Limiting and excess reagents

By the end of this lesson

By the end you can identify the limiting reagent in a reaction, use it to calculate the amount of product, and find how much of the excess reagent is left over.

The burger problem

You have 10 buns and 6 patties. Each burger needs 1 bun and 1 patty. You make 6 burgers, not 10, and 4 lonely buns are left over.

The patties ran out first, so they limited how many burgers you could make. The buns were in excess. Reactions work exactly the same way, except the "recipe" is the balanced equation.

Definition

Limiting reagent and excess reagent

The limiting reagent is the reactant that is completely used up first. It decides the maximum amount of product.

The excess reagent is any reactant that is left over when the reaction stops.

There's more hydrogen than oxygen, but hydrogen still runs out first. Fewer moles doesn't mean limiting.

The test that never fails

Find the moles of each reactant.
Divide each by its coefficient in the balanced equation.
The smallest answer is the limiting reagent.
Use the limiting reagent, and only the limiting reagent, to calculate the product.

Why dividing works: it tells you how many "equation's worth" of each reactant you have. The reactant with the fewest runs out first, just like the patties.

Quick check

Multiple choiceQuestion 1 · Recall

Multiple choiceQuestion 2 · Apply

Answer the 2 questions above to continue.