Limiting reagents are the substances that are completely consumed in the completion of a chemical reaction. They are also referred to as limiting agents or limiting reactants.
According to the stoichiometry of chemical reactions, a fixed amount of reactants are required for the completion of the reaction. Let us consider the following reaction of formation of ammonia:
3H2 + N2 → 2NH3
In the reaction given above, 3 moles of Hydrogen gas are required to react with 1 mole of nitrogen gas to form 2 moles of ammonia. But what if, during the reaction, only 2 moles of hydrogen gas is available along with 1 mole of nitrogen. In that case, the entire quantity of nitrogen cannot be used (because the entirety of nitrogen requires 3 moles of hydrogen gas to react). Hence, the hydrogen gas is limiting the reaction and is therefore called the limiting reagent for this reaction.
What is Limiting Reagent?
The reactant that is entirely used up in a reaction is called as limiting reagent. This reactant generally determines when the reaction will stop. The exact amount of reactant which will be needed to react with another element can be calculated from the reaction stoichiometry. The limiting reagent depends on the mole ratio, not on the masses of the reactants present.
From the illustration shown above, it can be observed that the limiting reactant is the reason the reaction cannot continue since there is nothing left to react with the excess reactant. it is the reactant that entirely consumed over the course of the reaction.
Limiting Reagent Examples
Consider 1 mol of oxygen and 1 mol of hydrogen are present to undergo the following reaction.
2H2 + O2 → 2H2O
Since the reaction uses up hydrogen twice as fast as oxygen, the limiting reactant would be hydrogen.
Example: 100g of hydrochloric acid is added to 100g of zinc. Find the volume of hydrogen gas evolved under standard laboratory conditions.
The chemical equation for this reactions is given below.
2HCl(aq) + Zn(s) → ZnCl2(aq) + H2(g)
Zinc chloride is formed in excess so the limiting reagnet here is hydrochloric acid.
73g of HCl = 24.5l of H2
100g of HCl = yL of H2
y/24.5 = 100/73
y = (100 x 24.5)/73
y = 33.6L
Therefore 33.6L of H2 is produced under standard laboratory conditions.
How to find Limiting Reagent?
The determination of the limiting reactant is typically just a piece of a larger puzzle. In most limiting reactant stoichiometry problems, the real goal is to determine how much product could be formed from a particular reactant mixture. The limiting reactant or reagent can be determined by two methods.
- Using the mole ration
- Using the product approach
In order to calculate the mass of the product first, write the balanced equation and find out which reagent is in excess. Using the limiting reagent calculate the mass of the product.
The Following points should be considered while attempting to identify the limiting reagent:
- When there are only two reactants, write the balanced chemical equation and check the amount of reactant B required to react with reactant A. When the amount of reactant B is greater, the reactant A is the limiting reagent.
- The reactant which is in a lesser amount than is required by stoichiometry is the limiting reactant.
- In an alternate method of finding the limiting agent, the amount of product formed by each reactant is calculated.
- The limiting reactant is the reactant from which the minimum amount of product is formed.
- Also if we calculate the amount of one reactant needed to react with another reactant, then the reactant which is in shortage would be the required limiting reactant.
Thus the required limiting reagent for the reaction can be identified using the points provided above. These reagents are very important while calculating the percentage yield of a given reaction.