Explain why cracking is carried out in industry. In your answer, refer to the feedstock used, the conditions required, and the products formed.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Long‑chain hydrocarbons, usually heavy fractions from crude oil, are used as the feedstock because they are abundant but of limited commercial value. The industry requires shorter‑chain fractions, which are more useful as fuels and as raw materials for chemicals, so cracking is carried out to increase their supply.
Cracking is performed at high temperature – either by heating the hydrocarbons to vapourise them or by using a hot powdered aluminium oxide catalyst in catalytic cracking, or by passing the feed through steam at very high temperature in steam cracking. The high temperature breaks the C–C bonds.
The main products are shorter‑chain alkanes, which are more flammable and therefore more valuable as fuels, and alkenes, which contain a C=C double bond that makes them reactive and useful as monomers for polymers and other chemicals.
Cracking is performed at high temperature – either by heating the hydrocarbons to vapourise them or by using a hot powdered aluminium oxide catalyst in catalytic cracking, or by passing the feed through steam at very high temperature in steam cracking. The high temperature breaks the C–C bonds.
The main products are shorter‑chain alkanes, which are more flammable and therefore more valuable as fuels, and alkenes, which contain a C=C double bond that makes them reactive and useful as monomers for polymers and other chemicals.
Examiner tips
- Use the word "cracking" early to show you understand the process; mention both catalytic and steam cracking to cover all options.
- Show the link between high temperature and bond breaking, and explain why the products are more useful.
- Keep the answer concise – 5 marks, so limit to 3–4 sentences.
- Use UK spelling (e.g. "favourable").
Common mistakes
- Confusing cracking with distillation; forgetting to mention the high temperature or catalyst.
- Listing only alkanes as products and omitting alkenes.
- Using vague terms like "more useful" without explaining the reason (flammability, monomer potential).
Mark scheme (5 marks)
- Long-chain hydrocarbons / large molecules are used as the feedstock (accept: heavy fractions from crude oil)
- There is greater demand for shorter-chain fractions / smaller molecules than supply from fractional distillation (accept: long-chain molecules are less useful / less in demand)
- High temperature is required / hydrocarbons are heated to vaporise them (accept: catalytic cracking uses a hot powdered aluminium oxide catalyst; steam cracking uses steam and very high temperature)
- Products include shorter-chain alkanes, which are more useful as fuels (accept: more flammable than longer-chain alkanes)
- Alkenes are also produced, which are used to make polymers / other chemicals (accept: alkenes have a C=C double bond making them reactive / useful as monomers)
Key terms in this question
Related
- All AQA A-Level Chemistry (7405) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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