CBSE 2025 · Region 5 · Set 1 · Q33 · 5 marks
(i)When pyrolusite ore is fused with KOH, in presence of air, a dark green coloured product 'A' is obtained which changes to purple coloured compound 'B' in acidic medium.(I)Write the formulae of 'A' and 'B'.(II)Write the ionic equation for the reaction when compound 'B' reacts with $\displaystyle \mathrm{Fe}^{2+}$ in acidic medium.(ii)Give reasons :(I)$\displaystyle \mathrm{Ce}^{4+}$ in aqueous solution is a good oxidising agent.(II)The actinoid contraction is greater from element to element than lanthanoid contraction.(III)$\displaystyle \mathrm{E}_{\mathrm{Zn}^{2+} / \mathrm{Zn}}^{\circ}$ value is more negative than expected, whereas $\displaystyle \mathrm{E}_{\mathrm{Cu}^{2+} / \mathrm{Cu}}^{\circ}$ is positive.(i)While studying the periodic properties, Arti came across an abnormal behaviour in the atomic size of Hf. She found that, even though Hf is placed below Zr in the same group, both have almost similar atomic sizes.(I)Which phenomenon is responsible for the above behaviour ? Define it.(II)Mention any other consequence of the above phenomenon.(ii)Give reasons for the following :(I)Transition metals exhibit catalytic properties.(II)Transition metals have high enthalpy of atomisation.(III)Sc is a transition element, while Zn is not.
(i)
When pyrolusite ore is fused with KOH, in presence of air, a dark green coloured product 'A' is obtained which changes to purple coloured compound 'B' in acidic medium.
(I)
Write the formulae of 'A' and 'B'.
(II)
Write the ionic equation for the reaction when compound 'B' reacts with $\displaystyle \mathrm{Fe}^{2+}$ in acidic medium.
(ii)
Give reasons :
(I)
$\displaystyle \mathrm{Ce}^{4+}$ in aqueous solution is a good oxidising agent.
(II)
The actinoid contraction is greater from element to element than lanthanoid contraction.
(III)
$\displaystyle \mathrm{E}_{\mathrm{Zn}^{2+} / \mathrm{Zn}}^{\circ}$ value is more negative than expected, whereas $\displaystyle \mathrm{E}_{\mathrm{Cu}^{2+} / \mathrm{Cu}}^{\circ}$ is positive.
(i)
While studying the periodic properties, Arti came across an abnormal behaviour in the atomic size of Hf. She found that, even though Hf is placed below Zr in the same group, both have almost similar atomic sizes.
(I)
Which phenomenon is responsible for the above behaviour ? Define it.
(II)
Mention any other consequence of the above phenomenon.
(ii)
Give reasons for the following :
(I)
Transition metals exhibit catalytic properties.
(II)
Transition metals have high enthalpy of atomisation.
(III)
Sc is a transition element, while Zn is not.
Marking-scheme solution
(i)
(I)
A - K2MnO4 B- KMnO4
(II)
(ii)
(I)
Gets reduced to +$\displaystyle 3$ common oxidation state.
(II)
Due to poorer shielding offered by 5f electrons than 4f.
(III)
Due to completely filled d- subshell (d10) in zinc whereas in Cu, due to high enthalpy of
atomization and low enthalpy of hydration.
(i)
(I)
Lanthanoid contraction.
The steady decrease in atomic and ionic radii in lanthanoid series.
(II)
Decrease in basic character from left to right in lanthanoid series.
(ii)
(I)
They have the ability to exhibit variable oxidation states/ tendency to form complex
compounds/ large surface area.
(II)
Due to involvement of (n-$\displaystyle 1$) d and ns electrons which results in strong metallic bond and
strong interatomic bonding.
(III)
Sc has incompletely filled d orbital (3d1) in its ground state whereas Zn has completely filled
d orbital (3d10) in ground state as well as in its oxidized state.
The d-and f-Block ElementsSome Important Compounds of Transition ElementsUnderstandlong_answerhard
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CBSE Class 12 Chemistry past-paper question from the 2025board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.