How to Find the Limiting Reactant in a Chemical Reaction (Honors Chem, Step by Step)

In every chemical reaction, substances combine in specific proportions. Sometimes, one reactant is used up before the others, which prevents the reaction from continuing. This reactant is called the limiting reactant because it limits the amount of product that can form.

The concept of a limiting reactant is similar to making sandwiches. If you have 8 slices of bread and 5 slices of cheese, you can only make 4 sandwiches. Once you run out of bread, you cannot make more sandwiches, even if you still have cheese left. Chemistry follows the same idea.

Step 1: Write and Balance the Chemical Equation

Every limiting reactant problem begins with a balanced equation.

For example:

N₂ + 3H₂ → 2NH₃

This equation shows that 1 mole of nitrogen reacts with 3 moles of hydrogen to produce 2 moles of ammonia.

Step 2: Convert the Given Amounts to Moles

Suppose we start with 5.0 moles of N₂ and 12.0 moles of H₂. To determine which reactant will run out first, we compare the amount of each substance based on the balanced equation.

Step 3: Compare the Mole Ratios

According to the equation, 1 mole of N₂ reacts with 3 moles of H₂.

If we have 5.0 moles of N₂, we would need:

5.0 mol N₂ × (3 mol H₂ / 1 mol N₂) = 15.0 mol H₂

However, we only have 12.0 mol H₂, which is not enough to completely react with all the nitrogen. Therefore, H₂ is the limiting reactant, and N₂ is the excess reactant.

Step 4: Use the Limiting Reactant to Find the Product Formed

Because hydrogen is the limiting reactant, we use its amount to calculate how much ammonia will form.

From the equation, 3 mol H₂ produce 2 mol NH₃.

So:

12.0 mol H₂ × (2 mol NH₃ / 3 mol H₂) = 8.0 mol NH₃ produced

This means 8.0 moles of ammonia will be formed before the hydrogen runs out.

Why It Matters

Understanding limiting reactants helps chemists predict the amount of product formed in a reaction and avoid waste. In industries such as fertilizer or medicine production, knowing which reactant limits the reaction ensures that materials are used efficiently and safely

Practice Problem

If 4.0 moles of aluminum react with 10.0 moles of chlorine gas according to the equation

2Al + 3Cl₂ → 2AlCl₃,

which reactant is limiting, and how many moles of AlCl₃ can be produced?

(Hint: Start by comparing the mole ratios just like in the example above.)

Want More Practice?

Watch this helpful tutorial from @TheOrganicChemistryTutor for a step-by-step explanation of limiting reactant problems.