Q.Permanganate ion changes to ________ in acidic medium

KMnO₄'s oxidising behaviour depends sharply on the reaction medium. On heating it disproportionates: 2KMnO₄ →(Δ) K₂MnO₄ + MnO₂ + O₂ (the standard lab method for generating O₂ gas). With cold conc. H₂SO₄ it forms unstable, explosive Mn₂O₇; with hot conc. H₂SO₄ it forms MnSO₄.

Neutral medium (reduced only to MnO₂, 3-electron change): MnO₄⁻ + 2H₂O + 3e⁻ → MnO₂ + 4OH⁻ -- e.g. oxidises H₂S to S, and thiosulphate to sulphate. Alkaline medium (via a manganate intermediate, MnO₄⁻ + e⁻ → MnO₄²⁻, then further reduced to MnO₂ -- overall the same 3-electron equation as neutral): cold dilute alkaline KMnO₄ is Baeyer's reagent, oxidising alkenes to vicinal diols (e.g. ethylene → ethylene glycol), the classic test for C=C unsaturation. Acidic medium (the strongest pathway, 5-electron change, all the way to Mn²⁺): MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O -- oxidises Fe²⁺→Fe³⁺, I⁻→I₂, oxalic acid→CO₂ (the basis of standardising KMnO₄ against oxalic acid/oxalate), S²⁻→S, NO₂⁻→NO₃⁻, ethanol→acetaldehyde (a MILDER, partial oxidation versus dichromate's complete oxidation to acetic acid), and SO₃²⁻→SO₄²⁻. …