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KMnO4 - Potassium Permanganate - Practice Questions & MCQ

Edited By admin | Updated on Sep 18, 2023 18:35 AM | #JEE Main

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MEDIUM

Amount of oxalic acid present in a solution can be determined by its titration with $KMnO_{4}$ solution in the presence of $H_{2}SO_{4}$ . The titration gives unsatisfactory result when carried out in the presence of $HCl$ , because $HCl$

oxidises oxalic acid to carbon dioxide and water

gets oxidised by oxalic acid to chlorine

Furnishes $H^{+}$ ions in addition to those from oxalic acid

reduces permanganate to $Mn^{2+}$

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Concepts Covered - 1

KMnO4

Potassium permanganate is prepared by fusion of MnO₂ with an alkali metal hydroxide and an oxidising agent like KNO₃. This produces the dark green K₂MnO₄ which disproportionates in a neutral or acidic solution to give permanganate.

$\mathrm{2MnO_{2}\: +\: 4KOH\:+\: O_{2}\: \rightarrow \: 2K_{2}MnO_{4}\: +\: 2H_{2}O}$

$\mathrm{3MnO_{4}^{2-}\: +\: 4H^{+}\:+\: O_{2}\: \rightarrow \: 2MnO_{4}^{-}\: +\:MnO_{2}\: +\: 2H_{2}O}$

Commercially it is prepared by the alkaline oxidative fusion of MnO₂ followed by the electrolytic oxidation of manganate (Vl).

In the laboratory, a manganese (II) ion salt is oxidised by peroxodisulphate to permanganate.

$\mathrm{2Mn^{2+}\: +\: 5S_{2}O_{8}^{2-}\: +\: 8H_{2}O\: \rightarrow \: 2MnO_{4}\: +\: 10SO_{4}^{2-}\: +\: 16H^{+}}$

Potassium permanganate forms dark purple (almost black) crystals which are isostructural with those of KClO₄. The salt is not very soluble in water (6.4 g/100 g of water at 293 K), but when heated it decomposes at 513 K.

$\mathrm{2KMnO_{4}\: \rightarrow \: K_{2}MnO_{4}\: +\: MnO_{2}\: +\: O_{2}}$

It has two physical properties of considerable interest: its intense colour and its diamagnetism along with temperature-dependent weak paramagnetism. These can be explained by the use of molecular orbital theory which is beyond the present scope.
The manganate and permanganate ions are tetrahedral; the π-bonding takes place by overlap of p orbitals of oxygen with d orbitals of manganese. The green manganate is paramagnetic because of one unpaired electron but the permanganate is diamagnetic due to the absence of unpaired electron.

A few important oxidising reactions of KMnO₄ are given below:
(1) In acidic solutions:

Iodine is liberated from potassium iodide:
$\mathrm{10I^{-}\: +\: 2MnO_{4}^{-}\: +\: 16H^{+}\: \rightarrow \: 2Mn^{2+}\: +\: 8H_{2}O\: +\: 5I_{2}}$
Fe²⁺ ion (green) is converted to Fe³⁺ (yellow):
$5 \mathrm{Fe}^{2+}+\mathrm{MnO}_{4}^{-}+8 \mathrm{H}^{+} \rightarrow \mathrm{Mn}^{2+}+4 \mathrm{H}_{2} \mathrm{O}+5 \mathrm{Fe}^{3+}$

(2) In neutral or faintly alkaline solutions:

A notable reaction is the oxidation of iodide to iodate:
$2 \mathrm{MnO}_{4}^{-}+\mathrm{H}_{2} \mathrm{O}+\mathrm{I^{-}} \longrightarrow 2 \mathrm{MnO}_{2}+2 \mathrm{OH}^{-}+\mathrm{IO}_{3}^{-}$
Thiosulphate is oxidised almost quantitatively to sulphate:
$8 \mathrm{MnO}_{4}^{-}+3 \mathrm{S}_{2} \mathrm{O}_{3}^{2-}+\mathrm{H}_{2} \mathrm{O} \longrightarrow 8 \mathrm{MnO}_{2}+6 \mathrm{SO}_{4}^{2-}+2 \mathrm{OH}^{-}$