Question

Describe the preparation of potassium permanganate. How does the acidified permanganate solution react with

(i) iron(II) ions 

(ii) SO₂ and 

(iii) oxalic acid? 

Write the ionic equations for the reactions.

Answer 1

Potassium permangnate is prepared by fusion of MnO₂ with an alkali metal hydroxide and an oxidising agent like KNO₃. This produces the dark green k₂MnO₄ which disproportionates in a neutral or acidic solution to give permanganate. 

2MnO₂ + 4 KOH + O₂ → 2K₂MnO₄ + 2H₂O 

3MnO₄^2- + 4H⁺ → 2MnO₄^- + MnO₂ + 2H₂O 

Commercially it is prepared by the alkaline oxidative fusion of MnO₂ followed by the electrolytic oxidation of manganate (VI)

In laboralory a manganese (II) ion salt is oxidised by peroxidisulphate to permanganate.

2Mn²⁺ + 5S₂O₈^2- + 8H₂O → 2MnO₄^– + 10SO₄^2- + 16H⁺
(i) 5Fe²⁺ + MnO₄^– + 8H⁺ → Mn²⁺ + 4H₂O + 5Fe₃⁷

(ii) 2MnO₄²+ 10SO₂ + 5O₂ + 4H₂O → 2Mn²⁺ + 10SO₄^2- + 8H⁺

(iii) 5C₂O₄^2- + 2MnO₄^– + 16H⁺ → 2Mn²⁺ + 8H₂O + 10CO₂