Calcium carbonate reacts with hydrochloric acid according to the equation: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂. A student has 10 g of calcium carbonate and adds it to a large excess of hydrochloric acid. Explain what is meant by the term 'limiting reactant' in this reaction and explain why the calcium carbonate is the limiting reactant in this experiment.

Eduqas A-Level Chemistry — 3.6 Amines and polymers (A-Level only) · Explain · 5 marks · View as Markdown

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 limits or controls the amount of product that can be formed.
In this experiment the hydrochloric acid is in excess, meaning there is more than enough HCl to react with all of the calcium carbonate.
Because HCl is in excess, all of the calcium carbonate is used up or reacts completely.
Therefore the calcium carbonate limits the amount of product formed, making it the limiting reactant.

Examiner tips

  • Define limiting reactant first, then state which is in excess, explain complete consumption of CaCO3, link to product amount.
  • Use the exact wording from the mark scheme: ‘completely used up’, ‘limits the amount of product’.”]
  • common_mistakes
  • :

Mark scheme (5 marks)

  1. The limiting reactant is the reactant that is completely used up during the reaction
  2. The limiting reactant limits/controls the amount of product formed
  3. The hydrochloric acid is in excess, meaning there is more than enough HCl to react with all of the calcium carbonate
  4. Because HCl is in excess, all of the calcium carbonate is used up / reacts completely
  5. Therefore the calcium carbonate limits the amount of product formed, making it the limiting reactant

Key terms in this question

limiting reactant · excess

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