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.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Magnesium is added to excess hydrochloric acid. The reaction produces hydrogen gas, which is collected and its volume measured at RTP.
Model answer (5 marks)
The limiting reactant is the reactant that is completely used up during the reaction.
The limiting reactant limits the amount of product that can be formed.
In this reaction magnesium is the limiting reactant because hydrochloric acid is in excess.
Once all the magnesium has reacted, no further reaction can occur, so no more hydrogen gas is produced.
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 the limiting reactant.
The limiting reactant limits the amount of product that can be formed.
In this reaction magnesium is the limiting reactant because hydrochloric acid is in excess.
Once all the magnesium has reacted, no further reaction can occur, so no more hydrogen gas is produced.
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 the limiting reactant.
Examiner tips
- Define limiting reactant and state its role in controlling product amount.
- Identify which reactant is limiting and justify using the excess condition.
- Explain that reaction stops when the limiting reactant is consumed.
- Mention the 24 dm³ per mole at RTP to link moles to volume.
Common mistakes
- Confusing excess reactant with limiting reactant.
- Failing to explain why the reaction stops after the limiting reactant is used up.
- Using the wrong gas volume (e.g., 22.4 dm³) instead of 24 dm³ at RTP.
Mark scheme (5 marks)
- The limiting reactant is the reactant that is completely used up during the reaction
- The limiting reactant limits / controls the amount of product formed
- In this reaction, magnesium is the limiting reactant (because hydrochloric acid is in excess)
- Once the magnesium is completely used up, no further reaction can occur / no more hydrogen gas is produced
- 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 in this question
limiting reactant · RTP · excess
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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