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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.
heat
2KMnO4(s) ————→ K2MnO4(s) + MnO2 + O2(g)
Oxidizing 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 MnO4–) 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 MnO4–
, 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.,
5Fe2+ + 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 + 4H2O
In faintly acidic and neutral solutions, manganous ion is oxidized to manganese oxidized to manganese dioxide by permanganate.
2MnO4– + 3Mn+2 + 2H2O → 5MnO2 + 4H+
In strongly basic solutions, permangante oxidizes manganese dioxide to manganate ion.
MnO2 + 2MnO4– + 4OH– → 3MnO42– + 2H2O
In acidic medium, KMnO4 oxidizes,
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] × 3
2KMnO4 + 3H2SO4 + 5(COOH)2 → K2SO4 + 2MnSO4 + 10CO2 + 8H2O
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
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 + 10 KI → 6K2SO4 + 2MnSO4 + 5I2 + 8H2O
Ionic equation
2MnO4– + 16H+ + 10I– → 2Mn2+ + 5I2 + 8H2O
Hydrogen peroxide to oxygen
2KMnO4 + 3H2SO4 → K2SO4 + 2MnSO4 + 3H2O + 5[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
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 oxidizing 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. (v) Dilute alkaline KMnO4
solution known as Baeyer’s reagent.
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.
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