# Ethene is an alkene produced during the cracking of crude oil fractions. Explain why cracking is carried out in the petroleum industry and describe what happens to the bonds in ethene during an addition reaction with hydrogen.

> WJEC A-Level Chemistry (Wales) — 2.5 Hydrocarbons · Explain · 5 marks

> Cracking is an industrial process used to convert large hydrocarbon molecules into smaller, more useful ones. Ethene is one of the products of cracking and is an important raw material in the chemical industry.

## Mark scheme (5 marks)

1. Cracking is carried out because there is greater demand for shorter-chain / smaller hydrocarbons (such as fuels) than for longer-chain ones
2. Cracking also produces alkenes / unsaturated molecules which are useful as monomers / starting materials for plastics
3. Ethene contains a carbon–carbon double bond (C=C)
4. During the addition reaction, the double bond breaks / opens up
5. The hydrogen atoms bond to / add across the two carbon atoms, forming ethane (a single C–C bond remains)

## Key terms

- [cracking](https://www.gradenine.co.uk/glossary/cracking)
- [addition reaction](https://www.gradenine.co.uk/glossary/addition-reaction)
- [alkene](https://www.gradenine.co.uk/glossary/alkene)

## Related

- [Revision notes for WJEC A-Level Chemistry (Wales)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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