Question

Describe the preparation of potassium permanganate. How does the acidified permanganate solution react with
(i) iron (II) ions
(ii) ${SO}_2$ and
(iii) Oxalic acid?
Write the ionic equations for the reactions

Answer 1

Potassium permanganate is prepared by fusion of $Mn{O}_2$ with an alkali metal hydroxide and an oxidising agent like $K{NO}_3$. This produces the dark green $K_{2}Mn{O}_4$ which disproportionates in a neutral or acidic solution to give permanganate.

$2Mn{O}_2+4KOH+O_2\to 2K_{2}Mn{O}_4+2H_{2}O$

$3Mn{O}_4^{2-}+4H^{+}\to 2Mn{O}_4^{-}+Mn{O}_2+2H_{2}O$

Commercially it is prepared by the alkaline oxidative fusion of $Mn{O}_2$ followed by the electrolytic oxidation of manganate (VI).

$Mn{O}_2\underset{oxidisedwithair}{\overset{fusedwithKOH}{\to }}Mn{O}_4^{2-}$

$Mn{O}_4^{2-}\underset{inalkalinesolution}{\overset{electrolyticoxidation}{\to }}Mn{O}_4^{-}$

In the laboratory manganese (II) ion salt is oxidised by peroxidisulphate to permanganate

$2{Mn}^{2+}+5S_2{O}_8^{2-}+8H_{2}O\to 2Mn{O}_4^{-}+10{SO}_4^{2-}+16H^{+}$

Reaction of acidified permanganate solution:

(i) $5{Fe}^{2+}+Mn{O}_4^{-}+8H^{+}\to {Mn}^{2+}+4H_{2}O+5{Fe}^{3+}$

(ii) $2Mn{O}_4^{2-}+5{SO}_2+2H_{2}O\to 2{Mn}^{2+}+5{SO}_4^{2-}+4H^{+}$

(iii) $5C_2{O}_4^{2-}+2Mn{O}_4^{-}+16H^{+}\to 2{Mn}^{2+}+8H_{2}O+10{CO}_2$