Cracking is used to convert large hydrocarbon molecules into smaller, more useful ones. One product of cracking is propene. Explain why cracking is carried out in industry and describe the two methods by which cracking can be done.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Cracking is carried out in industry because there is a greater demand for shorter‑chain hydrocarbons, which are more useful as fuels and as feed‑stocks for other chemicals, than for the long‑chain hydrocarbons that are present in crude oil.
Two methods are used:
1. Catalytic cracking – the long‑chain hydrocarbons are heated to vapourise them and the vapour is passed over a hot powdered aluminium oxide catalyst, which breaks the C–C bonds.
2. Steam cracking – the hydrocarbon vapour is mixed with steam and heated to a very high temperature, causing the C–C bonds to break and producing alkenes such as propene.
Two methods are used:
1. Catalytic cracking – the long‑chain hydrocarbons are heated to vapourise them and the vapour is passed over a hot powdered aluminium oxide catalyst, which breaks the C–C bonds.
2. Steam cracking – the hydrocarbon vapour is mixed with steam and heated to a very high temperature, causing the C–C bonds to break and producing alkenes such as propene.
Examiner tips
- Use the word ‘greater demand’ to justify the need for cracking
- Mention both catalytic and steam cracking and the key step of heating the vapour
- Show that catalytic cracking uses a catalyst, steam cracking does not
Common mistakes
- Confusing catalytic cracking with steam cracking (e.g. saying steam is used in catalytic cracking)
- Omitting the fact that catalytic cracking uses a powdered aluminium oxide catalyst
- Failing to state that the vapour is heated to a very high temperature in steam cracking
Mark scheme (5 marks)
- There is greater demand for shorter-chain hydrocarbons (as fuels / for making other chemicals) than for longer-chain hydrocarbons
- Shorter-chain hydrocarbons are more flammable / better fuels OR alkenes produced can be used to make polymers / other chemicals
- In catalytic cracking, the long-chain hydrocarbons are heated to vaporise them
- In catalytic cracking, the vapour is passed over a hot powdered aluminium oxide catalyst
- In steam cracking, the hydrocarbon vapour is mixed with steam and heated to a very high temperature
Key terms in this question
Related
- All AQA A-Level Chemistry (7405) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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