CIE AS Chemistry (9701) exams from 2022

Revision Notes

3.2.4 Cracking of Alkanes

Obtaining Useful Compounds by Cracking

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

Fractional-Distillation, IGCSE & GCSE Chemistry revision notes

Crude oil is initially separated into fractions with similar boiling points in a process called fractional distillation

  • 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


  • 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 alkene molecules are formed
    • Eg. octane and ethene from decane


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

Long hydrocarbon fraction is cracked into two smaller ones


  • The low-molecular mass alkanes formed make good fuels and are in high demand
  • The low-molecular mass alkenes are more reactive than alkanes due to their double bond
    • This makes them useful for the chemical industry as the starting compounds (feedstock) for making new products
    • Eg. they are used as monomers in polymerisation reactions


Hydrocarbons Reactions of Alkenes, downloadable AS & A Level Chemistry revision notes

Alkenes are reactive molecules and can undergo many different types of reactions making them useful as starting compounds


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