Butene is an alkene with the molecular formula C₄H₈. Explain why butene belongs to the alkenes homologous series and describe how a student could use a chemical test to distinguish butene from butane.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Butene and butane are both hydrocarbons that can be produced from crude oil. They have similar molecular formulae but different chemical properties.
Model answer (5 marks)
Butene contains a carbon–carbon double bond, so it is unsaturated and therefore an alkene.
The general formula for alkenes is CₙH₂ₙ. For butene, n=4 gives C₄H₈, which matches the formula.
All alkenes share the same C=C functional group, giving them similar chemical behaviour.
To distinguish butene from butane a student can add bromine water to each sample. Shake the mixture. The orange bromine water will decolourise to colourless when it reacts with the double bond in butene, but it will remain orange when mixed with the saturated hydrocarbon butane.
The general formula for alkenes is CₙH₂ₙ. For butene, n=4 gives C₄H₈, which matches the formula.
All alkenes share the same C=C functional group, giving them similar chemical behaviour.
To distinguish butene from butane a student can add bromine water to each sample. Shake the mixture. The orange bromine water will decolourise to colourless when it reacts with the double bond in butene, but it will remain orange when mixed with the saturated hydrocarbon butane.
Examiner tips
- Mention the C=C bond first to justify the alkene classification. Include the general formula CₙH₂ₙ and show the calculation for n=4. Describe the bromine water test and the expected colour change for each compound. Use correct UK spelling and chemical symbols.
Common mistakes
- Saying butene is an alkane instead of an alkene. Forgetting to mention the double bond or the general formula. Describing the test incorrectly (e.g., saying bromine water turns colourless with butane).
Mark scheme (5 marks)
- Butene contains a carbon-carbon double bond / is unsaturated
- Butene fits the general formula CₙH₂ₙ (e.g. C₄H₈ fits with n=4)
- Alkenes have the same functional group / similar chemical properties to other members of the series
- Add bromine water to each sample / shake each hydrocarbon with bromine water
- Bromine water turns from orange to colourless with butene but remains orange with butane
Key terms in this question
Related
- All AQA A-Level Chemistry (7405) revision notes →
- How to answer a "Explain and Describe" question →
- Decode the mark scheme abbreviations →
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