Describe the process of catalytic cracking and explain why cracking is a necessary industrial process.

AQA A-Level Chemistry (7405) — 3.3.15 NMR spectroscopy (A-Level only) · Describe and Explain · 5 marks · View as Markdown

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.
On the catalyst surface the long‑chain molecules split apart into smaller molecules.
This produces shorter‑chain fractions such as petrol and fuels, which are in higher demand.
Cracking also yields alkenes that can be used as monomers for polymers or other useful chemicals.

Examiner tips

  • Use the sequence of steps in order; mention the catalyst explicitly.
  • Explain the demand for shorter chains and the value of alkenes.
  • Show the process in clear, concise sentences.
  • Use UK spelling (e.g. ‘vapourise’).

Common mistakes

  • Omitting the catalyst or its temperature.
  • Confusing cracking with distillation.
  • Not mentioning the production of alkenes or the demand for shorter chains.

Mark scheme (5 marks)

  1. Long-chain hydrocarbons are heated to vaporise / turn them into a gas
  2. The vapour is passed over a hot powdered aluminium oxide catalyst
  3. The long-chain molecules split apart / break down into smaller molecules on the surface of the catalyst
  4. There is greater demand for shorter-chain fractions (e.g. petrol / fuels) than for longer-chain fractions, so cracking converts less useful long-chain hydrocarbons into more useful shorter-chain ones
  5. Cracking also produces alkenes, which can be used as monomers to make polymers / to produce other useful chemicals

Key terms in this question

catalytic cracking · cracking

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