Limiting reagent

High School

Definition

The reactant that is completely used up first in a chemical reaction, which therefore limits the amount of product that can form. Once the limiting reagent runs out the reaction stops, leaving any other reactants in excess.

Worked examples

\(2\) H\(_2 + \)O\(_2 \)→\( 2\) H\(_2\)O\(\): 3 mol H₂ and 1 mol O₂ are mixed.

H₂ is the limiting reagent because it runs out first (needs 1.5 mol O₂, but 1 mol O₂ only needs 2 mol H₂).

\(\)N\(_2 + 3\) H\(_2 \)→\( 2\) NH\(_3\): 1 mol N₂ and 2 mol H₂ available.

H₂ is limiting; it can only make 1.33 mol NH₃, while N₂ could make 2 mol if enough H₂ were present.

Common mistakes

The reactant with the smallest mass or moles is always the limiting reagent. → Compare mole ratios from the balanced equation to find which runs out first. A small amount of one reactant may still be in excess if the stoichiometry requires even less of it.
All reactants are used up completely when the reaction stops. → Only the limiting reagent is used up; other reactants remain in excess. The reaction stops as soon as the limiting reagent is gone, leaving leftover excess reactants.
The limiting reagent produces the least product by itself. → The limiting reagent determines the maximum product for the whole reaction. You must use stoichiometry with all reactants together, not treat them separately.

Where you'll use it next

You'll apply limiting reagent calculations in stoichiometry problems, lab yield predictions, and later in chemistry when studying reaction efficiency, percent yield, and industrial process optimization.

Found in 1 StudyPug lesson

Mastering Moles, Excess and Limiting Reagents

Chemistry 11

Dive into the world of stoichiometry! Learn to identify limiting reagents, calculate excess reactants, and predict reaction outcomes. Perfect for students aiming to excel in chemistry and real-world applications.

11th Grade11th