Explain why cracking is an important industrial process for the oil industry.

OCR GCSE Chemistry A: Gateway Science (J248) — C6.1 Improving processes and products · Explain · 4 marks · View as Markdown

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

Crude oil is separated by fractional distillation into fractions. However, some fractions are in much greater demand than others, and the oil industry must find ways to meet this demand.

Model answer (4 marks)

Long‑chain hydrocarbons are produced in large quantities by fractional distillation but are of low commercial value. Cracking cleaves these long chains into smaller molecules, producing useful fuels such as petrol and diesel and also alkenes. The smaller alkanes are in high demand as fuels, while the alkenes can be polymerised to make plastics or used as feedstocks for other chemicals.

Examiner tips

Use the word "cracking" and explain that it breaks long chains into short chains. Mention both fuels and alkenes as products. Show the link between demand for fuels and the need to crack heavy fractions.

Common mistakes

Confusing cracking with distillation. Forgetting to mention the production of alkenes. Using vague terms like "smaller molecules" without specifying fuels or alkenes.

Mark scheme (4 marks)

Long-chain hydrocarbons / large molecules are in low demand (or there is a surplus of large/heavy fractions)
Cracking breaks (long-chain) hydrocarbons into smaller, more useful molecules
Smaller hydrocarbons such as petrol / diesel / fuels are in higher demand (accept named short-chain alkane fraction)
Cracking also produces alkenes which can be used as monomers (for plastics / polymers) or as feedstocks for other chemicals

Key terms in this question

cracking