Limiting Reagent

Limiting Reagent

Introduction

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 above reaction, 3 moles of Hydrogen gas is required to react with 1 mole of nitrogen gas to form 2 units 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 whole of nitrogen cannot be used as for that 3 moles of hydrogen gas is required. Hence, the hydrogen gas is limiting the reaction and so is called limiting reagent.

What is Limiting Reagent?

The reactant that is entirely used up in a reaction is called as limiting reagent. It 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 limits the reaction from continuing as there is nothing left to react with the excess reactants. it is the one that gets totally consumed.

Limiting Reactant Example

Given: 1 mol of oxygen and 1 mol of hydrogen in the reaction.

2 H2 + O2 → 2 H2O

The reaction uses up hydrogen twice as fast as oxygen, so the limiting reactant would be hydrogen.

How to find the Limiting Reagent?

Question on limiting reagent is of much importance in chemistry, so identifying limiting reagent is important. Students often make mistakes in identifying this reagent and so can’t proceed correctly with these questions. Following points should be considered while identifying limiting reagent:

  • Write the balanced chemical equation and if the amount of an available reactant is not in the ratio as in the balanced chemical equation, then that reaction will be limiting reaction.
  • The reactant which is in a lesser amount than is required by stoichiometry is the limiting reagent.
  • There’s one more method of calculation of limiting reagent in the reaction. In this method, the amount of product formed by each reactant is calculated. The limiting reagent is the one 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 is known as limiting reagent.

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Practise This Question

Decreasing order of first ionization energy is