(s) about the chemistry of manganese…

(s) about the chemistry of manganese and chromium.

One or more options correct

Choose the correct statement(s) about the chemistry of manganese and chromium.

A. In strongly alkaline solution $\mathrm{MnO_4^-}$ can be reduced to green $\mathrm{MnO_4^{2-}}$ , and on acidification $\mathrm{MnO_4^{2-}}$ disproportionates to $\mathrm{MnO_4^-}$ and $\mathrm{MnO_2}$ .
B. The conversion of yellow $\mathrm{CrO_4^{2-}}$ to orange $\mathrm{Cr_2O_7^{2-}}$ on acidification is a redox reaction in which chromium changes oxidation state.
C. $\mathrm{K_2MnO_4}$ is diamagnetic.
D. The aqua ion $\mathrm{Mn^{3+}}$ is stable towards disproportionation in acidic aqueous solution and is therefore a common stable oxidant in water.

Show answer and solution

Answer: A

(A) True. Permanganate is reduced to manganate in strong base (for example by excess alkali with a mild reductant, or by

\mathrm{OH^-}

itself under boiling). In acid,

3\mathrm{MnO_4^{2-}}+4\mathrm{H^+}\to2\mathrm{MnO_4^-}+\mathrm{MnO_2}+2\mathrm{H_2O}

: Mn(VI) goes to Mn(VII) and Mn(IV).
(B) False.

2\mathrm{CrO_4^{2-}}+2\mathrm{H^+}\rightleftharpoons\mathrm{Cr_2O_7^{2-}}+\mathrm{H_2O}

is a condensation; chromium remains

+6

throughout.
(C) False. Manganate contains Mn(VI), a

d^1

ion, so it has one unpaired electron and is paramagnetic (

\mu\approx1.73

BM).
(D) False.

\mathrm{Mn^{3+}}

disproportionates:

2\mathrm{Mn^{3+}}+2\mathrm{H_2O}\to\mathrm{Mn^{2+}}+\mathrm{MnO_2}+4\mathrm{H^+}

, because the

d^4

ion is a strong oxidant and its reduction potential to

\mathrm{Mn^{2+}}

(about

+1.5

V) exceeds that for the

\mathrm{MnO_2/Mn^{3+}}

couple (about

+0.95

V).
Only (A) is correct.

Original question written in the JEE Advanced 2022 Paper 2 pattern and checked twice for the correct answer. Not an official question paper. How we write and check questions · Corrections policy · Report a mistake