# 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.

> OCR A-Level Chemistry B: Salters (H433) — 4.1 Basic concepts and hydrocarbons · Explain · 5 marks

> 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.

## Mark scheme (5 marks)

1. A homologous series is a family/group of compounds with the same general formula
2. Members of a homologous series differ by CH₂ each time / successive members differ by one CH₂ unit
3. Alkenes contain a C=C double bond whereas alkanes only have C–C single bonds (alkanes are saturated; alkenes are unsaturated)
4. Alkenes decolourise bromine water whereas alkanes do not (alkenes react with bromine water / bromine water stays orange/brown with alkanes)
5. Alkenes undergo addition reactions (e.g. with hydrogen/bromine/steam) whereas alkanes undergo substitution reactions / alkenes are more reactive than alkanes

## Key terms

- [homologous series](https://www.gradenine.co.uk/glossary/homologous-series)
- [alkane](https://www.gradenine.co.uk/glossary/alkane)
- [alkene](https://www.gradenine.co.uk/glossary/alkene)

## Related

- [Revision notes for OCR A-Level Chemistry B: Salters (H433)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/a-student-is-studying-a-series-8ade7a18) · Published by Druglandscape Ltd.