When Sulphur(IV)oxide is passed into solution of acidified tetraoxomanganate(VII), the colour changes from

Analysis

The question asks for the color change observed when Sulphur(IV) oxide ($\text{SO}_2$) reacts with an acidified solution of tetraoxomanganate(VII) ($\text{KMnO}_4$). This is a classic redox reaction frequently tested in JAMB Chemistry.

Concept Explanation

1. The Reactants: • Acidified potassium tetraoxomanganate(VII) ($\text{KMnO}_4$) is a powerful oxidizing agent. In its acidic state, the manganese ion is in the $+7$ oxidation state, which gives the solution a deep purple color. • Sulphur(IV) oxide ($\text{SO}_2$) acts as a reducing agent.

2. The Reaction: When $\text{SO}_2$ is bubbled through the acidified $\text{KMnO}_4$ solution, the $\text{SO}_2$ is oxidized to sulphate ions ($\text{SO}_4^{2-}$), and the manganese(VII) ions are reduced to manganese(II) ions ($\text{Mn}^{2+}$).

The balanced ionic equation is: 2\text{MnO}_4^-_{(aq)} + 5\text{SO}_{2(g)} + 2\text{H}_2\text{O}_{(l)} \rightarrow 2\text{Mn}^{2+}_{(aq)} + 5\text{SO}_4^{2-}_{(aq)} + 4\text{H}^+_{(aq)}

3. The Color Change: • The $\text{Mn}^{2+}$ ion in aqueous solution is virtually colourless (or very pale pink, which is often considered colourless in the context of this specific reaction). • Therefore, the intense purple color of the permanganate solution disappears as the reaction proceeds.

Step-by-Step Solution

• Step 1: Identify the initial state. Acidified $\text{KMnO}_4$ is purple.
• Step 2: Identify the reaction type. This is a redox reaction where $\text{KMnO}_4$ is reduced.
• Step 3: Determine the final state. The reduction of $\text{Mn}^{7+}$ to $\text{Mn}^{2+}$ results in a loss of color.
• Step 4: Match with options. The change is from purple to colourless.

Conclusion

The reaction causes the purple color of the permanganate to be discharged.

Exam Tip: Always remember that acidified $\text{KMnO}_4$ is a "color-killer" for reducing agents. If you see it in a question, look for the option that involves the disappearance of the purple color!