3.2.4 Limiting Reactants (HT only)

The limiting reactant is the reactant that is completely consumed and determines the maximum amount of product that can be obtained. In this lesson, we will delve into the details and learn how to identify and calculate limiting reactants.

Understanding Limiting Reactants

When two or more reactants are involved in a chemical reaction, the reactant present in the lesser amount (in terms of stoichiometry or molar ratio) limits the amount of product that can be formed. The limiting reactant is entirely consumed, while the excess reactant remains unreacted.

Identifying the Limiting Reactant

To identify the limiting reactant, follow these steps:

1. Write the balanced chemical equation for the reaction.
2. Determine the moles or masses of the reactants.
3. Calculate the moles of the products that can be obtained from each reactant, assuming complete reaction.
4. The reactant that yields the smaller amount of product is the limiting reactant.

Calculating the Maximum Amount of Product

Once you have identified the limiting reactant, you can calculate the maximum amount of product that can be obtained using the stoichiometry of the balanced equation.

• Determine the molar ratio between the limiting reactant and the desired product.
• Multiply the moles of the limiting reactant by the stoichiometric coefficient of the desired product to calculate the moles of product that can be formed.
• Convert the moles of product to grams, if necessary, using the molar mass of the product.

Example

Let's consider the reaction between hydrogen gas (H₂) and oxygen gas (O₂) to form water (H₂O):

2H₂ + O₂ → 2H₂O

• Given 4 moles of H₂ and 3 moles of O₂.
• Calculate the moles of water that can be produced from each reactant.
• Determine that 3 moles of water can be produced from 3 moles of O₂, while only 4 moles of water can be produced from 4 moles of H₂.
• The limiting reactant is oxygen gas (O₂).

Calculating Excess Reactant

Once the limiting reactant is determined, you can calculate the amount of excess reactant that remains unreacted. This can be done by subtracting the amount of the limiting reactant consumed from the initial amount of the excess reactant.

Conclusion

Understanding and identifying the limiting reactant is essential for predicting the maximum amount of product that can be obtained in a chemical reaction. By comparing the amounts of reactants and applying stoichiometry, we can determine which reactant is consumed completely and which is in excess. Calculating the maximum amount of product allows us to optimise reactions and avoid wasting reactants.