Ammonia is produced industrially by the Haber process: N₂(g) + 3H₂(g) → 2NH₃(g). In a particular run, nitrogen gas is identified as the limiting reagent. Explain what is meant by the term 'limiting reagent' and describe how identifying the limiting reagent allows the theoretical yield of ammonia to be calculated.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The limiting reagent is the reactant that is completely consumed first, i.e. the one present in an insufficient molar amount relative to the stoichiometric ratio.
Because the reaction stops when the limiting reagent is used up, the amount (in moles) of that reagent determines the maximum moles of product that can be formed.
Using the balanced equation, 1 mol N₂ gives 2 mol NH₃. Therefore
moles NH₃ = 2 × moles N₂ (the limiting reagent).
The theoretical yield in grams is found by multiplying the moles of NH₃ by its molar mass (17.04 g mol⁻¹).
Because the reaction stops when the limiting reagent is used up, the amount (in moles) of that reagent determines the maximum moles of product that can be formed.
Using the balanced equation, 1 mol N₂ gives 2 mol NH₃. Therefore
moles NH₃ = 2 × moles N₂ (the limiting reagent).
The theoretical yield in grams is found by multiplying the moles of NH₃ by its molar mass (17.04 g mol⁻¹).
Examiner tips
- Define limiting reagent clearly; link it to stoichiometry.
- Show the stoichiometric calculation step‑by‑step.
- Convert moles to grams using the correct molar mass.
- Use correct units and significant figures.
Mark scheme (4 marks)
- The limiting reagent is the reactant that is completely consumed first / is present in insufficient molar quantity relative to the stoichiometric ratio.
- The amount (in moles) of the limiting reagent determines the maximum moles of product that can form, because once it is consumed the reaction cannot proceed further.
- The stoichiometric mole ratio from the balanced equation is used: moles of NH₃ = moles of N₂ × 2 (from the 1:2 ratio of N₂ to NH₃).
- The theoretical yield (in grams) is then obtained by multiplying the calculated moles of NH₃ by its molar mass (17.04 g mol⁻¹).
Key terms in this question
limiting reagent · theoretical yield
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