In many chemical reactions, one reactant is used up first. This is called the limiting reactant – it limits the amount of product that can be made.

What is a limiting reactant?

• The limiting reactant is the reactant that runs out first
• Once it’s used up, the reaction stops – no more product can form
• The other reactant is in excess – there’s more than needed

💡 The amount of product formed is directly proportional to the amount of limiting reactant

Why do reactions have a limiting reactant?

In real life, we often add extra of one reactant to make sure another one fully reacts.
This ensures:

• A cheaper or more available reactant is in excess
• A more expensive reactant is used efficiently

Example: hydrogen + oxygen → water

Balanced equation:
2H₂ + O₂ → 2H₂O

If you had:

• 4 moles of H₂
• 1 mole of O₂

You can only make 2 moles of H₂O because O₂ is limiting
(Hydrogen is in excess – only 2 moles are needed)

Steps to identify the limiting reactant

1. Write a balanced equation
2. Calculate moles of each reactant
3. Use the mole ratio from the equation
4. The reactant that runs out first = limiting reactant

Effect on product

The mass of product depends on:

• The amount of limiting reactant
• The mole ratio in the balanced equation

If you double the limiting reactant, you can double the amount of product.