Iron reacts with hydrochloric acid. The reaction produces iron(II) chloride and hydrogen gas. A student carries out this reaction using 0.5 mol of iron and an excess of hydrochloric acid. Explain what is meant by the term 'limiting reactant' and identify the limiting reactant in this reaction, giving a reason for your answer.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The balanced equation for the reaction is: Fe + 2HCl → FeCl₂ + H₂
Model answer (5 marks)
A limiting reactant is the reactant that is completely consumed in a reaction and therefore determines the maximum amount of product that can be formed.
In the equation Fe + 2HCl → FeCl₂ + H₂, one mole of iron reacts with two moles of hydrochloric acid.
Only 0.5 mol of iron is available, whereas the acid is in excess. Therefore iron is the limiting reactant.
Because all of the iron is used up, the reaction stops and no further product can be formed.
In the equation Fe + 2HCl → FeCl₂ + H₂, one mole of iron reacts with two moles of hydrochloric acid.
Only 0.5 mol of iron is available, whereas the acid is in excess. Therefore iron is the limiting reactant.
Because all of the iron is used up, the reaction stops and no further product can be formed.
Examiner tips
- Define limiting reactant and state its role in determining product yield.
- Show the stoichiometric ratio from the balanced equation.
- Identify the reactant present in the smallest stoichiometric amount.
- Explain why the reaction stops when this reactant is consumed.
Common mistakes
- Confusing excess reactant with limiting reactant.
- Failing to mention that the reaction stops when the limiting reactant is used up.
- Using vague language such as "smallest amount" instead of stoichiometric reasoning.
Mark scheme (5 marks)
- The limiting reactant is the reactant that is completely used up
- The limiting reactant limits the amount of products formed
- Iron is the limiting reactant
- Because iron is not in excess / hydrochloric acid is in excess
- Once all the iron is used up, the reaction stops / no more product can be formed
Key terms in this question
Related
- All Eduqas A-Level Chemistry revision notes →
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