Use the half-reaction method to balance the following equation which is in an acidic
solution:
clo3-(aq) + i- (aq)
+i2 (s) + cl- (aq)

ClO₃⁻ + 6I⁻ + 6H⁺ ⟶ Cl⁻ + 3I₂ + 3H₂O

Explanation:

Step 1. Write the skeleton equation

ClO₃⁻ + I⁻ ⟶ I₂ + Cl⁻

Step 2. Separate into two half-reactions.

ClO₃⁻ ⟶ Cl⁻

      I⁻ ⟶ I₂

Step 3. Balance all atoms other than H and O

ClO₃⁻ ⟶ Cl⁻

   2I⁻ ⟶ I₂

Step 4. Balance O by adding H₂O molecules to the deficient side.

ClO₃⁻ ⟶ Cl⁻ + 3H₂O

   2I⁻ ⟶ I₂

Step 5. Balance H by adding H⁺ ions to the deficient side.

ClO₃⁻ + 6H⁺ ⟶ Cl⁻ + 3H₂O

              2I⁻ ⟶ I₂

Step 6. Balance charge by adding electrons to the deficient side.

ClO₃⁻ + 6H⁺ + 6e⁻ ⟶ Cl⁻ + 3H₂O

                        2I⁻ ⟶ I₂ + 2e⁻

Step 7. Multiply each half-reaction by a number to equalize the electrons transferred.

1 × [ClO₃⁻ + 6H⁺ + 6e⁻ ⟶ Cl⁻ + 3H₂O]

3 × [                         2I⁻ ⟶ I₂ + 2e⁻]

Step 8. Add the two half-reactions.

        ClO₃⁻ + 6H⁺ + 6e⁻ ⟶ Cl⁻ + 3H₂O

                                6I⁻ ⟶ 3I₂ + 6e⁻                      

ClO₃⁻ + 6I⁻ + 6H⁺ + 6e⁻ ⟶ Cl⁻ + 3I₂ + 3H₂O + 6e⁻

Step 9. Cancel species that occur on each side of the equation

ClO₃⁻ + 6I⁻ + 6H⁺ + 6e⁻ ⟶ Cl⁻ + 3I₂ + 3H₂O + 6e⁻

becomes

ClO₃⁻ + 6I⁻ + 6H⁺ ⟶ Cl⁻ + 3I₂ + 3H₂O

Step 10. Check that all atoms are balanced.

Step 11. Check that charge is balanced

Everything checks. The balanced equation is

ClO₃⁻ + 6I⁻ + 6H⁺ ⟶ Cl⁻ + 3I₂ + 3H₂O