OCR Gateway GCSE Chemistry

Revision Notes

6.2.9 Cracking

Cracking

  • Saturated molecules contain single bonds only whereas unsaturated molecules contain double bonds between their carbon atoms
  • Alkanes are saturated compounds and alkenes are unsaturated compounds
  • Long chain alkane molecules are further processed to produce other products consisting of smaller chain molecules
  • A process called cracking is used to convert them into short chain molecules which are more useful
  • Small alkenes and hydrogen are produced using this process
  • Kerosene and diesel oil are often cracked to produce petrol, other alkenes and hydrogen

Cracking-Decane, IGCSE & GCSE Chemistry revision notes

Decane is cracked to produce octane for petrol and ethene for ethanol synthesis

  • Cracking involves heating the hydrocarbon molecules to around 600 – 700°C to vaporise them
  • The vapours then pass over a hot powdered catalyst of alumina or silica
  • This process breaks covalent bonds in the molecules as they come into contact with the surface of the catalyst, causing thermal decomposition reactions
  • The molecules are broken up in a random way which produces a mixture of smaller alkanes and alkenes
  • Hydrogen and a higher proportion of alkenes are formed at higher temperatures and higher pressure

Writing Equations for Cracking

  • We can use the general formulae for alkanes and alkenes to check that we have correctly balanced equations for cracking
  • Hexane for example, can be cracked to form butane and ethene, both of which are very useful molecules
  • Ethene as the starting material for the production of alcohol and butane is used as a fuel
  • The equation for this cracking reaction is:

C6H14  C4H10 + C2H4

  • Note that the starting compound for this reaction is an alkane and thus the general formula CnH2n+2 applies
  • Butane is also an alkane and so the same rule applies
  • Ethene is an alkene and so its formula will follow the CnH2n rule

Exam Tip

Always check that sum of the carbons and hydrogens adds up on each side of the equation AND that you have made alkanes or alkenes.

You've read 0 of your 0 free revision notes

Get unlimited access

to absolutely everything:

  • Downloadable PDFs
  • Unlimited Revision Notes
  • Topic Questions
  • Past Papers
  • Model Answers
  • Videos (Maths and Science)

Join the 80,663 Students that ❤️ Save My Exams

the (exam) results speak for themselves:

Did this page help you?

Stewart

Author: Stewart

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.