# A chemist reacts magnesium with excess hydrochloric acid. The balanced equation for the reaction is: Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g). Explain what is meant by the term 'limiting reactant' in this reaction, and explain why the volume of hydrogen gas produced at room temperature and pressure (RTP) stops increasing once the reaction is complete.

> WJEC A-Level Chemistry (Wales) — 3.7 Spectroscopy and chromatography · Explain · 5 marks

> Magnesium is added to excess hydrochloric acid. The reaction produces hydrogen gas, which is collected and its volume measured at RTP.

## Mark scheme (5 marks)

1. The limiting reactant is the reactant that is completely used up during the reaction
2. The limiting reactant limits / controls the amount of product formed
3. In this reaction, magnesium is the limiting reactant (because hydrochloric acid is in excess)
4. Once the magnesium is completely used up, no further reaction can occur / no more hydrogen gas is produced
5. One mole of any gas occupies 24 dm³ at RTP, so the total volume of hydrogen is fixed by the number of moles of magnesium / the moles of H₂ produced are determined by the moles of limiting reactant

## Key terms

- [limiting reactant](https://www.gradenine.co.uk/glossary/limiting-reactant)
- [RTP](https://www.gradenine.co.uk/glossary/rtp)
- [excess](https://www.gradenine.co.uk/glossary/excess)

## 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/a-chemist-reacts-magnesium-with-excess-3836b5ff) · Published by Druglandscape Ltd.