Iron reacts with sulfur to form iron sulfide. A student carries out this reaction and notices that the total mass of the products is equal to the total mass of the reactants. Explain why the mass of the products equals the mass of the reactants in this reaction, and explain what is meant by the term 'limiting reactant' in a chemical reaction where one reactant is used in excess.

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).

Iron reacts with sulfur according to the equation: Fe + S → FeS. A student measures the mass of iron and sulfur before the reaction, and the mass of iron sulfide after the reaction, and finds the total mass does not change.

Model answer (5 marks)

The reaction Fe + S → FeS obeys the law of conservation of mass – no atoms are lost or created, so the total mass of Fe and S before the reaction equals the mass of FeS after.

In a reaction where one reactant is present in excess, the reactant that is completely consumed first is called the limiting reactant. It determines the maximum amount of product that can be formed, because once it is used up the reaction stops even though other reactants remain.

Examiner tips

  • Use the phrase "law of conservation of mass" to link the first part to the mark scheme.
  • Define the limiting reactant and state its role in determining product yield.
  • Show the logical link between atoms being conserved and mass equality.
  • Mention that excess reactant remains unused after the limiting one is consumed.

Common mistakes

  • Saying the mass is the same because the reaction is exothermic or endothermic – ignores conservation of mass.
  • Confusing the limiting reactant with the excess reactant – not recognising which one is used up first.
  • Omitting the term "law of conservation of mass" or mis‑spelling it.

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 (conservation of mass)
  3. This is described by the law of conservation of mass
  4. The limiting reactant is the reactant that is completely used up in the reaction
  5. The limiting reactant limits / determines the amount of product formed

Key terms in this question

limiting reactant · reactant · product

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