Question

When pyrolusite ore is fused with KOH, in presence of air, a dark green coloured product ‘A’ is obtained which changes to purple coloured compound ‘B’ in acidic medium.
(II) Write the formulae of ‘A’ and ‘B’.

Hint:

In alkaline medium, manganese exists as manganate (\( MnO_4^{2-} \)), which is green in color, while in acidic medium, it exists as permanganate (\( MnO_4^{-} \)), which is purple.

Answer 1

(I) The compound 'A' formed is Potassium manganate (\( K_2MnO_4 \)). When pyrolusite ore (which contains manganese dioxide, \( MnO_2 \)) is fused with KOH in the presence of air, manganese in \( MnO_2 \) gets oxidized, forming the manganate ion (\( MnO_4^{2-} \)) in alkaline solution. The dark green color is characteristic of the manganate ion.
(II) The compound 'B' formed is Potassium permanganate (\( KMnO_4 \)). When potassium manganate is dissolved in an acidic medium, the manganate ion (\( MnO_4^{2-} \)) is further oxidized to permanganate ion (\( MnO_4^{-} \)), which is purple in color. This oxidation occurs because the acidic medium provides protons that help to convert \( MnO_4^{2-} \) to \( MnO_4^{-} \).