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Chemistry · 2023 · 5 marks
The d-and f-Block ElementsSome Important Compounds of Transition Elements5 marksUnderstandlong answer
CBSE 2023 · Region 3 · Set 1 · Q33
(a) (i) Account for the following: (1) Transition metals form complex compounds. (2) The E_Mn^2+ / Mn^° value for manganese is highly negative…
(i)
Account for the following:
(1)
Transition metals form complex compounds.
(2)
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.
(3)
$\displaystyle \mathrm{Cu}^{+}$ion is unstable in aqueous solution.
(ii)
Write the equations involved in the preparation of $\displaystyle \mathrm{KMnO}_{4}$ from Pyrolusite ore $\displaystyle \left(\mathrm{MnO}_{2}\right)$.
(i)
Identify the following:
(1)
Transition metal of 3d series that exhibits only one oxidation state.
(2)
Transition metal of 3d series that acts as a strong reducing agent in+$\displaystyle 2$ oxidation state in aqueous solution.
(ii)
Complete and balance the following equations:
(1)
$\displaystyle \mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}+14 \mathrm{H}^{+}+6 \mathrm{Fe}^{2+} \longrightarrow$
(2)
$\displaystyle \mathrm{KMnO}_{4} \xrightarrow{\text { heat }}$
(iii)
What is Misch metal? Write its one use.
Marking-scheme solution
(a) (i)
(1) Because of small size, high ionic charge and availability of d-orbital.
(2) Because of stable half-filled \(\displaystyle 3 \mathrm{~d}^{5}\) configuration in \(\displaystyle \mathrm{Mn}^{2+}\).
(3) \(\displaystyle \mathrm{Cu}^{+}\)ion (aq.) undergoes disproportionation to \(\displaystyle \mathrm{Cu}^{2+}\) (aq.) and Cu /
\[2 \mathrm{Cu}^{+} \text {(aq.) ⟶ } \mathrm{Cu}^{2+} \text { (aq.) }+\mathrm{Cu} \text {. }
\]
(ii) \(\displaystyle \mathbf{2} \mathbf{M n O}_{\mathbf{2}} \boldsymbol{+} \mathbf{4} \mathbf{~ K O H} \boldsymbol{+} \mathbf{O}_{\mathbf{2}} \boldsymbol{\longrightarrow} \mathbf{2 ~ K}_{\mathbf{2}} \mathbf{M n O}_{\mathbf{4}} \boldsymbol{+} \mathbf{2 ~ H}_{\mathbf{2}} \mathbf{O}\)
\[\mathbf{3} \mathrm{MnO}_{4}^{2-}+\mathbf{4} \mathbf{H}^{+} \longrightarrow \mathbf{2} \mathrm{MnO}_{4}^{-}+\mathbf{M n O}_{\mathbf{2}}+\mathbf{2} \mathbf{H}_{\mathbf{2}} \mathbf{O}
\]
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CBSE Class 12 Chemistry past-paper question from the 2023 board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.