Syllabus Edition

First teaching 2023

First exams 2025

|

Producing Alkanes (CIE AS Chemistry)

Revision Note

Test yourself
Philippa

Author

Philippa

Last updated

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
    • E.g. butane from 1-butene

The hydrogenation reaction

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

Hydrogen gas is added to 1-butene which is 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, 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
    • E.g. octane and ethene from decane

An example of cracking

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

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

Examiner Tip

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

You've read 0 of your 10 free revision notes

Unlock more, it's free!

Join the 100,000+ Students that ❤️ Save My Exams

the (exam) results speak for themselves:

Did this page help you?

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.