Iron reacts with sulfur according to the equation: Fe + S → FeS. A student heats 5.6 g of iron with an excess of sulfur. After the reaction is complete, the student finds that the total mass of the reaction vessel and its contents has not changed. Explain why the mass does not change, identify the limiting reactant in this reaction, and explain what the term limiting reactant means.

WJEC A-Level Chemistry (Wales) — 3.5 Aromatic chemistry (A-Level only) · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Iron reacts with sulfur according to the equation: Fe + S → FeS. A student heats 5.6 g of iron with an excess of sulfur. After the reaction is complete, the student finds that the total mass of the reaction vessel and its contents has not changed.

Model answer (5 marks)

The mass of the reaction vessel and its contents does not change because no atoms are lost or gained during the reaction – the law of conservation of mass states that the mass of the products equals the mass of the reactants.

The iron is the limiting reactant. Five‑point calculation: 5.6 g Fe × (1 mol Fe/55.85 g) = 0.100 mol Fe. The reaction requires 1 mol S per 1 mol Fe, so 0.100 mol S is needed. Because sulfur is in excess, Fe is used up first.

The limiting reactant is the reactant that is completely consumed in a reaction and determines the maximum amount of product that can be formed.

Examiner tips

  • Use the law of conservation of mass to explain the unchanged mass.
  • Show the molar mass calculation to identify Fe as the limiting reactant.
  • Define the limiting reactant as the one that is fully used and limits product formation.
  • Keep the answer concise and use exact terminology from the mark scheme.

Common mistakes

  • Claiming mass changes due to evaporation or loss of gas, ignoring the closed system.
  • Identifying sulfur as the limiting reactant without calculation.
  • Using vague terms like ‘the reaction stops’ instead of ‘the reactant is fully consumed’.

Mark scheme (5 marks)

  1. No atoms are lost or made during a chemical reaction
  2. So the mass of the products equals the mass of the reactants (law of conservation of mass)
  3. Iron (Fe) is the limiting reactant
  4. The limiting reactant is the reactant that is completely used up
  5. The limiting reactant limits the amount of products formed

Key terms in this question

limiting reactant · excess

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