CBSE Class 11-science Answered

Let us understand ion exchange method for balancing the chemical equation by considering the following unbalanced reaction.

Cr(OH)₃(s) + ClO₃ ⁻( aq) →  CrO₄ ²⁻(aq) + Cl⁻(aq) (basic)

 Step-1 Split the reaction in two half cell reaction and balance the central atom and other elements on the basis of priority of metal>non-metal>oxygen>hydrogen. Oxygen and hydrogen will be balanced by H₂O and OH⁻ only.

Cr(OH)₃ + 5HO⁻ → CrO₄ ²⁻ + 4H₂O

ClO₃ ⁻ + 3H₂O  → Cl⁻ + 6OH⁻

Step-2 Balance the charge by number of electron. Consider 1e ⁻ = -1

Cr(OH)₃ + 5HO⁻ →  CrO₄ ²⁻ + 4H₂O + 3e⁻

ClO₃ ⁻ + 3H₂O + 6e⁻  → Cl⁻ + 6OH⁻

Step-3 Cross multiplies the equation by the coefficient of electron i.e. 3 and 6 or 1and 2. Add the equation

2Cr(OH)₃ + ClO₃ ⁻ + 3H₂O + 10 OH⁻ →  2CrO₄ ²⁻ + Cl⁻ + 8H₂O + 6OH⁻

Step- 4 Simplify the equation to get the balanced equation.

2Cr(OH)₃(s) + ClO₃  ⁻  (aq) + 4OH  ⁻  (aq)  →  2CrO₄ ^{2  −}  (aq) + Cl  ⁻  (aq) + 5H₂O(l)