CBSE 2025, Set 3

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.

(I) Write the formulae of 'A' and 'B'.

(II) Write the ionic equation for the reaction when compound 'B' reacts with Fe$^{2+}$ in acidic medium.

Edvaya · Accepted Answer

The correct answer is Subjective. **(I) Formulae of A and B:** **A = K$_2$MnO$_4$ (Potassium manganate)** — dark green colour $$2	ext{MnO}_2 + 4	ext{KOH} + 	ext{O}_2 ightarrow 2\underbrace{	ext{K}_2	ext{MnO}_4}_{A	ext{ (dark green)}} + 2	ext{H}_2	ext{O}$$ In acidic medium, manganate disproportionates to permanganate: **B = KMnO$_4$ (Potassium permanganate)** — purple colour $$3	ext{K}_2	ext{MnO}_4 + 2	ext{H}_2	ext{SO}_4 ightarrow 2\underbrace{	ext{KMnO}_4}_{B	ext{ (purple)}} + 	ext{MnO}_2 + 2	ext{K}_2	ext{SO}_4 + 2	ext{H}_2	ext{O}$$ **(II) Ionic equation for KMnO$_4$ + Fe$^{2+}$ in acidic medium:** $$	ext{MnO}_4^- + 5	ext{Fe}^{2+} + 8	ext{H}^+ ightarrow 	ext{Mn}^{2+} + 5	ext{Fe}^{3+} + 4	ext{H}_2	ext{O}$$ Mn goes from +7 (in MnO$_4^-$) to +2 (in Mn$^{2+}$): gain of 5e$^-$ Fe goes from +2 to +3: loss of 1e$^-$ × 5 = 5e$^-$

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Chemistry > Inorganic Chemistry > D And F Block Elements

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