AQA GCSE Chemistry

Revision Notes

7.1.4 Cracking & Alkenes

Cracking Hydrocarbons

  • 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
  • There are two methods used to crack alkanes: catalytic cracking and steam cracking
  • As the names suggest, one method uses a catalyst and the other uses steam

Cracking-Decane, IGCSE & GCSE Chemistry revision notes

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

  • Catalytic cracking involves heating the hydrocarbon molecules to around 470 – 550°C to vaporise them
  • The vapours then pass over a hot powdered catalyst of aluminium oxide
  • 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
  • In steam or thermal cracking the process is carried out at slightly higher temperatures and produces more ring structures and unsaturated compounds
  • The vaporised hydrocarbons are mixed with steam and heated to a high temperature which induces cracking

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 C2H2n 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.

Alkenes

  • Alkenes are a homologous series of hydrocarbon compounds with at least one double bond between two of the carbon atoms on the chain
  • The double bond can be written as carbon carbon double bond or as C=C
  • The general formula for alkenes is:

CnH2n

  • Alkenes are generally more desirable than alkanes as they are more reactive due to the presence of the carbon-carbon double bond, so they can take part in reactions in which alkanes cannot, making them more useful than alkanes
  • They are used to make polymers and are the starting materials for the production of many other chemicals
  • Two useful reactions are the bromination of alkenes and polymerisation

Bromination of Ethene

  • Alkenes undergo addition reactions in which atoms of a simple molecule add across the C=C double bond
  • The reaction between bromine and ethene is an example of an addition reaction
  • The same process works for any halogen and any alkene in which the halogen atoms always add to the carbon atoms across the C=C double bond

Bromine-Addition-to-Ethene, IGCSE & GCSE Chemistry revision notes

Bromine atoms add across the C=C in the addition reaction of ethene and bromine

Bromine Water Test

  • Alkanes and alkenes have different molecular structures
  • All alkanes are saturated and alkenes are unsaturated
  • The presence of the C=C double bond allows alkenes to react in ways that alkanes cannot
  • This allows us to tell alkenes apart from alkanes using a simple chemical test called the bromine water test

Bromine-Test Alkenes, IGCSE & GCSE Chemistry revision notes

Diagram showing the result of the test using bromine water with alkanes and alkenes

  • Bromine water is an orange coloured solution
  • When bromine water is added to an alkane, it will remain as an orange solution as alkanes do not have double carbon bonds (C=C) so the bromine remains in solution
  • But when bromine water is added to an alkene, the bromine atoms add across the C=C bond, hence the solution no longer contains free bromine so it loses its colour

Exam Tip

Alkenes are more reactive than alkanes due to the presence of the carbon carbon double bond which contains an area of high electron density.

Author: Francesca

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.
Close

Join Save My Exams

Download all our Revision Notes as PDFs

Try a Free Sample of our revision notes as a printable PDF.

Join Now
Already a member?
Go to Top