POTASSIUM PERMANGANATE , Class - 12

POTASSIUM PERMANGANATE ( KMnO₄)

PREPARATION

Potassium permanganae is prepared from the mineral pyrolusite (MnO₂) in two steps:
(i)Conversion of MnO₂ into K₂MnO₄ (potassium manganate) by fusing
with KOH or K₂CO₃ in presence of air or oxidizing agent like KNO₃ , KClO₃ etc.
     2MnO₂ + 4KOH + O₂ → 2K₂MnO₄ + 2H₂O
or, 3MnO₂ + 6KOH + KClO₃ → 3K₂MnO₄+ KCl + 3H₂O

 
(ii)Oxidation of K₂MnO₄ to KMnO₄ chemically with CO₂,Cl₂, O₃ etc, or electrolytically(by electrolysis)
3K₂MnO₄ + 2CO₂ → 2 KMnO₄ + MnO₂ + 2 K₂CO₃

Electrolysis :-
K₂MnO₄ ⇋ 2K⁺ + MnO₄^2-
H₂O ⇋ H⁺ + OH ^-
MnO₄^2- → MnO₄^- + e^- (at cathode)
H⁺ + e^- → 1/2H₂( at cathode)

PROPERTIES

1.Colour:

Potassium permanganate exists as deep purple black prism with a greenish lusture which become dull in air due to superficial reduction.

2.Solubility:

It is moderately soluble in water at room temperature and it is more soluble in hot water.

3.Action of heat:

When heated to 513K, it readily decomposes giving oxygen.
2KMnO₄ → K₂MnO₄ + MnO₂ + O₂
At red heat , potassium permanganate formed decomposes into potassium manganite ( K₂MnO₃ )
2KMnO₄ → K₂MnO₃ + O₂

4.Oxidising property:

Potassium permanganate is a powerful oxidising agent.It behaves in different way in neutral , alkaline and acidic solutions.

(i) In neutral solution:

Potassium permanganate acts as a moderate oxidising agent in neutral aqueous solution because of the reaction :
2KMnO₄ + H₂O → 2KOH + 2MnO₂ + 3O
MnO₄^- + 2H₂O + 3e^- → MnO₂ + 4OH^-
Thus, MnO₄^- ion gains 3 electrons. Also , according to the above equation, in neutral medium, from two moles of permanganate, three oxygen atoms are available. In fact, during the course of reaction, the alkali generated renders the medium alkaline even when we start with neutral solutions.
Equivalent weight of KMnO₄ in neutral or weakly alkaline medium = molar weight/3 = ......
In the neutral medium,
(a) It oxidises hot manganous sulphate (MnSO₄ ) to manganese dioxide (MnO₂ ).
2KMnO₄ + MnSO₄ + 2H₂O → K₂SO₄ + 2H₂SO₄ + 5MnO₂
(b) It oxidises sodium thiosulphate (Na₂S₂O₃ ) to sodium sulphate (Na₂SO₄ )
8KMnO₄ + 3Na₂S₂O₃ + H₂O → K₂SO₄ + 8MnO₂ + 3 Na₂SO₄ + 2KOH
(c) It oxidises hydrogen sulphide (H₂S ) to sulphur.
2KMnO₄ + 4H₂S → 2MnS + S + K₂SO₄ + 4H₂O

(ii) In alkaline solution:

In strongly alkaline solution, MnO₄^2-(manganate) ion is produced.
2KMnO₄ + 2KOH → 2 K₂MnO₄ + H₂O + O
or, MnO₄^- + e^- → MnO₄^2-
Equivalent weight of KMnO₄ = Mol. wt/ 1 = ......
Potassium manganate is also further reduced to MnO₂ when a reducing agent is present.
K₂MnO₄ + H₂O → MnO₂ + 2KOH + O
or, MnO₄^2- + 2H₂O + 2e^- → MnO₂ + 4OH^-
So the complete reaction is :
2KMnO₄ + H₂O → 2MnO₂ + 2KOH + 3O
MnO₄^- + 2H₂O + 3e^- → MnO₂ + 4OH^-
Which is the same as that for neutral medium . Hence, equivalent weight of KMnO₄ in weakly alkaline medium is same as that in the neutral medium, viz, 52.67

(iii) In acidic solution: