Magnesium reacts with hydrochloric acid according to the equation: Mg + 2HCl → MgCl₂ + H₂. A student adds an excess of hydrochloric acid to a small amount of magnesium. Explain what is meant by the term 'limiting reactant' and explain why the magnesium is the limiting reactant in this reaction.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
The limiting reactant is the reactant that is completely used up during the reaction.
The limiting reactant controls the amount of product that can be formed.
In this reaction the hydrochloric acid is in excess, meaning there is more acid than is needed to react with all of the magnesium.
The magnesium is the limiting reactant because it is completely used up before the hydrochloric acid.
Once the magnesium is used up, no more hydrogen or magnesium chloride can be produced, so magnesium limits the amount of product.
The limiting reactant controls the amount of product that can be formed.
In this reaction the hydrochloric acid is in excess, meaning there is more acid than is needed to react with all of the magnesium.
The magnesium is the limiting reactant because it is completely used up before the hydrochloric acid.
Once the magnesium is used up, no more hydrogen or magnesium chloride can be produced, so magnesium limits the amount of product.
Examiner tips
- Define limiting reactant and state its role in product formation.
- Explain that acid is in excess and magnesium runs out first.
- Show that product formation stops when magnesium is consumed.
- Use clear, concise language and correct chemical symbols.
Common mistakes
- Confusing the roles of excess and limiting reactants.
- Failing to mention that product formation stops when the limiting reactant is consumed.
- Using vague terms like "more" instead of "excess" or "completely used up."
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 products formed
- The hydrochloric acid is in excess, meaning there is more than enough acid to react with all of the magnesium
- The magnesium is completely used up / runs out before the hydrochloric acid
- Once the magnesium is used up, no more hydrogen / magnesium chloride can be produced, so magnesium limits the amount of product
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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