This deep dive covers Limiting Reactants within Moles & Calculations for GCSE Chemistry. Revise Moles & Calculations in Quantitative Chemistry for GCSE Chemistry with 27 exam-style questions and 20 flashcards. This topic shows up very often in GCSE exams, so students should be able to explain it clearly, not just recognise the term. It is section 13 of 17 in this topic. Use this deep dive to connect the idea to the wider topic before moving on to questions and flashcards.
⚗️ Limiting Reactants
In any reaction, the limiting reactant is the reactant that is completely used up first — it determines how much product can form, because the reaction stops when it runs out.
Imagine making cheese sandwiches. You have 10 slices of bread but only 3 slices of cheese. Each sandwich needs 2 slices of bread and 1 slice of cheese. You can only make 3 sandwiches before the cheese runs out — the cheese is the limiting reactant. The bread is in excess (you have leftover bread).
Worked example: In the reaction Mg + 2HCl → MgCl₂ + H₂, suppose you have 0.5 mol of Mg and 0.8 mol of HCl.
So 0.5 mol Mg needs 0.5 × 2 = 1.0 mol HCl
But you only have 0.8 mol HCl — not enough!
Therefore HCl is the limiting reactant. The Mg is in excess.
To find the limiting reactant: calculate how much of each reactant you need based on the mole ratio, then compare with what you actually have.
Practice questions for Moles & Calculations
One mole of any substance contains how many particles?
Explain why the percentage yield of a reaction is never 100% in practice.