CBSE Class 11-science Answered

Balancing of a chemical equation means making the number of atoms of each element equal on both sides of the equation.

The following methods are generally used for balancing of chemical equations:

(1)  Hit and trial method or trial and error method

(2)  Partial equation method

Hit and Trial Method

The simplest method to balance a chemical equation is by the hit and trial method also called the trial and error method.

The method involves the following steps:

Step 1: Write the correct formulae of the reactants and the products with plus signs in between with an arrow pointing from the reactants to the products. This is called the skeleton equation.

Step 2: Select the largest formula from the skeleton equation and equalise the number of atoms of each of its constituent elements on both sides of the chemical equation by suitable multiplications.

Or
Balance the atoms of that element which occurs at the minimum number of places on both sides of the chemical equation first. Atoms which occur at a maximum number of places are balanced last of all.

Step 3: When an elementary gas (diatomic) such as hydrogen, oxygen, chlorine or nitrogen appears as a reactant or a product, the equation is balanced more easily by keeping the elementary gas in the atomic state. The balanced atomic equation is then made molecular by multiplying the whole equation by two.

Step 4: Verify that the number of atoms of each element is balanced in the final equation.

Step 5: The chemical equation can be made more informative by mentioning the physical states of the reactants and the products. Thus, the gaseous, liquid, aqueous and solid states of the reactants and products are represented by the notations (g), (l), (aq) and (s), respectively.

Note: Subscripts in the formulae of reactants and products cannot be changed for the sake of convenience to balance an equation. An integer placed in front of chemical formula multiples every atom of that formula by that integer. Thus,

2 NH₃ is 2 N atoms and 6 H atoms.
3 H₂O is 6 H atoms and 3 O atoms.

Partial Equation Method

The hit and trial method is useful only for balancing simple chemical reactions. For a reaction in which the same element is repeated in several compounds, the partial equation method is more helpful.

The method involves the following steps:

Step 1: The chemical reaction represented by the equation is supposed to proceed in two or more steps.

Step 2: The skeleton equations representing each step are written and then balanced by the hit and trial method. These equations are known as partial equations.

Step 3: If necessary, the partial equations are multiplied by suitable integers so as to cancel those intermediate products which do not occur in the final equation.

Step 4: The partial equations are added to get the final balanced equation.