Explain why cracking is an important industrial process.

AQA GCSE Chemistry (8462) — 4.7.1 Carbon compounds as fuels and feedstock · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Crude oil is separated into fractions by fractional distillation. Some fractions are in greater demand than others. Cracking is used to convert less useful fractions into more useful products.

Model answer (4 marks)

Cracking breaks long‑chain hydrocarbons into shorter chains.

Shorter chains are more flammable, so they are better fuels.

Cracking also produces alkenes, which are unsaturated hydrocarbons.

Alkenes can be polymerised to make useful chemicals such as plastics.

Examiner tips

  • Use the word ‘break’ or ‘convert’ to show the process. Mention both fuel and chemical uses. Keep each point short and to the point.
  • Use the exact terms: ‘long‑chain’, ‘short‑chain’, ‘alkenes’, ‘polymers’.

Common mistakes

  • Confusing cracking with distillation. Failing to mention alkenes or their use in polymers. Using vague terms like ‘smaller molecules’ without specifying fuel or chemical value.

Mark scheme (4 marks)

  1. Long-chain hydrocarbons are broken down into smaller, more useful molecules
  2. Shorter-chain hydrocarbons are more flammable, making them better fuels
  3. Cracking produces alkenes, which are unsaturated hydrocarbons
  4. Alkenes can be used to make other useful chemicals, such as polymers

Key terms in this question

cracking

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