What is a Limiting Reactant?
In any chemical reaction, one reactant often runs out before the others, stopping the reaction. This is the limiting reactant. It determines how much product can form — no matter how much excess reactant you have, you're limited by the one that runs out first.
The limiting reactant is the reactant that runs out first in a chemical reaction, determining the maximum amount of product that can form. Other reactants are present in excess.
- 1↓Add Reactants A & BA: 10 moles, B: 5 moles (stoichiometry 2:1)
- 2↓React A + B → ProductUses 1 B for every 2 A
- 3↓B Runs Out FirstB is limiting; 2.5 A remains unreacted
- 4Product FormsMaximum determined by limiting reactant B
Step-by-step worked examples
In the reaction 2H₂ + O₂ → 2H₂O, you have 5 moles of H₂ and 3 moles of O₂. Which is the limiting reactant?
Stoichiometry ratio H₂:O₂ = 2:1 H₂ requires: 5 mol ÷ 2 = 2.5 mol O₂ You have 3 mol O₂, which is more than 2.5 mol Therefore, H₂ is the limiting reactant
4 mol N₂ and 10 mol H₂ react: N₂ + 3H₂ → 2NH₃. Find the limiting reactant.
Stoichiometry ratio N₂:H₂ = 1:3 H₂ needed: 4 mol × 3 = 12 mol You have 10 mol H₂, which is less than 12 mol needed H₂ is the limiting reactant
A reaction uses 6 mol of reactant X and 4 mol of reactant Y. If the ratio is X:Y = 2:3, what is limiting?
Stoichiometry X:Y = 2:3 (or 1:1.5) X requires: 6 mol ÷ 2 = 3 mol Y needed You have 4 mol Y, which is more than 3 mol Therefore, X is the limiting reactant
Flashcards
Quick quiz
Q1.In 2A + B → C, you have 8 mol A and 5 mol B. Which is limiting?
Q2.How do you find the limiting reactant?
Q3.What limits product formation?
Q4.3H₂ + N₂ → 2NH₃. You have 6 mol H₂ and 3 mol N₂. Limiting?
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Common mistakes
The reactant with the most moles is limiting. — Correct: The reactant that runs out first (after dividing by stoichiometry) is limiting — even if you started with fewer moles.
All reactants run out at the same time. — Correct: Typically one runs out first (limiting); others remain in excess.
Limiting reactant is the one with the highest mass. — Correct: It depends on stoichiometry and molar amounts, not mass alone.
Ignoring stoichiometric coefficients. — Correct: You must divide moles by coefficients to find the limiting reactant.
FAQ
What is the definition of a limiting reactant?
The reactant that runs out first in a chemical reaction, determining the maximum amount of product formed.
How do you calculate which reactant is limiting?
Divide the moles of each reactant by its stoichiometric coefficient; the smallest result is the limiting reactant.
Why is it important to find the limiting reactant?
It tells you the maximum product that can form and helps predict what remains unreacted.
Can there be no limiting reactant?
In theory, if all reactants run out simultaneously (from perfectly mixed proportions), but this is rare.