, the complete combustion of octane to carbon dioxide and water

Hydrocarbons Complete Combustion, downloadable AS & A Level Chemistry revision notes

The complete combustion of alkanes

Incomplete combustion

  • When alkanes are burnt in only a limited supply of oxygen, incomplete combustion will take place and not all the carbon is fully oxidised
  • Some carbon is only partially oxidised to form carbon monoxide
    • For example, the incomplete combustion of octane to form carbon monoxide

 Hydrocarbons Incomplete Combustion, downloadable AS & A Level Chemistry revision notes

The incomplete combustion of alkanes

  • Carbon monoxide is a toxic gas as it will bind to haemoglobin in blood which can then no longer bind oxygen
  • As no oxygen can be transported around the body, victims will feel dizzy, lose consciousness and if not removed from the carbon monoxide, they can die
  • Carbon monoxide is extra dangerous as it is odourless (it doesn’t smell) and will not be noticed
  • Incomplete combustion often takes place inside a car engine due to a limited amount of oxygen present

 Free-radical substitution of alkanes

  • Alkanes can undergo free-radical substitution in which a hydrogen atom gets substituted by a halogen (chlorine/bromine)
  • Since alkanes are very unreactive, ultraviolet light (sunlight) is needed for this substitution reaction to occur
  • The free-radical substitution reaction consists of three steps:
    • In the initiation step, the halogen bond (Cl-Cl or Br-Br) is broken by UV energy to form two radicals
    • These radicals create further radicals in a chain type reaction called the propagation step
    • The reaction is terminated when two radicals collide with each other in a termination step

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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