CBSE 2026 · Region 3 · Set 1 · Q31 · 5 marks
(i)Describe giving reason which one of the following pairs has the property indicated :(I)Fe or Cu - higher melting point(II)$\displaystyle \mathrm{Ti}^{3+}$ or $\displaystyle \mathrm{Sc}^{3+}$ - coloured in aqueous solution(III)Cr or Zn - higher third ionisation enthalpy(ii)Write the ionic equations for the oxidizing action of $\displaystyle \mathrm{MnO}_{4}^{-}$in acidic medium with :(I)$\displaystyle \mathrm{Fe}^{2+}$ ion(II)$\displaystyle \mathrm{I}^{-}$ion(i)A black-brown coloured solid (A) when fused with KOH in the presence of air, produces a dark green coloured compound (B) which on electrolytic oxidation in alkaline medium gives a dark purple coloured compound (C). Identify(A), (B) and (C). Write the reactions involved.(ii)What happens when an acidic solution of the green compound (B) is allowed to stand for some time ? Also write the equation involved. What is this type of reaction called ?
(i)
Describe giving reason which one of the following pairs has the property indicated :
(I)
Fe or Cu - higher melting point
(II)
$\displaystyle \mathrm{Ti}^{3+}$ or $\displaystyle \mathrm{Sc}^{3+}$ - coloured in aqueous solution
(III)
Cr or Zn - higher third ionisation enthalpy
(ii)
Write the ionic equations for the oxidizing action of $\displaystyle \mathrm{MnO}_{4}^{-}$in acidic medium with :
(I)
$\displaystyle \mathrm{Fe}^{2+}$ ion
(II)
$\displaystyle \mathrm{I}^{-}$ion
(i)
A black-brown coloured solid (A) when fused with KOH in the presence of air, produces a dark green coloured compound (B) which on electrolytic oxidation in alkaline medium gives a dark purple coloured compound (C). Identify
(A)
, (B) and (C). Write the reactions involved.
(ii)
What happens when an acidic solution of the green compound (B) is allowed to stand for some time ? Also write the equation involved. What is this type of reaction called ?
Marking-scheme solution
(i)
(I)
Fe, Because of more unpaired electrons than Cu.
(II)
Ti$\displaystyle ^{3+}$, Because of the presence of one unpaired electron in 3d-orbital.
(III)
Zn, Because of stable configuration 3d$\displaystyle ^{10}$ in +$\displaystyle 2$ state.
(ii)
(I)
$\displaystyle 5\mathrm{Fe}^{2+} + \mathrm{MnO}_4^{-} + 8\mathrm{H}^{+} \rightarrow \mathrm{Mn}^{2+} + 4\mathrm{H}_2\mathrm{O} + 5\mathrm{Fe}^{3+}$
(II)
$\displaystyle 10\mathrm{I}^{-} + 2\mathrm{MnO}_4^{-} + 16\mathrm{H}^{+} \rightarrow 2\mathrm{Mn}^{2+} + 8\mathrm{H}_2\mathrm{O} + 5\mathrm{I}_2$
(i)
$\displaystyle 2\mathrm{MnO}_2 + 4\mathrm{KOH} + \mathrm{O}_2 \rightarrow 2\mathrm{K}_2\mathrm{MnO}_4 + 2\mathrm{H}_2\mathrm{O}$
$\displaystyle \mathrm{MnO}_4^{2-} \xrightarrow{\text{electrolytic oxidation in alkaline solution}} \mathrm{MnO}_4^{-}$
(A)
: MnO$\displaystyle _2$ , (B): K$\displaystyle _2$MnO$\displaystyle _4$ , (C): KMnO$\displaystyle _4$
(ii)
$\displaystyle 3\mathrm{MnO}_4^{2-} + 4\mathrm{H}^{+} \rightarrow 2\mathrm{MnO}_4^{-} + \mathrm{MnO}_2 + 2\mathrm{H}_2\mathrm{O}$
On standing compound (B) changes to permanganate ion in acid solution.
Disproportionation reaction.
The d-and f-Block ElementsGeneral Properties of the Transition Elements (d-Block)Understandlong_answerhard
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CBSE Class 12 Chemistry past-paper question from the 2026board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.