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Quiz yourself on Moles & CalculationsGet your flashcardsBuild my revision planTrack my progress⚗️ 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.
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