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