3.Potassium Permanganatic (KMnO4)

Potassium Permanganatic (KMnO₄)

Physical Properties:

Purple coloured crystalline compound.

Moderately soluble in water at room temperature.

Structure of manganate and permanganate ion.

Chemical Properties

(i) Heating effect

MnO₄^2– in dilute alkaline, water and acidic solutions is unstable and disproportionates to give MnO₄^– and MnO₂.

3MnO₄^2– + 4H⁺ ¾®2MnO₄^– + MnO₂ ¯ + 2H₂O

3MnO₄^2– + 2H₂O ¾® 2MnO₄^– + MnO₂ ¯ + 4OH^–        

or

3MnO₄^2– + 3H₂O ¾® 2MnO₄^– + MnO (OH)₂ ¯+ 4OH^– 

On heating in current of H₂ , solid KMnO₄ gives MnO

2KMnO₄  + 5H₂  2KOH + 2MnO + 4H₂O

(ii) On treatment with concentrated H₂SO₄ (KMnO₄ is taken in excess), it forms manganese heptoxide via permanganyl sulphate which decomposes explosively on heating.

2KMnO₄ + 3H₂SO₄ ¾® 2KHSO₄ + (MnO₃)₂SO₄ + 2H₂O

(MnO₃)₂SO₄ + H₂O  ¾®  Mn₂O₇ + H₂SO₄

Mn₂O₇ ¾®  2MnO₂ + O₂

 KMnO₄ + 3H₂SO₄ (conc.) ¾® K⁺ + MnO₃⁺ (green) + 3HSO₄^– + H₃O⁺.

 (iii)  Potassium permanganate is a powerful oxidising agent.

Potassium permanganate acts as an oxidising agent in alkaline, neutral or acidic solutions.

A mixture of sulphur, charcoal and KMnO₄ forms an explosive powder. A mixture of oxalic acid and KMnO₄ catches fire spontaneous after a few seconds. The same thing happens when glycerine is poured over powdered KMnO₄.

In alkaline & neutral medium :

In strongly alkaline medium KMnO₄ is reduced to manganate.

2KMnO₄ + 2KOH (conc.)  ¾® 2K₂ MnO₄ + H₂O + [O]

or    e^– + MnO₄^– ¾® MnO₄^2–

if solution is dilute then K₂MnO₄ is converted in to MnO₂ which appears as a  brownish precipitate.

2K₂MnO₄ + 2H₂O ¾® 2MnO₂ + 4KOH + 2[O]

2e^– + 2H₂O + MnO₄^2– ¾® MnO₂ + 4OH^–

This type of behaviour is shown by KMnO4 itself in neutral medium.

3e^– + 2H₂O + MnO₄^– ¾® MnO₂ + 4OH^–

In alkaline or neutral medium KMnO₄ shows following oxidising properties.

(a) It oxidises ethene to glycol.

In alkaline medium KMnO₄ solution is also known as Bayer’s reagent (1% alkaline KMnO₄ solution).

(b) It oxidises iodide into iodate.

(c)  H₂S is oxidised into sulphur :

2MnO₄^– + 3H₂S  ¾®  2MnO₂ + 2OH^– +2H₂O + 3S

(d) Manganous salt is oxidised to MnO₂ ; the presence of zinc sulphate or zinc oxide catalyses the oxidation :

2MnO₄^– + 3Mn₂⁺ + 2H₂O ¾® 5MnO₂ + 4H⁺

(e) In neutral/ faintly alkaline solution thiosulphate is quantitatively oxidised to sulphate.

8MnO₄^– + 3S₂O₃^2– + H₂O  ¾® 8MnO₂ + 6SO₄^2– + 2OH^–

(f) In the presence of sodium hydroxide, sodium sulphite is oxidised in to sodium sulphate.   

2MnO₄^– + 3SO₃^2– + 3H₂O ¾® 2MnO(OH)_2¯+ 3SO₄^2– + 2OH^–.

In acidic medium (in presence of dilute H₂SO₄) :

Manganous sulphate is formed. The solution becomes colourless.

2KMnO₄ + 3H₂SO₄ ¾® K₂SO₄ + 2MnSO₄ + 3H₂O + 5[O]

or       MnO4¯ + 8H+ + 5e^– ¾® Mn²⁺ + 4H₂O

This medium is used in quantitative (volumetric) estimations. The equivalent mass of KMnO4 in acidic medium is = .

(a) Ferrous salts are oxidised to ferric salts.

MnO₄¯ + 5Fe²⁺ + 8H+ ¾® 5Fe³⁺ + Mn²⁺ + 4H₂O

(b) Iodine is evolved from potassium iodide.

2MnO4¯ + 10I¯ + 16H⁺ ¾® 2Mn²⁺ + 5I₂ + 8H₂O

(c) H₂S is oxidised to sulphur :

2MnO₄^– + 16H⁺+ 5S^2– ¾® 2Mn²⁺ + 8H₂O + 5S

(d) SO₂ is oxidised to H₂SO₄ :

2MnO₄^– + 5SO₂ + 2H₂O ¾® 5SO₄^2– + 2Mn²⁺ + 4H⁺

(e) Nitrites are oxidised to nitrates :

2MnO₄^– + 5NO₂^– + 6H⁺ ¾® 2Mn²⁺ + 3H₂O + 5NO₃^–

(f) Oxalic acid is oxidised to CO₂ :

This reaction is slow at room temperature, but is rapid at 60ºC.

Mn(II) ions produced catalyse the reaction; thus the reaction is autocatalytic.

2MnO₄^– + 16H⁺ + 5C₂O₄^2–2Mn²⁺ + 8H₂O + 10CO₂

(g) HCHO is oxidised to HCOOH

2MnO₄^– + 5HCHO + 6H⁺ ¾® 2Mn²⁺ + 5HCOOH + 3H₂O.

(h) It oxidises hydrogen halides (HCl, HBr or HI) into X₂ (halogen)

2MnO4^– + 16H⁺ + 10X^– ¾® 2Mn²⁺ + 8H₂O + 5X₂ 

(i)  H₂O₂ is oxidised to O₂.

 2MnO₄^– + 6H⁺ + 5H₂O₂ ¾® 5O₂ + Mn2⁺ + 8H₂O.

Uses :It is used

1. KMnO₄ is used as an oxidising agent in laboratory and industry.
2. Alkaline potassium permanganate is called Bayer's reagent. This reagent is used in organic chemistry for the test of unsaturation. KMnO₄ is used in the manufacture of saccharin, benzoic acid, acetaldehyde etc.
3. KMnO₄ is used in qualitative analysis for detecting halides, sulphites, oxalates, etc.