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Preparation of Sulphuric Acid: Contact Process

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Question

(a) A blackish brown coloured solid ‘A’ when fused with alkali metal hydroxides in presence of air, produces a
dark green coloured compound ‘B’, which on electrolytic oxidation in alkaline medium gives a dark purple coloured
compound C. Identify A, B and C and write the reactions involved.
(b) What happens when an acidic solution of the green compound (B) is allowed to stand for some time? Give the
equation involved. What is this type of reaction called?

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Solution

a)
A= MnO₂ B= K₂ MnO₄ C= KMnO₄
MnO₂ + 4KOH + O₂â†’ K₂ MnO₄ + 2H₂ O
MnO₄^2- â†’(electrolytic oxidation in alkaline solution) MnO₄^-
^{(green) (purple)}
b) 3MnO₄^2- + H⁺ â†’ 2MnO₄^-+ MnO₂+ 2H₂ O
Here, MnO₄^- so formed is purple in colour and MnO₂ is the black precipitate.

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