5.11 - Limiting Reactants

Excess and the limiting reactant

When two reactants react, the reaction stops once one of them is completely used up.

Adding calcium carbonate to dilute hydrochloric acid makes the mixture fizz, because carbon dioxide is given off:

CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)

The fizzing stops when one of the two reactants has been used up completely, however much of the other is left in the beaker.

Key terms

• Limiting reactant - The reactant completely used up. It limits how much product can be made.
• In excess - Any reactant still left over when the reaction stops.

Product depends on the limiting reactant

The amount of product formed is directly proportional to the amount of the limiting reactant.

• Double the limiting reactant → twice as much product.
• Halve it → half as much product.

Why a reactant is often added in excess

Using a surplus of the cheaper or more easily supplied reactant makes sure the other reactant is used up completely, so none of the expensive one is left unreacted.