KMnO4 - Potassium Permanganate - Practice Questions & MCQ

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.

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.

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.

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:
  
• Fe²⁺ ion (green) is converted to Fe³⁺ (yellow):
  

(2) In neutral or faintly alkaline solutions:

• A notable reaction is the oxidation of iodide to iodate:
  
• Thiosulphate is oxidised almost quantitatively to sulphate: