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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 1 · Set 1 · Q32
(a) (i) From the given data of E^° values, answer the following questions: (I) Why E_M^2+ / M^° show irregular trend in the above values? (II) Why is…
(i)
From the given data of $\displaystyle \mathrm{E}^{\circ}$ values, answer the following questions :
| $\displaystyle \mathrm{E}_{\mathrm{M}^{2+} / \mathrm{M}}^{\circ}$ | V | Cr | Mn | Fe | Co | Ni | Cu |
| -$\displaystyle 1.18$ | -$\displaystyle 0.91$ | -$\displaystyle 1.18$ | -$\displaystyle 0.44$ | -$\displaystyle 0.28$ | -$\displaystyle 0.25$ | +$\displaystyle 0.34$ |
(I)
Why $\displaystyle \mathrm{E}_{\mathrm{M}^{2+} / \mathrm{M}}^{\circ}$ show irregular trend in the above values ?
(II)
Why is $\displaystyle \mathrm{E}_{\mathrm{Cu}^{2+} / \mathrm{Cu}}^{\circ}$ value exceptionally positive ?
(III)
Why $\displaystyle \mathrm{E}_{\mathrm{Mn}^{2+} / \mathrm{Mn}}^{\circ}$ value is highly negative ?
(ii)
Write the ionic equations for the oxidising action of potassium permanganate for its reaction with I $\displaystyle { }^{-}$in both acidic and alkaline solutions.
Answer the following questions : $\displaystyle 1+1+1+2$
(i)
Name a member of the lanthanoid series
(I)
which exhibits +$\displaystyle 4$ oxidation state
(II)
which exhibits +$\displaystyle 2$ oxidation state.
(ii)
Why transition metals act as good catalyst ?
(iii)
Why Cr has higher melting point than Mn ?
(iv)
What happens when acidic solution of potassium permanganate is allowed to stand for sometime ? Give the equation involved. What is this type of reaction called ?
Marking-scheme solution
(a)
.
(i)
(I) Due to the irregular variation of ionisation enthalpies ( ΔiH1 + ΔiH2 )
and sublimation enthalpies.
(II)
Due to low Δhyd
o
H
and high Δa
o
H of Cu.
(III)
Because Mn2+ is highly stable due to half-filled 3d5 configuration.
(ii)
2MnO− + 10I− + 16H+ →
2Mn ++ 8H2O + 5I2
2MnO− + I− + H2O →
2MnO +
IO− + 2OH−
OR
$\displaystyle 32$(b).
(i)
(I)
Cerium
(II)
Europium / Ytterbium
(ii)
Because of their ability to show multiple oxidation states / complex
formation / Due to large surface area.
(iii)
Involvement of greater number of electrons from (n-$\displaystyle 1$)d and ns electrons in
interatomic metallic bonding.
(iv)
Reduction reaction
Redox 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.