Question

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.

Answer 1

Compound A is manganese dioxide (MnO₂), B is manganate (MnO₄²⁻), and C is permanganate (MnO₄⁻). The reactions include fusion of MnO₂ with KOH and O₂ to form K₂MnO₄, and electrolytic oxidation of K₂MnO₄ to form MnO₄⁻.

Step-by-step explanation:

The question describes a chemical process involving oxidation and reduction reactions of an amphoteric oxide in the presence of air and under electrolytic conditions. Based on the descriptions given, solid A is likely a blackish brown colored amphoteric oxide such as manganese dioxide (MnO₂), compound B is the dark green colored manganate (MnO₄²⁻), and compound C is the dark purple colored permanganate (MnO₄⁻).

The chemical reactions involved in this process can be outlined as follows:

1. Fusion of the amphoteric oxide (MnO₂) with an alkali metal hydroxide such as potassium hydroxide (KOH) in the presence of air produces the manganate ion:

2 MnO₂(s) + 4 KOH(s) + O₂(g) → 2 K₂MnO₄(aq) + 2 H₂O(l)

1. The manganate ion (MnO₄²⁻) undergoes electrolytic oxidation in an alkaline medium to form permanganate ion (MnO₄⁻):

3 MnO₄²⁻(aq) + H₂O(l) → 2 MnO₄⁻(aq) + MnO₂(s) + 2 OH⁻(aq)

During this electrochemical process, manganese goes through changes in oxidation states from +4 in manganese dioxide to +6 in manganate and finally to +7 in permanganate.