Manganese ions on reaction with peroxodisulphate ions form:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → \(MnO_4^-\)
The peroxodisulphate ion, \( \ce{S2O8^{2-}} \), is a very strong oxidising agent. It oxidises the manganese(II) ion right up to the highest oxidation state of Mn, +7, in the permanganate ion; this is the laboratory method of making permanganate.
Writing the two half-reactions:
\( \ce{Mn^2+ + 4H2O -> MnO4- + 8H+ + 5e-} \) (Mn: +2 → +7)
\( \ce{S2O8^2- + 2e- -> 2SO4^2-} \)
Multiplying the first by 2 and the second by 5 balances the electrons (10 each):
\( \ce{2Mn^2+ + 5S2O8^2- + 8H2O -> 2MnO4- + 10SO4^2- + 16H+} \)
Why the other options are incorrect
Option (1) \(MnO_4^{2-}\) — Incorrect. The green manganate ion (Mn +6) is made by fusing \( \ce{MnO2} \) with KOH and an oxidising agent such as \( \ce{KNO3} \); in acid it disproportionates to \( \ce{MnO4^{-}} \) and \( \ce{MnO2} \).
Option (2) \(MnO_2\) — Incorrect. \( \ce{MnO2} \) (Mn +4) is what permanganate is reduced to in neutral or faintly alkaline solution; \( \ce{S2O8^{2-}} \) takes \( \ce{Mn^{2+}} \) further, to +7.
Option (3) \(Mn_2O_7\) — Incorrect. Manganese heptoxide is the acidic oxide formed when \( \ce{KMnO4} \) reacts with concentrated \( \ce{H2SO4} \); it is not formed in aqueous solution.
NCERT (2026-27 edition):
Potassium Permanganate – …
