What is a Limiting Reactant?
A limiting reactant is the substance that is completely used up first in a chemical reaction. Once it's gone, the reaction stops, even if other reactants remain — this reactant determines how much product forms.
To find the limiting reactant, calculate how many moles of product each reactant would form, treating each as the limiting one. Whichever makes the least product is the limiting reactant.
- •Some remains after reaction
- •Doesn't limit product formation
- •Can calculate how much is left
- •Completely consumed
- •Controls the amount of product
- •Determines reaction completion
Step-by-step worked examples
In 2H₂ + O₂ → 2H₂O, if you have 5 mol H₂ and 2 mol O₂, which is limiting?
If H₂ is limiting: 5 mol H₂ × (1 mol O₂/2 mol H₂) = 2.5 mol O₂ needed. Only 2 available, so H₂ is in excess. If O₂ is limiting: 2 mol O₂ × (2 mol H₂/1 mol O₂) = 4 mol H₂ needed. 5 available, so O₂ is limiting.
For N₂ + 3H₂ → 2NH₃, with 3 mol N₂ and 8 mol H₂, find the limiting reactant.
If N₂ is limiting: 3 mol N₂ × (3 mol H₂/1 mol N₂) = 9 mol H₂ needed. Only 8 available, so H₂ is limiting. H₂ is the limiting reactant (needs 9, have 8).
A + 2B → C, with 4 mol A and 6 mol B. Which limits the reaction?
If A is limiting: 4 mol A × (2 mol B/1 mol A) = 8 mol B needed. Only 6 available, so B is limiting. B is the limiting reactant.
Flashcards
Quick quiz
Q1.2A + B → 2C. With 10 mol A and 4 mol B, which is limiting?
Q2.What does a limiting reactant do?
Q3.In 2H₂ + O₂ → 2H₂O with 5 mol H₂ and 3 mol O₂, what is limiting?
Q4.After reaction stops, limiting reactant…
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Common mistakes
Assuming more grams = more limiting. — Correct: Compare moles, not grams — molar mass matters.
Forgetting to use mole ratios. — Correct: Always apply coefficients when calculating limiting reactant.
Thinking the first to run out is always limiting. — Correct: Calculate for both; the one giving fewer products is limiting.
Confusing limiting with excess reactant. — Correct: Limiting = runs out first; excess = has leftover.
FAQ
How do you identify a limiting reactant?
Calculate moles of product from each reactant (treating each as limiting). The one yielding the smallest amount of product is the limiting reactant.
Why is limiting reactant important?
It determines the maximum amount of product that can form — you can't make more than what the limiting reactant allows.
Can limiting reactant change?
Yes — if you start with different amounts of reactants, a different one may be limiting.
Real example of limiting reactant?
In cooking, if a recipe needs 2 cups flour and 1 egg, and you have 3 cups flour and 1 egg, the egg is limiting.