CBSE 2024 · Region 3 · Set 1 · Q29 · 4 marks
The involvement of $\displaystyle (\mathrm{n}-1) \mathrm{d}$ electrons in the behaviour of transition elements impart certain distinct characteristics to these elements. Thus, in addition to variable oxidation states, they exhibit paramagnetic behaviour, catalytic properties and tendency for the formation of coloured ions. The transition metals react with a number of non-metals like oxygen, nitrogen and halogens. $\displaystyle \mathrm{KMnO}_{4}$ and $\displaystyle \mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}$ are common examples. The two series of inner transition elements, lanthanoids and actinoids, constitute the f-block of the periodic table. In the lanthanoids, there is regular decrease in atomic size with increase in atomic number due to the imperfect shielding effect of 4f-orbital electrons which causes contraction. Answer the following questions :(a)Why do transition metals and their compounds act as good catalysts ?(b)What is the cause of contraction in the atomic size of lanthanoids ?Define lanthanoid contraction. How does it affect the atomic radii of the third transition series and the second transition series?In aqueous media, which is a stronger reducing agent - $\displaystyle \mathrm{Cr}^{2+}$ or $\displaystyle \mathrm{Fe}^{2+}$ and why ?
The involvement of $\displaystyle (\mathrm{n}-1) \mathrm{d}$ electrons in the behaviour of transition elements impart certain distinct characteristics to these elements. Thus, in addition to variable oxidation states, they exhibit paramagnetic behaviour, catalytic properties and tendency for the formation of coloured ions. The transition metals react with a number of non-metals like oxygen, nitrogen and halogens. $\displaystyle \mathrm{KMnO}_{4}$ and $\displaystyle \mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}$ are common examples. The two series of inner transition elements, lanthanoids and actinoids, constitute the f-block of the periodic table. In the lanthanoids, there is regular decrease in atomic size with increase in atomic number due to the imperfect shielding effect of 4f-orbital electrons which causes contraction. Answer the following questions :
(a)
Why do transition metals and their compounds act as good catalysts ?
(b)
What is the cause of contraction in the atomic size of lanthanoids ?
Define lanthanoid contraction. How does it affect the atomic radii of the third transition series and the second transition series?
In aqueous media, which is a stronger reducing agent - $\displaystyle \mathrm{Cr}^{2+}$ or $\displaystyle \mathrm{Fe}^{2+}$ and why ?
Marking-scheme solution
(a)
Due to their ability to show multiple oxidation states and to form complexes / provide large surface area.
(b)
Due topoor shielding effect of \(\displaystyle 4 f\) orbital .
The overall decrease in atomic and ionic radii from La to Lu is known as lanthanoid contraction. Atomic radii of second and third transition series are very similar.
\(\displaystyle \mathrm{Cr}^{2+}\) is stronger reducing agent than \(\displaystyle \mathrm{Fe}^{2+}\) Reason: \(\displaystyle \mathrm{d}^{4} \rightarrow \mathrm{~d}^{3}\) occurs in case of \(\displaystyle \mathrm{Cr}^{2+}\) to \(\displaystyle \mathrm{Cr}^{3+}\) But \(\displaystyle \mathrm{d}^{6} \rightarrow \mathrm{~d}^{5}\) occurs in case of \(\displaystyle \mathrm{Fe}^{2+}\) to \(\displaystyle \mathrm{Fe}^{3+}\) In a medium (like water) \(\displaystyle \mathrm{d}^{3}\) is more stable as compared to \(\displaystyle \mathrm{d}^{5}\)
The d-and f-Block ElementsGeneral Properties of the Transition Elements (d-Block)Understandcase_studymedium
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CBSE Class 12 Chemistry past-paper question from the 2024board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.