Explain why alkenes are produced during cracking and describe how you could distinguish an alkene from an alkane using a simple chemical test.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Cracking is a thermal decomposition of long‑chain hydrocarbons, producing smaller molecules. The process generates unsaturated hydrocarbons, notably alkenes, which contain at least one carbon–carbon double bond.
To distinguish an alkene from an alkane, add a few drops of bromine water to each sample. Shake the alkene – the orange bromine water turns colourless as the double bond reacts. The alkane shows no change; the solution remains orange.
To distinguish an alkene from an alkane, add a few drops of bromine water to each sample. Shake the alkene – the orange bromine water turns colourless as the double bond reacts. The alkane shows no change; the solution remains orange.
Examiner tips
- Mention that cracking is a thermal decomposition of long chains. Explain that alkenes contain a C=C double bond. Describe the bromine water test: alkene → colourless, alkane → remains orange. Use clear, concise language and the exact terminology.
- common_mistakes
- :
- Failing to state that cracking produces unsaturated hydrocarbons. Confusing the colour change direction (alkane should stay orange). Using vague terms like "reacts" without specifying bromine water test.
Mark scheme (4 marks)
- Cracking involves breaking down long-chain hydrocarbons into smaller molecules / thermal decomposition occurs
- The smaller molecules produced include unsaturated hydrocarbons / alkenes have at least one carbon-carbon double bond
- Add bromine water to both substances / shake each substance with bromine water
- Bromine water turns from orange to colourless with the alkene but remains orange with the alkane
Key terms in this question
Related
- All AQA GCSE Chemistry (8462) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →