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Chemistry · 2025 · 5 marks
The d-and f-Block ElementsGeneral Properties of the Transition Elements (d-Block)5 marksAnalyselong answer
CBSE 2025 · Region 7 · Set 3 · Q33
(a) (i) Account for the following: (I) The E_Mn^2+ / Mn^° value for manganese is highly negative, whereas E_Mn^3+ / Mn^2+^° is highly positive. (II)…
(a)(i)
Account for the following :
(I)
The $\displaystyle \mathrm{E}_{\mathrm{Mn}^{2+} / \mathrm{Mn}}^{\circ}$ value for manganese is highly negative, whereas $\displaystyle \mathrm{E}_{\mathrm{Mn}^{3+} / \mathrm{Mn}^{2+}}^{\circ}$ is highly positive.
(II)
Actinoids show wide range of oxidation states.
(III)
Transition metals have high melting points.
(ii)
Complete the following ionic equations :
(I)
$\displaystyle 5 \mathrm{SO}_{3}{ }^{2-}+2 \mathrm{MnO}_{4}{ }^{-}+6 \mathrm{H}^{+} \longrightarrow$
(II)
$\displaystyle 2 \mathrm{MnO}_{4}{ }^{-}+\mathrm{H}_{2} \mathrm{O}+\mathrm{I}^{-} \longrightarrow$
OR(b) Answer the following questions :
(i)
Name two elements of 3d series for which the third ionisation enthalpies are quite high.
(ii)
Out of $\displaystyle \mathrm{KMnO}_{4}$ and $\displaystyle \mathrm{K}_{2} \mathrm{MnO}_{4}$, which one is paramagnetic and why ?
(iii)
Write any one consequence of lanthanoid contraction.
(iv)
How do you prepare potassium manganate from pyrolusite ore ?
(v)
Why is the ability of oxygen more than fluorine to stabilise higher oxidation states of transition metals ?
Marking-scheme solution
(i)
(I)
Because Mn²⁺ is more stable than Mn³⁺ due to extra stable half-filled d⁵ configuration.
(II)
Due to comparable energies of 5f, 6d and 7s orbitals
(III)
Due to the involvement of greater number of electrons from (n-$\displaystyle 1$)d in addition to the ns electrons in the inter-atomic metallic bonding.
(ii)
(I)
$\displaystyle 5\mathrm{SO_3^{2-}} + 2\mathrm{MnO_4^{-}} + 6\mathrm{H^{+}} \longrightarrow 2\mathrm{Mn^{2+}} + 3\mathrm{H_2O} + 5\mathrm{SO_4^{2-}}$
(II)
$\displaystyle 2\mathrm{MnO_4^{-}} + \mathrm{H_2O} + \mathrm{I^{-}} \longrightarrow 2\mathrm{MnO_2} + 2\mathrm{OH^{-}} + \mathrm{IO_3^{-}}$
(i)
Mn, Zn, Ni, Cu
(ii)
K₂MnO₄, due to presence of one unpaired electron
(iii)
Similar radii of 4d and 5d series elements/ similar properties/ difficulty in separation of lanthanoids
(iv)
It is prepared by fusion of MnO₂ with an alkali metal hydroxide and an oxidising agent /
$\displaystyle 2\mathrm{MnO_2} + 4\mathrm{KOH} + \mathrm{O_2} \rightarrow 2\mathrm{K_2MnO_4} + 2\mathrm{H_2O}$
(v)
because of the ability of oxygen to form multiple bonds with metal
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CBSE Class 12 Chemistry past-paper question from the 2025 board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.