CIE AS Chemistry (9701) exams from 2022

Revision Notes

3.2.1 Alkanes

Production of Alkanes: Hydrogenation & Cracking

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

Production of alkanes from addition reactions

  • Alkenes are unsaturated organic molecules and contain C-C double bonds
  • When hydrogen gas and an alkene are heated and passed over a finely divided Pt/Ni catalyst, the addition reaction produces an alkane:
    • The Pt/Ni catalyst is finely divided to increase its surface area and therefore increase the rate of reaction
    • Eg. butane from 1-butene

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

Hydrogen gas is added to 1-butene which are then heated and passed over a Pt/Ni catalyst to produce butane

 

  • The addition reaction of alkenes with hydrogen is called hydrogenation
  • Hydrogenation is often used in the manufacture of margarine from vegetable oil
    • Vegetable oil is an unsaturated organic molecule with many C-C double bonds
    • When these are partially hydrogenated, their hydrocarbon chains become straighter
    • This raises the melting point of the oils which is why margarine is a soft solid and vegetable oil a liquid at room temperature

Production of alkanes from cracking

  • In cracking 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 doesn’t take place
  • 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 hydrocarbons are cracked by heating them and using aluminium oxide catalyst into smaller hydrocarbons and an alkene

Exam Tip

Remember that hydrogenation is an exothermic reaction and cracking is an endothermic reaction.

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