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(a) A blackish brown coloured solid 'A' ...

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

Text Solution

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(a) `underset("(A)Blackish brown")underset("Pyrolusite")(MnO_(2)) + 4KOH + O_(2) overset("Fuse")(rarr) underset("(B) - Green coloured")underset("Potassium manganate ")( 2K_(2)MnO_(4))+ 2H_(2)O`
`underset((B))(2K_(2)MnO_(4)) + H_(2)O+(O) underset("medium")overset("Alkaline")(rarr) underset("(C ) -Purple coloured")underset("Potassium permanganate")(2KMnO_(4)) + 2KOH`
or `MnO_(4)^(2-) rarrMnO_(4)^(-) + e^(-)`
(b) When acidic solution of green compound (B) , i.e., potassium manganate is allowed to stand for some time, it disproportionates to give permanganate as follows `:`
`3MnO_(4)^(2-) + 4H^(+) rarr 2MnO_(4)^(-) + MnO_(2) + 2H_(2)O`
This reaction is called disproportionation reaction.
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