AQA A Level Chemistry

Revision Notes

3.2.2 Modification of Alkanes by Cracking


  • Alkanes are hydrocarbons that can be produced by the addition reaction of hydrogen to an alkene or by cracking of longer alkane chains

Crude oil

  • Crude oil is a mixture of hydrocarbons containing alkanes, cycloalkanes and arenes (compounds with a benzene ring)
  • The crude oil is extracted from the earth in a drilling process and transported to an oil refinery
  • At the oil refinery the crude oil is separated into useful fuels by fractional distillation
  • This is a separating technique in which the wide range of different hydrocarbons are separated into fractions based on their boiling points
  • However, the smaller hydrocarbon fractions (such as gasoline fractions) are in high demand compared to the larger ones
  • Therefore, some of the excess heavier fractions are broken down into smaller, more useful compounds
  • These more useful compounds include alkanes and alkenes of lower relative formula mass (Mr)
  • This process is called cracking

Hydrocarbons Cracking, downloadable AS & A Level Chemistry revision notes

The heavier fractions that are obtained in fractional distillation are further cracked into useful alkane and alkenes with lower Mr values

Production of alkanes from cracking

  • In cracking these large, less useful hydrocarbon molecules found in crude oil are broken down into smaller, more useful molecules
  • The large hydrocarbon molecules are fed into a steel chamber and heated to a high temperature and then passed over an aluminium oxide (Al2O3) catalyst
    • The chamber does not contain any oxygen to prevent combustion of the hydrocarbon to water and carbon dioxide
  • When a large hydrocarbon is cracked, a smaller alkane and one (or more) alkene molecules are formed
    • E.g. octane and ethene from decane

Hydrocarbons Cracking of Long Alkanes, downloadable AS & A Level Chemistry revision notes

Long hydrocarbons are cracked by heating them and using aluminium oxide catalyst into a smaller alkane and an alkene

Exam Tip

Remember that cracking is an endothermic reaction.


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