Limiting Reactants
Simple Explanation
When two reactants don't combine in exactly the ratio the equation needs, one of them runs out first — this is the limiting reactant, and it determines the maximum amount of product that can form. The other reactant is left over in excess.
Why Do We Need It?
Real reactions rarely use perfectly matched amounts of every reactant, so identifying the limiting reactant is essential for predicting how much product you'll actually get.
Worked Example
Identify the limiting reactant
In the reaction 2H₂ + O₂ → 2H₂O, 4 mol of H₂ react with 1 mol of O₂. Which is the limiting reactant?
Why Does This Work?
Comparing how much of a reactant is actually needed (based on the mole ratio) to how much is actually available reveals which one will be used up first — and once it's gone, the reaction has to stop, regardless of how much of the other reactant remains.
Real-Life Example
Building furniture from a kit
A furniture kit needs 4 screws per shelf; you have 20 screws and 6 shelves.
20 screws only allow 5 shelves to be built, not 6 — the screws are the 'limiting reactant,' leaving one shelf's worth of wood unused, just like a limiting reactant leaves the other chemical in excess.
Practice
In N₂ + 3H₂ → 2NH₃, if 2 mol of N₂ react with 3 mol of H₂, which is limiting?
MediumCommon mistake
Assuming the reactant with the smaller number of moles is automatically the limiting one — you must compare using the balanced equation's mole ratio, not just raw mole amounts.
Quick Review
- The limiting reactant runs out first and determines the maximum product.
- The other reactant is left in excess.
- Compare using the mole ratio from the balanced equation, not raw amounts.