Cracking (Edexcel GCSE Chemistry)

Revision Note

Stewart Hird

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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

  • 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

Why Cracking is Necessary

  • Crude oils vary considerably in their composition and some need more refining than others

  • Supply is how much of a particular fraction can be produced from refining the crude oil

  • Demand is how much customers want to buy

  • General the demand for certain fractions outstrips the supply so this is why cracking is necessary to convert surplus unwanted fractions into more useful ones

  • This is mostly larger, heavier fractions that are cracked into smaller lighter fractions

Supply & Demand of Crude oil Fractions, downloadable IGCSE & GCSE Chemistry revision notes

Supply & demand graph for crude oil fractions

  • You can see from the chart that fuel oil and bitumen are surplus fractions so they are cracked and modified to produce petrol, kerosene and diesel

Examiner Tips and Tricks

Remember that cracking is an endothermic reaction.

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Stewart Hird

Author: Stewart Hird

Expertise: Chemistry Lead

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.