Question

Describe the preparation of potassium permanganate. How does the acidifiedpermanganate solution react with (i) iron(II) ions (ii) SO2and (iii) oxalic acid? Write the ionic equations for the reactions.

Answer 1

Potassium permanganate can be prepared from pyrolusite (MnO2). The ore is fused with KOH in the presence of either atmospheric oxygen or an oxidising agent, such as KNO3 or KClO4, to give K2MnO4.
2MnO2+4KOH+O2 $\underrightarrow{-heat}$ 2K2MnO4 +2H2O
(Green)
The green mass can be extracted with water and then oxidized either electrolytically or by passing chlorine/ozone into the solution. Electrolytic oxidation
K2MnO4$\leftrightarrow$2K++$MnO^{2-}_4$
H2O$\leftrightarrow$H++OH-
At anode, manganate ions are oxidized to permanganate ions
$MnO^{2-}_4$ $\leftrightarrow$ $MnO^{-}_4$+e-
Green Purple
Oxidation by chlorine
2K2MnO4+Cl2 $\rightarrow$ 2KMnO4+2KCl
2MnO4-+Cl2 $\rightarrow$ 2$MnO^{-}_4$+2Cl-
Oxidation by ozone
2K2MnO4+O3+H2O $\rightarrow$ 2KMnO4+2KOH+O2
2$MnO^{2-}_4$+O3+H2O $\rightarrow$ 2$MnO^{2-}_4$+2OH-+O2

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(i) Acidified KMnO4 solution oxidizes Fe (II) ions to Fe (III) ions i.e., ferrous ions to ferric ions
$MnO^{-}_4$+8H++5e-$\rightarrow$ Mn2++4H2O
Fe2+$\rightarrow$ Fe3++e-] x 5
$MnO^{-}_4$+5Fe2++8H+$\rightarrow$ Mn2++5Fe3++4H2O

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(ii) Acidified potassium permanganate oxidizes SO2 to sulphuric acid.
$MnO^{-}_4$+6H++5e-$\rightarrow$ Mn2++3H2O] x 2
2H2O+2SO2+O2$\rightarrow$ 4H++$2So^{2-}_4$+2e-] x 5
2$MnO^{-}_4$+10SO2+5O2+4H2O $\rightarrow$ 2Mn2++$10So^{2-}_4$+8H+

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(iii) Acidified potassium permanganate oxidizes oxalic acid to carbon dioxide.
$MnO^{-}_4$+8H++5e-$\rightarrow$ Mn2++4H2O] x 2
C_2O^{2-}_4$$\rightarrow 2CO2+2e-] x 5
2MnO4+$5C_2O^{2-}_4$+16H+$\rightarrow$ 2Mn2++10CO2+8H2O