Limiting Reactant
The limiting reactant (or limiting reagent) is the reactant that runs out first in a chemical reaction. It gets used up completely, stops the reaction, and therefore controls how much product can form. Any other reactant is left over — it is in excess.
The key idea: compare reactants in moles and use the balanced equation's ratio — never compare masses directly.
The method — four steps
Write the balanced equation and note the mole ratio of the reactants.
Convert each reactant to moles using .
Divide each reactant's moles by its coefficient in the equation — the smallest value is the limiting reactant.
Use the limiting reactant's moles (and the ratio) to work out the amount of product.
Worked example
14 g of nitrogen (N₂) reacts with 6.0 g of hydrogen (H₂): N₂ + 3H₂ → 2NH₃. Which reactant is limiting, and what mass of ammonia (NH₃) forms? (M: N₂ = 28, H₂ = 2.0, NH₃ = 17 g mol⁻¹)
Convert to moles: mol; mol.
The equation needs 3 mol H₂ for every 1 mol N₂. So 0.50 mol N₂ needs mol H₂.
We have 3.0 mol H₂ but only need 1.5 mol — H₂ is in excess, so N₂ is the limiting reactant.
Base the product on N₂: the ratio N₂ : NH₃ is 1 : 2, so mol.
Convert to mass: g.
N₂ is limiting; 17 g of NH₃ forms (with H₂ left over in excess).
divide each reactant's moles by its coefficient — the smallest answer is the limiting reactant. Here: N₂ = 0.50/1 = 0.50 vs H₂ = 3.0/3 = 1.0, so N₂ (0.50) limits.
Why it matters
the theoretical yield is always calculated from the limiting reactant — the excess reactant's extra amount produces nothing.
you can find how much excess remains: subtract the amount that reacted (1.5 mol H₂ here) from the amount you started with (3.0 mol) → 1.5 mol H₂ unreacted.
Common mistakes
Comparing masses instead of moles — 14 g vs 6 g tells you nothing until you convert to moles.
Forgetting to divide by the coefficient — the reactant present in fewer moles isn't automatically limiting; the ratio decides.
Calculating the product from the excess reactant — always use the limiting reactant for yield.
Frequently asked questions
How do you find the limiting reactant?
Convert every reactant to moles, then divide each reactant's moles by its coefficient in the balanced equation. The reactant with the smallest result is the limiting reactant — it runs out first and controls how much product forms. The others are in excess.
What is the difference between limiting and excess reactant?
The limiting reactant is completely used up and determines the maximum amount of product. The excess reactant is left over after the reaction stops because there was more of it than the reaction needed. Only the limiting reactant is used to calculate the yield.
Why can't you just compare the masses of the reactants?
Different substances have different molar masses, so equal masses contain different numbers of moles. Reactions combine in mole ratios, not mass ratios, so you must convert to moles (and account for the equation's coefficients) before deciding which reactant is limiting.
This concept in your exam
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