# Limiting Reagent

## Key Questions

• The limiting reagent of a reaction is the reactant that runs out first. Once it is completely consumed, the reaction stops.

The limiting reagent is the only chemical that is used to calculate the theoretical yield. It is used up first. After that, any excess reagent will not be able to produce more products.

The $\text{limiting reagent}$ is the reagent in deficiency in a chemical reaction.

#### Explanation:

Consider a combustion reaction (of say methane):

$C {H}_{4} \left(g\right) + 2 {O}_{2} \left(g\right) \rightarrow C {O}_{2} \left(g\right) + 2 {H}_{2} O \left(g\right)$

There is lots of oxygen in the atmosphere; there is limited methane in your gas bottle. Clearly, here methane is the limiting reagent.

Many problems require you to identify the limiting reagent. If you have a stoichiometric equation, most of the time the limiting reagent is easily designated.