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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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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