Ethene can be produced by cracking long-chain hydrocarbons from crude oil. Describe the two methods by which cracking can be carried out, and explain why cracking is a useful industrial process.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
1. Catalytic cracking – long‑chain hydrocarbons are heated to vapourise them and the vapour is passed over a hot powdered aluminium oxide catalyst.
2. Steam cracking – long‑chain hydrocarbons are vapourised, mixed with steam and heated to a very high temperature.
3. Both processes break down large molecules into smaller, shorter‑chain hydrocarbons.
4. Shorter‑chain hydrocarbons are more flammable and therefore more useful as fuels.
5. Steam cracking also produces alkenes such as ethene, which are used to make polymers and other chemicals.
2. Steam cracking – long‑chain hydrocarbons are vapourised, mixed with steam and heated to a very high temperature.
3. Both processes break down large molecules into smaller, shorter‑chain hydrocarbons.
4. Shorter‑chain hydrocarbons are more flammable and therefore more useful as fuels.
5. Steam cracking also produces alkenes such as ethene, which are used to make polymers and other chemicals.
Examiner tips
- Use the exact phrases ‘catalytic cracking’ and ‘steam cracking’. Show the sequence of heating, vapourising and catalyst/steam. Mention both fuel and polymer uses of the products. Keep each point short and to the point.
Common mistakes
- Confusing the role of the catalyst with the role of steam. Omitting the fact that steam cracking produces alkenes. Using vague terms like ‘break down’ without specifying ‘shorter‑chain hydrocarbons’.
Mark scheme (5 marks)
- Catalytic cracking: long-chain hydrocarbons are heated to vaporise them and the vapour is passed over a hot powdered aluminium oxide catalyst
- Steam cracking: long-chain hydrocarbons are vaporised, mixed with steam and heated to a very high temperature
- Cracking breaks down larger/longer-chain hydrocarbons into smaller/shorter-chain molecules
- Shorter-chain hydrocarbons / products are more flammable, making them more useful as fuels
- Cracking also produces alkenes (e.g. ethene), which can be used to make polymers / other chemicals
Key terms in this question
Related
- All AQA A-Level Chemistry (7405) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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