Q. 66

When an oxide of manganese (A) is fused with KOH in the presence of an oxidising agent and dissolved in water, it gives a dark green solution of compound (B). Compound (B) disproportionates in neutral or acidic solution to give purple compound (C). An alkaline solution of compound (C) oxidizes potassium iodide solution to a compound (D) and compound (A) is also formed. Identify compounds A to D and also explain the reactions involved.

Answer :

A=MnO2, B=K2MnO4, C= KMnO4, D= KIO3

When Manganese dioxide(A) is fused with KOH, it gives a green solution of potassium manganate(B).


2MnO2 (A) + 4KOH +O2 2K2MnO4(B) +2H2O


Potassium manganate disproportionates to give purple potassium permanganate(C).


3MnO42- +H+2MnO4-(C)+MnO2 +2H2O


Potassium permanganate on reacting with KI gives potassium iodate(D) and manganese dioxide again


2MnO4- +H2O +KI 2MnO2 + 2OH- + KIO3(D)


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