Ø Properties
of KMnO4:
· Colour:
Potassium permanganate exists as deep
purple black colour with a greenish lustre which become dull
in air due to superficial reduction.
· Solubility: It is moderately soluble in water
at room temperature and it is more soluble in hot water.
· Action
of heat: When heated to 513 K, it readily decomposes giving
oxygen.
KMnO4 → K2MnO4
+ MnO2 + O2
At
red heat, potassium manganate formed decomposes into potassium manganite (K2MnO3)
and oxygen
K2MnO4 → K2MnO3
+ O2
· Action
of heat in current of Hydrogen: When heated in a current
of H2, solid KMnO4 gives KOH, MnO and water vapours.
2 KMnO4 + 5 H2 →
2 KOH + 2 MnO2 + 4 H2O
·
Oxidising Properties:
KMnO4 is a good oxidizing
agent in acidic, basic and neutral media. The oxidizing action in acidic medium
is due to the reaction:
MnO4- + 8H++ 5e-→
Mn2+ + 4H2O
Acidified
permanganate solution oxidises:
1) Oxalates
to carbon dioxide
5 x (C2O42-→2
CO2 + 2e–)
2
x (MnO4- + 8H++ 5e-→ Mn2+
+ 4H2O)
5 C2O42- +2 MnO4-
+ 16H+ → 10 CO2 +2 Mn2+ + 8H2O
2) Iron(II)
to iron(III)
5
Fe2+ → 5 Fe3+ + 5e–
MnO4-
+ 8H++ 5e-→ Mn2+ + 4H2O
5Fe2+ + MnO4-
+ 8H+ → 5Fe3+ + Mn2+ + 4H2O Overall Reaction
3) Nitrites
to nitrates
5NO2–
+ 5H2O → 5NO3- + 10H+ + l0e–
2
x (MnO4- + 8H++ 5e-→ Mn2+
+ 4H2O)
5NO2– +2 MnO4-
+ 6H+ → 5NO3- + 2 Mn2+ + 3H2O
In alkaline or neutral medium,
permanganate ion is reduced to
MnO2
MnO4- + 2H2O + 3e-→ MnO2
+ 4OH-
In
alkaline medium it oxidises:
1) Iodide
to iodate
(MnO4- + 2H2O 3e-→ MnO2
+ 4OH-) × 2
I-
+ 6OH- → IO3- + 3 H2O + 6 e–
2MnO4-
+ H2O + I-→ 2MnO2 + 2OH-+ IO3 Overall Reaction
2) Thiosulphate
to sulphate
(MnO4- + 2H2O
+ 3e-→ MnO2 + 4OH-) x 8
(S2O32-
+ 10OH- → 2SO42- + 5 H2O + 8e-) x 3
8MnO4-
+ H2O +3 S2O32- → 8MnO2
+ 2OH-+ 6SO42- Overall Reaction
Uses:
It is used as an oxidising agent in acidic, basic and neutral medium. It is
used as a primary standard in
volumetric analysis. It is used for the bleaching of wool, cotton, silk and
other textile fibres and also for the decolourisation of oils.
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