Explain why cracking is used in the petrochemical industry to produce alkenes, and describe how bromine water can be used to distinguish an alkene product from an alkane product.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
1. Cracking breaks large, long‑chain hydrocarbons into smaller, more useful molecules.
2. The process produces alkenes, which are key feedstocks for polymers and other chemicals.
3. Shorter‑chain products are more flammable and make better fuels than long‑chain hydrocarbons.
4. When bromine water is added to an alkene, the orange colour disappears, turning colourless.
5. Bromine water remains orange when added to an alkane, showing no colour change.
2. The process produces alkenes, which are key feedstocks for polymers and other chemicals.
3. Shorter‑chain products are more flammable and make better fuels than long‑chain hydrocarbons.
4. When bromine water is added to an alkene, the orange colour disappears, turning colourless.
5. Bromine water remains orange when added to an alkane, showing no colour change.
Examiner tips
- Use the exact terminology: ‘cracking’, ‘alkene’, ‘bromine water’.
- Show the sequence: cracking → alkenes → useful products.
- Explain the colour change mechanism for alkenes.
- Mention that alkanes do not react with bromine water.
Common mistakes
- Confusing alkanes with alkenes in the cracking step.
- Claiming bromine water reacts with alkanes.
- Using vague terms like ‘smaller molecules’ without specifying alkenes.
Mark scheme (5 marks)
- Large/long-chain hydrocarbons are broken down into smaller/more useful molecules
- Alkenes are produced (from cracking) and are used to make polymers / other chemicals
- Shorter-chain products are more flammable / better fuels than longer-chain hydrocarbons
- Bromine water turns from orange to colourless with an alkene
- Bromine water remains orange / does not change colour with an alkane
Key terms in this question
cracking · alkenes · bromine water
Related
- All AQA A-Level Chemistry (7405) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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