GCSE Combined Science revision · Chemistry: atoms, bonding and amounts

Higher: limiting reactants

Higher only

The idea

In a reaction one reactant is often used in excess so that all of the other reactant is used up. The reactant that is completely used up is the limiting reactant: it limits the amount of product formed. To find it, calculate the moles of each reactant and compare with the ratio in the balanced equation.

Find the moles of each reactant.
Divide by the number in the balanced equation.
The smaller result is the limiting reactant.

Watch out The limiting reactant is not always the one with the smaller mass.

See it worked

Worked example

2.4 g of magnesium (Mg = 24) reacts with 0.5 mol of hydrochloric acid: Mg + 2HCl → MgCl₂ + H₂. Which reactant is limiting?

Moles of Mg = 2.4 ÷ 24 = 0.1 mol, which needs 0.2 mol HCl.
0.5 mol HCl is more than enough, so magnesium is used up first.

Answer Magnesium is the limiting reactant

Your turn

Question 1Calculator allowed2 marks

2.4 g of magnesium (Mg = 24) reacts with 0.5 mol of hydrochloric acid: Mg + 2HCl → MgCl₂ + H₂. How many moles of hydrogen are made?

Question 2Calculator allowed1 mark

What is a limiting reactant?

Question 3Calculator allowed1 mark

Why is one reactant often added in excess?

In past papers

4 official questions in 4 papers since 2020. The latest:

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