Calcium carbonate reacts with hydrochloric acid according to the equation: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂. A student carries out this reaction using an excess of hydrochloric acid and a limited amount of calcium carbonate. Explain what is meant by the term 'limiting reactant' and explain how the law of conservation of mass applies to this reaction, even though a gas is produced.

WJEC A-Level Chemistry (Wales) — 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 (calcium carbonate) limits the amount of product(s) formed.
The law of conservation of mass states that no atoms are lost or made during a chemical reaction.
The mass of the products equals the mass of the reactants.
The carbon dioxide gas produced still has mass, so if it is collected and included, the total mass of all products (CaCl₂ + H₂O + CO₂) equals the total mass of the reactants.

Examiner tips

  • Use the exact phrase ‘limiting reactant’ and explain it is the one that is completely consumed. Show that the amount of CaCl₂, H₂O and CO₂ is limited by the amount of CaCO₃. State the law of conservation of mass and emphasise that CO₂ still has mass. Mention that the total mass of all products equals the total mass of all reactants.
  • common_mistakes
  • :
  • Confusing the excess reactant with the limiting reactant. Forgetting that CO₂ gas still contributes to the mass. Using vague wording such as ‘the reaction stops’ instead of ‘the reactant is completely used up’.

Mark scheme (5 marks)

  1. The limiting reactant is the reactant that is completely used up during the reaction
  2. The limiting reactant (calcium carbonate) limits the amount of product(s) formed
  3. The law of conservation of mass states that no atoms are lost or made during a chemical reaction
  4. The mass of the products equals the mass of the reactants
  5. The carbon dioxide gas produced still has mass, so if it is collected and included, the total mass of all products (CaCl₂ + H₂O + CO₂) equals the total mass of the reactants

Key terms in this question

limiting reactant · conservation of mass · excess

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