During the reaction of permanganate with thiosulphate, the change in oxidation of manganese occurs by value of 3. Identify which of the below medium will favour the reaction.

Solution

Chemistry ma’am · 3 steps

A change of 3 means Mn goes from +7 to +4, and that MnO₂ forms only when the solution is not acidic.

1. Read the change. Manganese in MnO₄⁻ is +7. A drop of 3 lands at +4, which is MnO₂. So the question is really asking where permanganate stops at MnO₂.

   Medium	Product of MnO₄⁻	Mn oxidation state	Change
   Acidic	Mn²⁺	+2	5
   Neutral or faintly alkaline	MnO₂	+4	3
   Strongly alkaline	MnO₄²⁻	+6	1

2. The reaction. In neutral solution, thiosulphate is oxidised to sulphate and permanganate goes to MnO₂. Electrons balance: 8 Mn gain 3 each (24), and 3 thiosulphates lose 8 each (24).

   8MnO₄⁻ + 3S₂O₃²⁻ + H₂O → 8MnO₂ + 6SO₄²⁻ + 2OH⁻

3. So. The reaction is favoured in aqueous neutral medium, option (a). Any option that includes acidic medium fails, because acid takes manganese all the way to +2.

The idea to keep. Read the medium from the Mn product: acidic gives Mn²⁺ (change 5), neutral or faintly alkaline gives MnO₂ (change 3), strongly alkaline gives MnO₄²⁻ (change 1).

Answer from the NTA final answer key. Solution by PrepWiser.