(a) Blackish brown coloured solid (A) which is an oxide of manganese, when fused with alkali metal hydroxide and an oxidising agent like `KNO_(3)`, produces a dark green coloured compound (B). Compound (B) on disporportionation in neutral and acidic solution gives a purple coloured compound (C). Identify (A), (B) and (C) and write the reaction involved when compound (C) is heated to 513 K.
(b) (i) `E^(@)_(M^(3+)//M^(2+))` values for the first series of transition elements are given below. Answer the question that follows.

Identify the two strongest oxidising agents in the aqueous solution from the above data.
(ii) Cu (I) ion in aqueous solution is not known
(iii) The highest oxidation state of a metal is exhibited in its oxide.

Explanation

(a) Compounds (A), (B) and (C) are
`underset((A))(MnO_(2))` `underset((B))(K_(2)MnO_(4))` `underset((C))(KMnO_(4))`
The reactions are explained as under
`underset((A))underset(("Blackish brown"))underset("dioxide")underset("Manganese")(2MnO_(2))+4KOH+O_(2)tounderset((B))underset(("Dark green"))underset("manganate")underset("Potassium")(2K_(2)MnO_(4))+2H_(2)O`
`underset((B))(3K_(2)MnO_(4))+4HCl to underset("permanganate")underset("Potassium")underset((C))(2KMnO_(4))+MnO_(2)+4KCl+2H_(2)O`
`underset((C))underset("permanganate")underset("Potassium")(2KMnO_(4))underset(Delta)overset(513K)to underset("manganate")underset("Potassium")(K_(2)MnO_(4))+MnO_(2)+O_(2)`
(b) (i) `Co(3+)` and `Mn^(3+)` are the strongest oxidising agents because they have the highest positive values for `E_(M^(3+)//M^(2+))^(0)`.
(ii) Copper (I) ions are unstable in aqueous solutions and undergo disproportionation.
`2Cu^(2+)to Cu^(2+)+Cu`
(iii) Oxygen has a small size and a high electronegativity. Hence they can oxidise the metal to high oxidation state by prompting all the valence electrons to participate in bonding.