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
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