3.2.4 Limiting reactants

One reactant runs out first and halts the reaction

1. Reactants are rarely mixed in the exact ratio of the equation, so usually one is used up before the other.
2. The reactant that runs out first is the limiting reactant, and the reaction stops once it is gone.
   1. The other reactant is in excess, so some of it is left over unreacted.

The limiting reactant fixes how much product forms

1. The amount of product is directly proportional to the amount of the limiting reactant.
2. Doubling the limiting reactant doubles the product, while the excess reactant has no effect on the yield.

Finding which reactant is limiting

1. Convert the mass of each reactant into moles.
2. Use the mole ratio in the equation to see which reactant is in short supply.
3. Base every amount-of-product calculation on the limiting reactant only.

Worked example: making water from hydrogen and oxygen

1. $4g$ of hydrogen and $40g$ of oxygen react in $2H_2+O_2\to 2H_{2}O$.
   1. This is $\frac{4}{2}=2mol$ of hydrogen and $\frac{40}{32}=1.25mol$ of oxygen.
   2. The $2mol$ of hydrogen needs only $1mol$ of oxygen, so hydrogen is limiting and oxygen is in excess.
   3. The $2mol$ of hydrogen makes $2mol$ of water, which is $2\times 18=36g$.