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ChemistryGCSE Higher & A Level

What is limiting reagent?

The limiting reagent is the reactant that is fully used up first and therefore determines the maximum yield of product.

Limiting reagent explained

Convert the mass of each reactant to moles, then divide by its coefficient in the balanced equation. The smallest result marks the limiting reagent.
All product calculations must be based on the limiting reagent; the other reactant is in excess and some remains unreacted.

Key formula

compare ncoefficient for each reactant\text{compare } \dfrac{n}{\text{coefficient}} \text{ for each reactant}

Worked example

With 22 mol H₂ and 22 mol O₂ for 2H2+O2→2H2O2\mathrm{H_2} + \mathrm{O_2} \rightarrow 2\mathrm{H_2O}, hydrogen limits and only 22 mol of water forms.

Examiner tip

Never assume the reactant with the smaller mass is limiting — always convert to moles and divide by the coefficient first.

Limiting reagent: common questions

Why use an excess of one reactant?
To make sure the more expensive or more valuable reactant reacts completely, improving the yield with respect to it.
How do I know which reactant is in excess?
It is the one that is not limiting — the reactant with the larger mole-to-coefficient ratio.

Related terms

Mole

A mole is the amount of substance containing 6.02 × 10²³ particles — the chemist's counting unit that links mass to number of particles.

Percentage yield

Percentage yield compares the mass of product actually obtained with the theoretical maximum predicted by the balanced equation.

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