# Zinc oxide reacts with carbon according to the balanced equation: 2ZnO + C → 2Zn + CO₂. A student reacts a small amount of carbon with excess zinc oxide. Explain what is meant by the terms 'limiting reactant' and 'conservation of mass', and describe how each applies to this reaction.

> WJEC A-Level Chemistry (Wales) — 3.1 Introduction to organic chemistry · Explain · 5 marks

## Mark scheme (5 marks)

1. The limiting reactant is the reactant that is completely used up during the reaction
2. Carbon is the limiting reactant in this reaction (because it is present in a small / non-excess amount)
3. Conservation of mass states that no atoms are lost or made during a chemical reaction
4. The mass of the products (zinc and carbon dioxide) equals the mass of the reactants (zinc oxide and carbon) that actually reacted
5. The equation is balanced (same number of each type of atom on both sides) which reflects conservation of mass

## Key terms

- [limiting reactant](https://www.gradenine.co.uk/glossary/limiting-reactant)
- [conservation of mass](https://www.gradenine.co.uk/glossary/conservation-of-mass)
- [excess](https://www.gradenine.co.uk/glossary/excess)
- [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)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/zinc-oxide-reacts-with-carbon-according-cfc26a90) · Published by Druglandscape Ltd.