Zinc reacts with sulfuric acid according to the balanced equation: Zn + H₂SO₄ → ZnSO₄ + H₂. A student carries out the reaction using a small amount of zinc powder and a large volume of sulfuric acid solution. Explain why one reactant is described as the limiting reactant in this reaction, identify which reactant is the limiting reactant in this experiment, and explain what effect using the other reactant in excess has on the amount of product formed.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
The limiting reactant is the one that is completely consumed during the reaction.
In the experiment zinc powder is used in a small amount while a large volume of sulphuric acid is present.
Because the amount of zinc is smaller than the amount required to react with all the acid, zinc is the limiting reactant.
Zinc limits the amount of ZnSO₄ (and H₂) that can be formed; once all the zinc has reacted no further product can be produced.
Sulphuric acid is in excess, so some of it remains unreacted after the zinc has been used up.
Using acid in excess guarantees that all of the zinc reacts and that the limiting reactant is fully consumed.
In the experiment zinc powder is used in a small amount while a large volume of sulphuric acid is present.
Because the amount of zinc is smaller than the amount required to react with all the acid, zinc is the limiting reactant.
Zinc limits the amount of ZnSO₄ (and H₂) that can be formed; once all the zinc has reacted no further product can be produced.
Sulphuric acid is in excess, so some of it remains unreacted after the zinc has been used up.
Using acid in excess guarantees that all of the zinc reacts and that the limiting reactant is fully consumed.
Examiner tips
- Define limiting reactant first, then state which is limiting and why; mention stoichiometry. Show that excess reagent remains. Keep answer concise and use correct terminology (e.g., ‘excess’).
Common mistakes
- Confusing which reactant is limiting; students often say acid is limiting. Failing to explain that excess reagent remains unreacted. Using vague language such as ‘more of one reactant’ instead of ‘excess’.
Mark scheme (5 marks)
- The limiting reactant is the reactant that is completely used up during the reaction.
- Zinc is the limiting reactant in this experiment.
- Zinc limits the amount of ZnSO₄ (or H₂) produced because no more product can form once the zinc is used up.
- The sulfuric acid is in excess, meaning not all of it reacts / some sulfuric acid remains unreacted at the end.
- Using the sulfuric acid in excess ensures that all of the zinc reacts / ensures the limiting reactant is fully used up.
Key terms in this question
Related
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