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Combustion & Free Radical Substitution of Alkanes (CIE AS Chemistry)

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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

The complete combustion of alkanes

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

Complete combustion involves a plentiful supply of oxygen

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

The incomplete combustion of alkanes

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

Incomplete combustion involves a limited supply of oxygen

  • Carbon monoxide is a toxic gas as it will bind to haemoglobin in blood which can then no longer bind to 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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Philippa

Author: Philippa

Expertise: Chemistry

Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener.