# How would you define the term "limiting reactant?"

May 21, 2017

In a reaction, once all of one reactant has been used up, there may be some of another left over. The reactant that is used up first is what limits the amount of product formed and is referred to as the 'limiting reagent' or 'limiting reactant'.

#### Explanation:

The limiting reactant may not be the one of which there is least. The balanced chemical reaction will show the ratio are which the reactants are used.

It's probably worth using a real-world example.

In the reaction of $50$ $g$ of ${H}_{2}$ with $200$ $g$ of ${O}_{2}$ to produce ${H}_{2} O$, which is the limiting reactant?

We might think it would be the ${H}_{2}$, but let's see.

First, we need the balanced chemical equation:

$2 {H}_{2} + {O}_{2} \to 2 {H}_{2} O$

Next, we need to know the number of moles we have of each element:

${n}_{{H}_{2}} = \frac{m}{M} = \frac{50}{2} = 25$ mol

${n}_{{O}_{2}} = \frac{m}{M} = \frac{200}{32} = 6.25$ mol

Now, looking back at the balanced equation, we see that $2$ mol of ${H}_{2}$ react with each $1$ mol of ${O}_{2}$... but that means once all $6.25$ mol of ${O}_{2}$ has been used, only $12.5$ mol of ${H}_{2}$ will have been used, and there will be a further $12.5$ mol left.

(we say that the reactant that is not the limiting reactant is 'in excess')