Iron reacts with hydrochloric acid according to the balanced equation: Fe + 2HCl → FeCl₂ + H₂. A student adds excess hydrochloric acid to a small amount of iron. Explain what is meant by the term 'limiting reactant' and explain why, in this reaction, the iron acts as the limiting reactant.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Iron reacts with hydrochloric acid according to the balanced equation: Fe + 2HCl → FeCl₂ + H₂. A student adds excess hydrochloric acid to a small amount of iron.
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(s) formed.
In this reaction, hydrochloric acid is added in excess, so there is more HCl than is needed to react with all the iron.
The iron is completely used up / all the iron reacts.
Once all the iron has been used up, the reaction stops and no more product can be formed even though HCl remains.
The limiting reactant limits the amount of product(s) formed.
In this reaction, hydrochloric acid is added in excess, so there is more HCl than is needed to react with all the iron.
The iron is completely used up / all the iron reacts.
Once all the iron has been used up, the reaction stops and no more product can be formed even though HCl remains.
Examiner tips
- Define limiting reactant first, then state why iron is limiting
- Show that HCl is in excess by comparing stoichiometric ratios
- Explain that reaction stops when iron is gone
- Use correct chemical equations and terminology
Common mistakes
- Confusing excess reactant with limiting reactant
- Not recognising that reaction stops when iron is consumed
- Using vague terms like ‘more’ instead of ‘excess’
Mark scheme (5 marks)
- The limiting reactant is the reactant that is completely used up during the reaction
- The limiting reactant limits the amount of product(s) formed
- The hydrochloric acid is added in excess, so there is more HCl than is needed to react with all the iron
- The iron is completely used up / all the iron reacts
- Once all the iron has been used up, the reaction stops and no more product can be formed even though HCl remains
Key terms in this question
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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