During a redox process in acid solution, chlorate ions, ClO3⁻(aq), are converted into chlorine, Cl2(g) - Scottish Highers Chemistry - Question 18 - 2016

To balance the equation for the reduction of chlorate ions to chlorine gas, we need to follow these steps:

1. Identify the half-reaction: The reduction half-reaction for converting chlorate ions (ClO3⁻) to chlorine gas (Cl2) must be balanced in both mass and charge.

2. Write the unbalanced equation: The basic reaction can be represented as: $\text{2 ClO3}^- \rightarrow \text{Cl2} + \text{other species}$

3. Balance chlorine: Since there are 2 Cl from Cl2, we have: $\text{2 ClO3}^- \rightarrow \text{Cl2}$

4. Balance oxygen atoms: Since we have 6 oxygen atoms on the left, we need 6 water molecules on the right: $\text{2 ClO3}^- \rightarrow \text{Cl2} + 6 \text{H2O}$

5. Balance hydrogen: This produces 12 H atoms from 6 water molecules, which requires 12 H⁺ ions: $\text{12 H}^+ + \text{2 ClO3}^- \rightarrow \text{Cl2} + 6 \text{H2O}$

6. Combine to form the final balanced equation: The final balanced equation now shows it takes 12 H⁺ ions and 6 water molecules to balance the reaction, confirming option D as correct (12 and 6).