(s) concerning oxidising agents of…

(s) concerning oxidising agents of chromium and manganese is/are correct?

One or more options correct

Which of the following statement(s) concerning oxidising agents of chromium and manganese is/are correct?

A. $0.10\ \mathrm{mol}$ of $\mathrm{K_2Cr_2O_7}$ in acidic solution oxidises $0.60\ \mathrm{mol}$ of $\mathrm{Fe^{2+}}$ to $\mathrm{Fe^{3+}}$
B. Strongly alkaline solutions of $\mathrm{K_2MnO_4}$ readily disproportionate to $\mathrm{KMnO_4}$ and $\mathrm{MnO_2}$
C. The change from orange $\mathrm{Cr_2O_7^{2-}}$ to yellow $\mathrm{CrO_4^{2-}}$ on adding alkali is accompanied by reduction of chromium
D. In neutral or weakly alkaline solution, two moles of $\mathrm{MnO_4^-}$ convert one mole of $\mathrm{I^-}$ into $\mathrm{IO_3^-}$ , with formation of $\mathrm{MnO_2}$

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Answer: A, D

(A) is correct: each dichromate takes up

6

electrons (

2\mathrm{Cr}^{6+}\to2\mathrm{Cr}^{3+}

) and each

\mathrm{Fe^{2+}}

gives one, so

0.10\times6=0.60\ \mathrm{mol}

\mathrm{Fe^{2+}}

is oxidised. (B) is incorrect: manganate is stable only in strongly alkaline medium; it disproportionates,

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

, when the solution is acidified or even neutral, so alkaline conditions prevent this. (C) is incorrect: the interconversion

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

is a pH-controlled equilibrium and chromium stays

+6

throughout, so there is no redox change. (D) is correct:

\mathrm{I^-}

goes to

\mathrm{IO_3^-}

(a loss of

6

electrons) while each

\mathrm{MnO_4^-}

going to

\mathrm{MnO_2}

gains

3

, giving

2\mathrm{MnO_4^-}+\mathrm{I^-}+\mathrm{H_2O}\to2\mathrm{MnO_2}+\mathrm{IO_3^-}+2\mathrm{OH^-}

Original question written in the JEE Advanced 2020 Paper 1 pattern and checked twice for the correct answer. Not an official question paper. How we write and check questions · Corrections policy · Report a mistake