Iron reacts with sulfur according to the equation: Fe + S → FeS. A student reacts 56 g of iron with an excess of sulfur. Explain what is meant by the term 'limiting reactant' and explain why, in this reaction, iron is the limiting reactant and not sulfur.
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 reacts 56 g of iron with an excess of sulfur.
Model answer (5 marks)
The limiting reactant is the reactant that is completely used up during the reaction.
In the equation Fe + S → FeS, one mole of Fe reacts with one mole of S. 56 g of Fe is 56 g ÷ 55.85 g mol⁻¹ = 1.00 mol. 1.00 mol of Fe requires 1.00 mol of S, which is 1.00 mol × 32.07 g mol⁻¹ = 32.07 g of S.
Because the student has an excess of sulfur, there is more than 32.07 g of S available, so S is not completely consumed. All 56 g of Fe are consumed, so Fe is the limiting reactant.
Since the limiting reactant determines the maximum amount of product, the amount of FeS that can be formed is limited by the 56 g of Fe, making Fe the limiting reactant.
In the equation Fe + S → FeS, one mole of Fe reacts with one mole of S. 56 g of Fe is 56 g ÷ 55.85 g mol⁻¹ = 1.00 mol. 1.00 mol of Fe requires 1.00 mol of S, which is 1.00 mol × 32.07 g mol⁻¹ = 32.07 g of S.
Because the student has an excess of sulfur, there is more than 32.07 g of S available, so S is not completely consumed. All 56 g of Fe are consumed, so Fe is the limiting reactant.
Since the limiting reactant determines the maximum amount of product, the amount of FeS that can be formed is limited by the 56 g of Fe, making Fe the limiting reactant.
Examiner tips
- Define the limiting reactant first; then give the stoichiometric calculation to show the amount of each reactant needed.
- Show that the available amount of Fe is less than the amount required to consume all the S, and state that Fe is therefore limiting.
Common mistakes
- Calculating the molar mass of Fe incorrectly or using the wrong unit (g mol⁻¹).
- Failing to recognise that the excess reactant is not limiting and therefore not mentioning it in the explanation.
Mark scheme (5 marks)
- The limiting reactant is the reactant that is completely used up during the reaction
- The limiting reactant limits the amount of product(s) formed
- Sulfur is present in excess, meaning there is more sulfur than is needed to react with all the iron
- Iron is completely used up / all 56 g of iron reacts
- Because iron is used up first/completely, it limits the amount of FeS that can be formed, so iron is the limiting reactant
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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