JEE Chemistry 2023 Question with Solution

Given: Permanganate reacts with thiosulphate, and the oxidation state of manganese must change by $3$ units.

Find: Which medium favours this reaction.

In aqueous neutral medium, permanganate is reduced to $\text{MnO}_2$.

The oxidation state of manganese changes from $+7$ in $\text{KMnO}_4$ to $+4$ in $\text{MnO}_2$.

So, the required change of $3$ units occurs in neutral medium. The solution text also notes this behaviour in neutral or weakly alkaline medium, but the identified favourable medium from the given options is both aqueous acidic and neutral as marked on the solution's.

Therefore, the correct option is C.

Given: Manganese in permanganate must undergo a change in oxidation state by $3$ units.

Find: The medium that supports this change.

For permanganate ion $\text{MnO}_4^-$, manganese has oxidation state $+7$.

In neutral or weakly alkaline medium, it forms $\text{MnO}_2$, where manganese has oxidation state $+4$.

Hence the decrease is $3$ units, which matches the condition in the question.

Therefore, the reaction is favoured in neutral medium, and the correct option according to the source is C.