# Ethene (C₂H₄) is a hydrocarbon. When ethene burns completely in oxygen, carbon dioxide and water are produced. The unbalanced equation for this reaction is: C₂H₄ + O₂ → CO₂ + H₂O. Explain why the total mass of the products is equal to the total mass of the reactants, and state the correctly balanced symbol equation for the complete combustion of ethene.

> WJEC A-Level Chemistry (Wales) — 3.2 Hydrocarbons · Explain · 5 marks

## Mark scheme (5 marks)

1. No atoms are lost or made during the reaction
2. The mass of the products equals the mass of the reactants (conservation of mass / law of conservation of mass stated or referenced)
3. Correct balancing of CO₂: 2CO₂
4. Correct balancing of H₂O: 2H₂O
5. Correct balancing of O₂: 3O₂, giving the full balanced equation C₂H₄ + 3O₂ → 2CO₂ + 2H₂O

## Key terms

- [hydrocarbon](https://www.gradenine.co.uk/glossary/hydrocarbon)
- [complete combustion](https://www.gradenine.co.uk/glossary/complete-combustion)
- [balanced equation](https://www.gradenine.co.uk/glossary/balanced-equation)

## 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)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/ethene-c-h-is-a-hydrocarbon-when-233dbe17) · Published by Druglandscape Ltd.