Alkanes are used as fuels. Explain why cracking is carried out on long-chain alkanes and describe what happens during catalytic cracking.

AQA A-Level Chemistry (7405) — 3.3.2 Alkanes · Explain and Describe · 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 alkanes are cracked because there is a greater demand for shorter‑chain hydrocarbons as fuels; shorter chains are more flammable and burn more readily. The long chains are first heated to vapourise them, then the vapour is passed over a hot powdered aluminium‑oxide catalyst. On the catalyst surface the long‑chain molecules split apart, forming smaller molecules, including alkenes.

Examiner tips

  • Use the word ‘cracking’ to link demand for fuels to the process
  • Mention the catalyst (aluminium oxide) and that it is hot and powdered
  • Explain that the long chains split into smaller molecules, especially alkenes

Common mistakes

  • Confusing cracking with distillation
  • Omitting the catalyst or its temperature
  • Failing to state that the products include alkenes

Mark scheme (5 marks)

  1. There is greater demand for shorter-chain hydrocarbons (as fuels) than for longer-chain hydrocarbons
  2. Shorter-chain alkanes are more flammable / burn more easily, making them better fuels
  3. Long-chain hydrocarbons are heated to vaporise them
  4. The vapour is passed over a hot powdered aluminium oxide catalyst
  5. The long-chain molecules split apart (on the surface of the catalyst) to form smaller molecules, including alkenes

Key terms in this question

cracking · catalytic cracking · alkanes

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