Zinc reacts with sulfuric acid according to the equation: Zn + H₂SO₄ → ZnSO₄ + H₂. A student adds a small amount of zinc powder to a large excess of sulfuric acid. Explain why the zinc is the limiting reactant in this reaction and describe what effect this has on the amount of hydrogen gas produced.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Zinc reacts with sulfuric acid according to the equation: Zn + H₂SO₄ → ZnSO₄ + H₂. A student adds a small amount of zinc powder to a large excess of sulfuric acid.
Model answer (5 marks)
The limiting reactant is the reactant that is completely used up during the reaction.
The zinc is the limiting reactant because only a small amount of Zn is added while a large excess of H₂SO₄ is present. Therefore the amount of product that can be formed is limited by the amount of Zn.
The amount of hydrogen gas produced is determined by the amount of zinc, not by the excess acid.
According to the equation, 1 mol of Zn produces 1 mol of H₂ (molar ratio Zn : H₂ = 1 : 1).
Once the zinc is completely consumed the reaction stops and no further H₂ is produced, even though H₂SO₄ remains in excess.
The zinc is the limiting reactant because only a small amount of Zn is added while a large excess of H₂SO₄ is present. Therefore the amount of product that can be formed is limited by the amount of Zn.
The amount of hydrogen gas produced is determined by the amount of zinc, not by the excess acid.
According to the equation, 1 mol of Zn produces 1 mol of H₂ (molar ratio Zn : H₂ = 1 : 1).
Once the zinc is completely consumed the reaction stops and no further H₂ is produced, even though H₂SO₄ remains in excess.
Examiner tips
- Use the definition of limiting reactant first. Show the stoichiometric ratio 1:1 to link Zn to H₂. Explain the consequence of Zn being exhausted. Keep the answer concise and use correct terminology.
Common mistakes
- Confusing excess reactant with limiting reactant. Ignoring the 1:1 molar ratio. Claiming that more H₂ can be produced after Zn is used up.
Mark scheme (5 marks)
- The limiting reactant is the reactant that is completely used up during the reaction
- The zinc is the limiting reactant because it limits the amount of products formed / the sulfuric acid is in excess so not all of it reacts
- The amount of hydrogen gas produced is determined by / limited by the amount of zinc, not the amount of sulfuric acid
- According to the equation, 1 mole of zinc produces 1 mole of hydrogen gas / the molar ratio of Zn to H₂ is 1:1
- Once the zinc is completely used up, the reaction stops and no more hydrogen is produced even though sulfuric acid remains
Key terms in this question
Related
- All Eduqas A-Level Chemistry revision notes →
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- Decode the mark scheme abbreviations →
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