Limiting Reactants [H]

This lesson covers limiting reactants and excess reactants as required by the AQA GCSE Chemistry specification (4.3.2). This is Higher Tier only content. You need to understand the difference between a limiting and an excess reactant, be able to identify which reactant is limiting, and calculate the mass of product formed based on the limiting reactant.

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What Is a Limiting Reactant?

In most chemical reactions, the reactants are not present in the exact amounts needed for them to react completely with each other. One reactant will be completely used up before the other — this is the limiting reactant. The other reactant is said to be in excess because some of it remains unreacted.

Term	Definition
Limiting reactant	The reactant that is completely used up in a reaction. It determines the maximum amount of product that can be formed.
Excess reactant	The reactant that is left over after the reaction is complete. There is more of it than is needed.

The limiting reactant controls:

• The amount of product formed
• When the reaction stops
• The extent of the reaction