What is limiting reactant? with examples

The limiting reactant or limiting reagent is a reactant in a chemical reaction that determines the amount of product that is formed. Identification of the limiting reactant makes it possible to calculate the theoretical yield of a reaction.

The reason there is a limiting reactant is that elements and compounds react according to the mole ratio between them in a balanced chemical equation. So, for example, if the mole ratio in the balanced equation states it takes 1 mole of each reactant to produce a product (1:1 ratio) and one of the reactants is present in a higher amount than the other, the reactant present in the lower amount would be limiting reactant. All of it would be used up before the other reactant ran out.

Limiting Reactant Example

Given 1 mol of hydrogen and 1 mol of oxygen in the reaction:

2 H2 + O2 → 2 H2O

The limiting reactant would be hydrogen because the reaction uses up hydrogen twice as fast as oxygen.

How to Find the Limiting Reactant

There are two methods used to find the limiting reactant. The first is to compare the actual mole ratio of the reactants to the mole ratio of the balanced chemical equation. The other method is to calculate the gram masses of the product resulting from each reactant. The reactant that yields the smallest mass of product is the limiting reactant.