Limiting Reactants (Higher Tier)

This lesson covers the concept of limiting reactants and excess reactants as required by the AQA GCSE Combined Science Trilogy specification (8464) at Higher tier. You will learn how to identify which reactant limits the reaction and how to calculate the mass of product formed when one reactant is in excess.

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Key Definitions

Term	Definition
Limiting reactant	The reactant that is completely used up first in a reaction. It determines the maximum amount of product that can be formed.
Excess reactant	The reactant that is not completely used up. Some of it remains after the reaction has finished.

When the limiting reactant is all used up, the reaction stops — even though there is still some excess reactant left.

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Why Does It Matter?

In the real world, you rarely mix reactants in perfect proportions. One reactant usually runs out first. The limiting reactant controls:

• The maximum amount of product formed.
• The amount of excess reactant left over.

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How to Identify the Limiting Reactant

Step-by-Step Method