In chemical reactions, not all reactants are consumed at the same time. One reactant is often used up first, stopping the reaction from continuing. This concept—central to stoichiometry—appears in IB Chemistry Topic 1 and in nearly every type of quantitative question. Understanding the limiting reagent helps you calculate theoretical yield, percent yield, and excess reactant amounts.
What Is the Limiting Reagent?
The limiting reagent (or limiting reactant) is the reactant that is completely consumed first in a chemical reaction, limiting the amount of product formed.
Once this reactant runs out, the reaction stops—even if other reactants remain.
This is why the limiting reagent determines the maximum amount of product.
Why the Limiting Reagent Matters
Identifying the limiting reagent is essential for:
- Calculating theoretical yield
- Finding percent yield
- Determining excess reactant left over
- Checking stoichiometric ratios
- Predicting how much reactant is needed
IB exam calculations almost always involve identifying this reactant before solving yield problems.
The Basic Method for Finding the Limiting Reagent
Write and Balance the Equation
Correct stoichiometric ratios are essential.
Example:
N₂ + 3H₂ → 2NH₃
Convert All Reactants to Moles
Use:
moles = mass ÷ molar mass
Or concentration × volume for solutions.
Compare Mole Ratios
Use the balanced equation to determine how much of each reactant is .
