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Chemistry · 2023 · 5 marks
The d-and f-Block ElementsGeneral Properties of the Transition Elements (d-Block)5 marksApplylong answer
CBSE 2023 · Region 5 · Set 3 · Q35
(a) Write the number of unpaired electrons in Cr^3+. (Atomic number of Cr =24 ) (b) Complete the reaction mentioning all the products formed: Cr_2…
(a)
Write the number of unpaired electrons in $\displaystyle \mathrm{Cr}^{3+}$. (Atomic number of $\displaystyle \mathrm{Cr}=24$ )
(b)
Complete the reaction mentioning all the products formed : $\displaystyle \mathrm{Cr}_{2} \mathrm{O}_{7}{ }^{2-}+3 \mathrm{H}_{2} \mathrm{~S}+8 \mathrm{H}^{+} \rightarrow$
(c)
Account for the following :
(i)
$\displaystyle \mathrm{Mn}^{2+}$ is more stable than $\displaystyle \mathrm{Fe}^{2+}$ towards oxidation to +$\displaystyle 3$ state.
(ii)
Copper has exceptionally positive $\displaystyle \mathrm{E}_{\mathrm{M}}^{\circ}{ }^{2+} / \mathrm{M}$ value.
(iii)
$\displaystyle \mathrm{Eu}^{2+}$ with electronic configuration $\displaystyle [\mathrm{Xe}] 4 \mathrm{f}^{7} 6 \mathrm{~s}^{2}$ is a strong reducing agent.
Marking-scheme solution
(a)
Three unpaired electrons.
(b)
\(\displaystyle \mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}+3 \mathrm{H}_{2} \mathrm{~S}+8 \mathrm{H}^{+} \longrightarrow 2 \mathrm{Cr}^{3+}+3 \mathrm{~S}+7 \mathrm{H}_{2} \mathrm{O}\)
(c)(i)
\(\displaystyle \mathrm{Mn}^{2+}\) has stable half-filled \(\displaystyle \mathrm{d}^{5}\) configuration. \(\displaystyle \mathrm{Fe}^{2+}\left(3 \mathrm{~d}^{6}\right)\) can lose one electron easily to give \(\displaystyle \mathrm{Fe}^{3+}\left(3 \mathrm{~d}^{5}\right.\), stable configuration).
(ii)
Due to high enthalpy of atomization and low hydration enthalpy.
(iii)
\(\displaystyle \mathrm{Eu}^{2+}\) has the tendency to change to common oxidation state of +$\displaystyle 3$ /
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