Describe the process of catalytic cracking and explain why cracking is carried out in industry. In your answer, refer to the products formed.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Long‑chain hydrocarbons are heated until they vapourise.
The vapour is passed over a hot powdered aluminium oxide catalyst.
The catalyst causes the long‑chain molecules to break apart into smaller molecules.
The products are mainly smaller alkanes, which are more useful as fuels and are more flammable.
Alkenes are also produced; these are used to make polymers and other chemicals.
The vapour is passed over a hot powdered aluminium oxide catalyst.
The catalyst causes the long‑chain molecules to break apart into smaller molecules.
The products are mainly smaller alkanes, which are more useful as fuels and are more flammable.
Alkenes are also produced; these are used to make polymers and other chemicals.
Examiner tips
- Use the word "vapourise" and "powdered aluminium oxide" to show knowledge of the process.
- Mention both alkanes and alkenes as products and explain their industrial use.
- Show why cracking is carried out – to meet demand for shorter‑chain fractions and produce useful fuels.
- Keep the answer concise and use correct terminology.
Mark scheme (5 marks)
- Long-chain hydrocarbons are heated to vaporise them / turned into a gas
- The vapour is passed over a hot powdered aluminium oxide catalyst
- The long-chain molecules break apart / split into smaller molecules
- Products include smaller alkanes (which are more useful as fuels / more flammable)
- Alkenes are also produced, which are used to make polymers / other chemicals (OR: demand for shorter-chain fractions is greater than for longer-chain fractions so cracking converts surplus long-chain hydrocarbons into more useful products)
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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