Limiting Reagent: Definition, Methods and Example

When one of the reactants in a chemical reaction is insufficient, the reaction stops abruptly. To calculate the amount of product produced, it is necessary to first determine which reactant will limit the chemical reaction (limiting reagent) and which reactant is in excess (excess reagent). One method for determining the limiting reagent is to calculate the amount of product that each reactant can produce. The one that produces the least amount of product is the limiting reagent. Let’s take a closer look at Limiting Reagents.

What is Limiting Reagents?

Limiting reagents are substances that are completely consumed during the course of a chemical reaction. They are also known as limiting agents or limiting reactants. According to the stoichiometry of the chemical reactions, a fixed amount of reactants is required for the reaction to complete.

Let us consider the following reaction for the formation of ammonia:

3H2 + N2 —> 2NH3

In the above reaction, 3 moles of hydrogen gas are required for the reaction with 1 mole of nitrogen gas to produce 2 moles of ammonia. But what if there are only 2 moles of hydrogen gas and 1 mole of nitrogen available during the reaction?

Here, the entire amount of nitrogen cannot be used as it requires 3 moles of hydrogen gas to react. As a result, the hydrogen gas limits the reaction and is thus referred to as the limiting reagent for this reaction.

Explanation: Generally, this reactant determines when the reaction would stop. The reaction stoichiometry determines the exact amount of reactant that would be needed to react with another element. The limiting reagent depends on the mole ratio and not on the masses of the reactants.

How to find Limiting Reagent?

The limiting reagent or reactant can be determined by two methods. One method is to calculate and compare the mole ratios of the reactants used in the reaction. And the other method is to calculate the grams of products produced from the quantities of reactants, in which the reactant that produces the smallest amount of product is the limiting reagent.

Method 1: Using the mole ratio

Identify the balanced chemical equation for the chemical reaction.
Convert all the given information into moles by using molar mass as a conversion factor.
Next, calculate the mole ratio from the given information. And then compare the calculated ratio to the actual ratio.
Calculate the amount of product produced using the amount of limiting reactant.
Calculate how much of the non-limiting agent is left in excess (if necessary).

Method 2: Using the product approach

For the given chemical reaction, balance the chemical equation.
Next, convert the given information into moles.
To determine the mass of the product produced, use stoichiometry for each individual reactant.
The reactant which produces a lesser amount of product would be the limiting reagent and the reactant that produces a larger amount of product would be the excess reagent.
Finally, to calculate the amount of remaining excess reactant, subtract the mass of excess reagent consumed from the total mass of excess reagent given.

Limiting Reagent Examples

What would be the limiting reagent if 78 grams of Na2O2 were reacted with 29.4 grams of H2O?

Method 1:

A. 78g x (1 mol/77.96g) = 1.001 moles of Na2O2

29.4g x (1 mol/18g)= 1.633 moles of H2O

B. Let us assume that all of the water is consumed, 1.633 x (2/2) or 1.633 moles of Na2O2 are required. As there are only 1.001 moles of Na2O2, it is the limiting reactant.

Method 2:

78g Na2O2 x (1 mol Na2O2)/(77.96g Na2O2) x (4 mol NaOH)/(2 mol Na2O2) x (40g NaOH)/(1 mol NaOH)

= 80.04g NaOH

As a result, we discover that either formula yields Na2O2 as the limiting reagent.

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