Explain why cracking is carried out in the petrochemical industry and describe the two methods used to crack hydrocarbons.

AQA A-Level Chemistry (7405) — 3.3.10 Aromatic chemistry (A-Level only) · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

Cracking is carried out to meet the higher demand for shorter‑chain hydrocarbons, such as gasoline and ethylene, which are more valuable than the longer‑chain fractions that are produced by distillation.

Cracking breaks down large, long‑chain hydrocarbons into smaller, shorter‑chain molecules.

Two common methods are:
1. Catalytic cracking – the hydrocarbon vapour is passed over a hot powdered aluminium oxide catalyst.
2. Steam cracking – the hydrocarbon vapour is mixed with steam and heated to a very high temperature.

The products of cracking include alkenes (and some alkanes), which can be used to make polymers or as fuels.

Examiner tips

  • Use the word ‘cracking’ first, then explain the demand for shorter chains.
  • Mention both catalytic and steam cracking and give the key detail for each.
  • Include that alkenes are produced and their uses.

Mark scheme (5 marks)

  1. There is greater demand for shorter-chain hydrocarbons / fractions than for longer-chain ones
  2. Cracking breaks down larger / longer-chain hydrocarbons into smaller / shorter-chain molecules
  3. Catalytic cracking: the hydrocarbon vapour is passed over a hot powdered aluminium oxide catalyst
  4. Steam cracking: the hydrocarbon vapour is mixed with steam and heated to a very high temperature
  5. Products of cracking include alkenes (as well as alkanes), which can be used to make polymers / as fuels

Key terms in this question

cracking · hydrocarbons

Related

More Aromatic chemistry (A-Level only) questions

▶ Try answering this question with AI marking (free) →