d Block Elements : Potassium Permanganate

 

Potassium Permanganate, (KMnO4)

Potassium permanganate is a salt of an unstable acid KMnO4 (permanganic acid). The Mn is and +7 state in this compound.

Preparation : Potassium permanganate is obtained from pyrolusite as follows.

Conversion of pyrolusite to potassium manganite : When manganese dioxide is fused with potassium hydroxide in the presence of air or an oxidising agent such as potassium nitrate or chlorate, potassium manganate is formed, possibly via potassium manganite.

MnO2 + 2KOH \underrightarrow { \quad fused\quad }   K2MnO3 + 4H2] × 2

                                                              Potassium manganite

2K2MnO3 + O2 → 2K2MnO4 + 2H2

\underset { pyrolusite }{ 2{ MnO }_{ 2 } } \quad +\quad 4KOH+{ O }_{ 2 }\quad \rightarrow \quad \underrightarrow { \quad fused\quad }

\underset { potassium\quad manganate\\ dark-green\quad mass }{ 2{ K }_{ 2 }{ MnO }_{ 4 } } +\quad { 2H }_{ 2 }O

 

Oxidation of potassium manganate to potassium permanganate : The potassium manganate so obtained is oxidised to potassium permanganate by either of the following methods.

By chemical method : The fused dark-green mass is extracted with a small quantity of water. The filtrate is warmed and treated with a current of ozone, chlorine or carbon dioxide. Potassium manganate gets oxidised to potassium permanganate and the hydrated manganese dioxide precipitates out. The reactions taking place are,

When CO2 is passed

3K2MnO4 + 2H2O → 2KMnO4 + MnO2↓ + 4KOH

Potassium manganate        potassium permanganate

2CO2 + 4KOH → 2K2CO3 + 2H2O

When chlorine or ozone is passed

2K2MnO4 + Cl2 → 2KMnO4 + 2KCl

2K2MnO4 + O3 + H2O → 2KMnO4 + 2KOH + O2(g)

The purple solution so obtained is concentrated and dark purple, needle-like crystals having metallic lustre are obtained.

 

Electrolytic method : Presently, potassium manganate (K2MnO4) is oxidised electrolytically. The electrode reactions are,

          At anode : 2MnO42– → 2MnO4– + 2e–

                                      Green                 purple

          At cathode : 2H+ + 2e– → H2(g)

The purple solution containing KMnO4 is evaporated under controlled condition to get crystalline sample of potassium permanganate.

 

Physical properties

KMnO4 crystallizes as dark purple crystals with greenish luster (m. p. 523 K).

It is soluble in water to an extent of 6.5g per 100g at room temperature. The aqueous solution of KMnO4 has a purple colour.

 

Chemical properties:

Some important chemical reactions of KMnO4 are given below,

Action of heat : KMnO4 is stable at room temperature, but decomposes to give oxygen at higher temperatures.

                   2KMnO4 (s) \underrightarrow { \quad heat\quad } K2MnO4(s) + MnO2 + O2(g)

 

Oxidising actions : KMnO4 is a powerful agent in neutral, acidic and alkaline media. The nature of reaction is different in each medium. The oxidising character of KMnO4 (to be more specific, of MnO) is indicated by high positive reduction potentials for the following reactions.

Acidic medium : MnO4– + 8H+ + 5e– → Mn2+ + 4H2O      Eo = 1.51 V

Alkaline medium : MnO4– + 2H2O + 3e– → MnO2 + 4OH–      Eo = 1.23 V

In strongly alkaline solutions and with excess of MnO, the reaction is

                   MnO4– + e– → MnO42–                   Eo = 0.56 V                  

There are a large number of oxidation-reduction reactions involved in the chemistry of manganese compounds. Some typical reactions are,

In the presence of excess of reducing agent in acidic solutions permanganate ion gets reduced to manganous ion, e.g.,

                   5F2+ + MnO4– + 8H+ → 5Fe3+ + Mn2+ + 4H2O

An excess of reducing agent in alkaline solution reduces permanganate ion only to manganese dioxide e.g.,

                   3NO2– + MnO4– + 2OH– → 3NO3– + MnO2 + H2O

In faintly acidic and neutral solutions, manganous ion is oxidised to manganese oxidised to manganese dioxide by permanganate.

                   2MnO4– + 3Mn+2 + 2H2O → 5MnO2 + 4H+

In strongly basic solutions, permangante oxidises manganese dioxide to manganate ion.

                   MnO2 + 2MnO4– + 4OH– → 3MnO42– + 2H2O

In acidic medium, KMnO4 oxidises,

 

Ferrous salts to ferric salts

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

                   2FeSO4 + H2SO4 + [O] → Fe2(SO4)3 + H2O] × 5

2KMnO4 + 8H2SO4 + 10FeSO4 → K2SO4 + 2MnSO4 + 5Fe2(SO4)3 + 8H2O

Ionic equation : 2MnO4– + 16H+ + 10Fe2+ → 2Mn2+ + 10Fe3+ + 8H2O

The reaction forms the basis of volumetric estimation of Fe2+ in any solution by KMnO4.

Oxalic acid to carbon dioxide

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

(COOH)2 + [O] → 2CO2 + H2O] × 5→

2KMnO4 + 3H2SO4 + 5(COOH)2 → K2SO4 + 2MnSO4 + 10CO2 + 8H2O  

Ionic equation : 2MnO4– + 6H+ + 5(COOH)2 → 2Mn2+ + 10CO2 + 8H2O

 

Sulphites to sulphates

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

                   Na2SO3 + [O] → Na2SO4] × 5        

2KMnO4 + 3H2SO4 + 5Na2SO3 → K2SO4 + 2MnSO4 + 5Na2SO4 + 3H2O

Ionic equation : 2MnO4– + 6H+ + 5SO32– → 2Mn2+ + 5SO42– + 3H2O

 

Iodides to iodine in acidic medium

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

                   2KI + H2O + [O] → I2 + 2KOH × 5

                   2KOH + H2SO4 → K2SO4 + 2H2O] × 5

2KMnO4 + 8H2SO4 + 10KI → 6K2SO4 + 2MnSO4 + 5I2 + 8H2O

Ionic equation : 2MnO4– + 16H+ + 10I– → 2Mn2+ 5I2 + 8H2O

 

Hydrogen peroxide to oxygen

                   2KMnO4 + 3H2SO4 → K2SO4 + 2MnSO4 + 3H2O + [O]

                   H2O2 + [O] → H2O + O2 ↑ × 5

2KMnO4 + 3H2SO4 + 5H2O2 → K2SO4 + 2MnSO4 + 8H2O + 5O2

 

Manganous sulphate (MnSO4) to manganese dioxide (MnO2)

                   2KMnO4 + H2O → 2KOH + 2MnO2 + 3[O]

MnSO4 + H2O + [O] → MnO2 + H2SO4 × 3

                   2KOH + H2SO4 → K2SO4 + 2H2O

                   2KMnO4 + 3MnSO4 + 2H2O → 5MnO2 + K2SO4 + 2H2SO4

Ionic equation : 2MnO4– + 3Mn2+ + 2H2O → 5MnO2 + 4H+

 

Ammonia to nitrogen

2KMnO4 + H2O → 2MnO2 + 2KOH + 3[O]

2NH3 + 3[O] → N2(g) + 3H2O

2KMnO4 + 2NH3 → 2MnO2 + 2KOH + 2H2O + N2(g)

                  

Uses : KMnO4 is used,

(i)      As an oxidising agent.

(ii)     As a disinfectant against disease-causing germs.

(iii)    For sterilizing wells of drinking water.

(iv)    In volumetric estimation of ferrous salts, oxalic acid etc.

 

Structure of Permanganate Ion (MnO4–) :

Mn in MnO4– is in +7 oxidation state. Mn7+ exhibits sp3 hybridisation in this ion. The structure of MnO4– is, shown in fig.

 

d Block Elements : Copper and its Compounds