# Alkenes are unsaturated hydrocarbons that contain a carbon-to-carbon double bond. Describe what is meant by the terms 'unsaturated' and 'homologous series', and explain why alkenes are more reactive than alkanes.

> Edexcel A-Level Chemistry (9CH0) — 6.3 Alkenes · Describe · 5 marks

> Ethene (C₂H₄) and propene (C₃H₆) are the first two members of the alkene homologous series. Ethane (C₂H₆) is the corresponding alkane with the same number of carbon atoms as ethene.

## Mark scheme (5 marks)

1. Definition of 'unsaturated': contains a carbon-to-carbon double bond (C=C) / not all bonds are single bonds
2. Definition of 'homologous series': a family/group of compounds with the same general formula / same functional group / similar chemical properties
3. Each successive member of a homologous series differs by CH₂ (from the previous member)
4. Alkenes are more reactive than alkanes because the C=C double bond can break / the double bond is available for reactions (addition reactions)
5. Alkanes only have C–C single bonds which are harder to break / alkanes are saturated so cannot undergo addition reactions

## Key terms

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

## Related

- [Revision notes for Edexcel A-Level Chemistry (9CH0)](https://www.gradenine.co.uk/learn)
- [How to answer "Describe" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/alkenes-are-unsaturated-hydrocarbons-that-contain-69a849f2) · Published by Druglandscape Ltd.