Limiting Reactant Definition (Limiting Reagent)

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.

Using the Mole Ratio:

1. Balance the equation for the chemical reaction.
2. Convert the masses of reactants to moles, if needed. If the quantities of reactants are given in moles, skip this step.
3. Calculate the mole ratio between reactants using the actual numbers. Compare this ratio to the mole ratio between reactants in the balanced equation.
4. Once you identify which reactant is the limiting reactant, calculate how much product it can make. You can check that you selected the correct reagent as the limiting reactant by calculating how much product the full amount of the other reactant would yield (which should be a larger number).
5. You can use the difference between the moles of non-limiting reactant that are consumed and the starting number of moles to find the amount of excess reactant. If necessary, convert the moles back to grams.

Using the Product Approach:

1. Balance the chemical reaction.
2. Convert the given quantities of reactants to moles.
3. Use the mole ratio from the balanced equation to find the number of moles of product that would be formed by each reactant if the full amount was used. In other words, perform two calculations to find the moles of product.
4. The reactant that yielded the smaller amount of product is the limiting reactant. The reactant that yielded the larger amount of produce is the excess reactant.
5. The amount of excess reactant may be calculated by subtracting the moles of excess reactant from the number of moles used (or by subtracting the mass of excess reactant from the total mass used). Mole to gram unit conversions may be necessary to provide answers for homework problems.