Limiting reactant
Reactants are rarely mixed in exactly the right ratio. The limiting reactant is used up first and stops the reaction. The other is the excess reactant, and some of it is left over.
It works like making sandwiches from 2 slices of bread and 1 slice of cheese. With 10 slices of bread and 3 of cheese, you can make only 3 sandwiches. Cheese is limiting and 4 slices of bread are left over.
To find the limiting reactant:
- Convert each reactant to moles.
- For each one, calculate how much product it could make.
- The reactant that gives the smaller amount of product is limiting. That smaller amount is what actually forms.
Worked example: 3.0 mol of H2 and 2.0 mol of O2
2 H2 + O2 → 2 H2O
From H2: 3.0 mol H2 × (2 mol H2O / 2 mol H2) = 3.0 mol H2O
From O2: 2.0 mol O2 × (2 mol H2O / 1 mol O2) = 4.0 mol H2O
H2 gives less product, so H2 is limiting and 3.0 mol of water forms. The reaction uses 1.5 mol of O2, leaving 0.5 mol.
Common mistake
Picking the reactant with the smaller amount. Here there were more moles of H2 than O2, yet H2 ran out first because the reaction uses twice as much of it.