Crude oil contains many long-chain hydrocarbons that are in low demand. Explain why the oil industry uses cracking to improve the usefulness of these long-chain hydrocarbons, and describe how catalytic cracking is carried out.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Crude oil is a finite resource. When crude oil is fractionally distilled, large quantities of long-chain hydrocarbons are produced. These fractions are less commercially valuable than shorter-chain products such as petrol and plastics feedstocks.
Model answer (4 marks)
Long‑chain hydrocarbons are in low demand, whereas shorter chains such as petrol are highly sought after.
Cracking breaks the long chains into smaller, more useful molecules.
The long‑chain hydrocarbons are heated until they vapourise.
The vapour is passed over a hot powdered aluminium‑oxide catalyst, where the molecules split on the catalyst surface.
Cracking breaks the long chains into smaller, more useful molecules.
The long‑chain hydrocarbons are heated until they vapourise.
The vapour is passed over a hot powdered aluminium‑oxide catalyst, where the molecules split on the catalyst surface.
Examiner tips
- Use the exact terms ‘cracking’, ‘long‑chain hydrocarbons’, ‘shorter‑chain’, ‘vapourise’, ‘catalyst’, ‘aluminium‑oxide’.
- Show the sequence: demand → cracking → heating → vapourisation → catalytic surface split.
- Mention that the catalyst is hot and powdered to maximise surface area.
Common mistakes
- Confusing cracking with distillation or saying it ‘mixes’ hydrocarbons instead of ‘breaks them down’.
- Omitting the catalyst material or the fact that the hydrocarbons are vapourised before contact with the catalyst.
Mark scheme (4 marks)
- Long-chain hydrocarbons are in low demand / there is greater demand for shorter-chain hydrocarbons (e.g. petrol / fuels)
- Cracking breaks down / splits long-chain hydrocarbons into smaller, more useful molecules
- The long-chain hydrocarbons are heated to vaporise / turn them into a gas
- The vapour is passed over a hot powdered aluminium oxide catalyst, causing the long-chain molecules to split apart on the surface of the catalyst
Key terms in this question
cracking · catalytic cracking · long-chain hydrocarbons
Related
- All OCR GCSE Chemistry A: Gateway Science (J248) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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