Alkanes are used as fuels. Describe what happens when a long-chain alkane undergoes cracking, and explain why cracking is carried out in the petroleum industry.

Cambridge International IGCSE Chemistry (0620) — 11.4 Alkanes · Describe and Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

A long‑chain alkane is broken down by cracking into smaller molecules.
The reaction produces a shorter‑chain alkane and an alkene (an unsaturated product).
Cracking is carried out in the petroleum industry because there is a greater demand for shorter‑chain, smaller fractions such as petrol or gasoline than for long‑chain fractions.
The alkenes produced are useful as monomers for making plastics and polymers, adding industrial value.

Examiner tips

  • Use the word "cracking" and mention both products (shorter alkane + alkene).
  • Explain the demand for shorter chains and the industrial use of alkenes.

Common mistakes

  • Confusing cracking with distillation or omitting the alkene product.
  • Failing to mention the demand for petrol or the use of alkenes in plastics.

Mark scheme (4 marks)

  1. A long-chain alkane is broken down / split into smaller molecules
  2. Produces a shorter-chain alkane AND an alkene (unsaturated product)
  3. Cracking is carried out because there is greater demand for shorter-chain / smaller fractions (e.g. petrol / gasoline) than for longer-chain fractions
  4. Alkenes produced by cracking are used as monomers / to make plastics / polymers, making cracking industrially valuable

Key terms in this question

cracking · alkane

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