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Chemistry · 2026 · 5 marks
The d-and f-Block ElementsGeneral Properties of the Transition Elements (d-Block)5 marksUnderstandlong answer
CBSE 2026 · Region 3 · Set 1 · Q31
(a) (i) Describe giving reason which one of the following pairs has the property indicated: (I) Fe or Cu - higher melting point (II) Ti^3+ or Sc^3+ -…
(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.
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CBSE Class 12 Chemistry past-paper question from the 2026 board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.