A student is studying a series of hydrocarbons found in crude oil. They learn that alkanes form a homologous series and that cracking can be used to produce alkenes from longer-chain alkanes. Explain what is meant by the term 'homologous series' and describe two chemical properties that distinguish alkenes from alkanes.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Crude oil contains a mixture of hydrocarbons. Shorter-chain hydrocarbons are more useful as fuels, so longer-chain alkanes are broken down by cracking to produce smaller, more useful molecules. Some of the products of cracking are alkenes.
Model answer (5 marks)
A homologous series is a group of compounds that share the same general formula and differ by a constant unit.
1. Successive members differ by one CH₂ group.
2. Alkenes contain a C=C double bond, whereas alkanes have only C–C single bonds (alkanes are saturated, alkenes unsaturated).
3. Alkenes decolourise bromine water (colourless), while alkanes do not (bromine remains orange).
4. Alkenes undergo addition reactions (e.g. H₂, Br₂, steam), whereas alkanes undergo substitution reactions.
1. Successive members differ by one CH₂ group.
2. Alkenes contain a C=C double bond, whereas alkanes have only C–C single bonds (alkanes are saturated, alkenes unsaturated).
3. Alkenes decolourise bromine water (colourless), while alkanes do not (bromine remains orange).
4. Alkenes undergo addition reactions (e.g. H₂, Br₂, steam), whereas alkanes undergo substitution reactions.
Examiner tips
- Use the term ‘homologous series’ and mention the CH₂ difference; this is worth 2 marks. State the double bond and the reaction with bromine water – each gives 1 mark. Show the addition reaction example to secure the final mark.
Common mistakes
- Writing ‘homologous series’ without explaining the CH₂ difference. Forgetting that alkanes do not react with bromine water. Mixing up addition for alkanes and substitution for alkenes.
Mark scheme (5 marks)
- A homologous series is a family/group of compounds with the same general formula
- Members of a homologous series differ by CH₂ each time / successive members differ by one CH₂ unit
- Alkenes contain a C=C double bond whereas alkanes only have C–C single bonds (alkanes are saturated; alkenes are unsaturated)
- Alkenes decolourise bromine water whereas alkanes do not (alkenes react with bromine water / bromine water stays orange/brown with alkanes)
- Alkenes undergo addition reactions (e.g. with hydrogen/bromine/steam) whereas alkanes undergo substitution reactions / alkenes are more reactive than alkanes
Key terms in this question
homologous series · alkane · alkene
Related
- All OCR A-Level Chemistry B: Salters (H433) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →