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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 5 · Set 1 · Q32
(a) (i) (I) Why do transition metals show variable oxidation states? (II) Out of Mn^2+ and Ti^2+ which will be more paramagnetic and why? [Atomic…
(i)
(I)
Why do transition metals show variable oxidation states ?
(II)
Out of $\displaystyle \mathrm{Mn}^{2+}$ and $\displaystyle \mathrm{Ti}^{2+}$ which will be more paramagnetic and why ? [Atomic No. : $\displaystyle \mathrm{Ti}=22, \mathrm{Mn}=25$ ]
(III)
Which ion is the strongest oxidising agent in the options given below : $\displaystyle \mathrm{Cr}^{3+}, \mathrm{V}^{3+}, \mathrm{Mn}^{3+}$ Give reason. [Atomic No. : $\displaystyle \mathrm{Cr}=24, \mathrm{~V}=23, \mathrm{Mn}=25$ ]
(ii)
Complete and balance the following equations :
(I)
$\displaystyle 2 \mathrm{MnO}_{2}+4 \mathrm{KOH}+\mathrm{O}_{2} \longrightarrow$
(II)
$\displaystyle 5 \mathrm{C}_{2} \mathrm{O}_{4}^{2-}+2 \mathrm{MnO}_{4}^{-}+16 \mathrm{H}^{+} \longrightarrow$
(i)
What is meant by lanthanoid contraction ?
(ii)
Why do transition metals form coloured compounds ?
(iii)
Why are $\displaystyle \mathrm{E}_{\mathrm{M}^{2+} / \mathrm{M}}^{\mathrm{o}}$ values for Mn and Zn more negative than expected ?
(iv)
Which is the most stable oxidation state of Cu and why ?
(v)
Why is $\displaystyle \mathrm{Ce}^{4+}$ in aqueous solution a good oxidising agent?
Marking-scheme solution
(i)
(I)
Electrons from ns and (n−$\displaystyle 1$)d orbitals take part in bonding / Due to
similar energy of ns and (n−$\displaystyle 1$)d orbitals.
(II)
Mn2+ , more number of unpaired electrons.
(III)
Mn3+, it has d4 configuration and can gain one electron to get stable d5
configuration
(ii)
(I)
2MnO2 + 4KOH + O2 2K2MnO4 + 2H2O
(II)
5C2
O −+ 2Mn
O− + 16H+ 2Mn2+ + 8H2O + 10CO2
(i)
As we move from left to right there is gradual decrease in the atomic radii of
elements of the lanthanoids / The filling of 4f before 5d orbital results in a regular
decrease in atomic radii.
(ii)
It is because of d-d transition / due to the presence of unpaired electrons
which undergoes d-d transition.
(iii)
Mn2+ : Half filled configuration
Zn2+ : Fully filled configuration
(iv)
Cu2+, Because of high ∆hydH°.
(v)
Ce4+ reverts to the most common oxidation state of Ce3+.
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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.