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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 4 · Set 1 · Q31
(a) (i) Account for the following: (I) MnO is basic while Mn_2 O_7 is acidic. (II) Iron has higher enthalpy of atomization than that of copper. (III)…
(i)
Account for the following :
(I)
MnO is basic while $\displaystyle \mathrm{Mn}_{2} \mathrm{O}_{7}$ is acidic.
(II)
Iron has higher enthalpy of atomization than that of copper.
(III)
$\displaystyle \mathrm{Mn}^{3+}$ is a stronger oxidising agent than $\displaystyle \mathrm{Cr}^{3+}$.
(ii)
How do you prepare potassium dichromate from sodium chromate ? Write balanced chemical equation for each step.
Answer the following questions :
(i)
Why is chemistry of actinoids more complicated as compared to lanthanoids ?
(ii)
$\displaystyle \mathrm{E}_{\mathrm{M}^{2+} / \mathrm{M}}^{\mathrm{o}}$ values are not regular for first row transition elements (3d series). Why ?
(iii)
Identify the following :
(I)
Oxoanion of chromium which is stable in acidic medium.
(II)
The lanthanoid element that exhibits +$\displaystyle 4$ oxidation state.
(iv)
Write the products obtained on heating $\displaystyle \mathrm{KMnO}_{4}$.
(v)
Predict which of the following ions will be coloured in aqueous solution : \[\mathrm{Cu}^{+}, \mathrm{Sc}^{3+}, \mathrm{Fe}^{2+}, \mathrm{Ti}^{3+}, \mathrm{Zn}^{2+} \]
Marking-scheme solution
(i)
(I)
Because Mn is in lower Oxidation state (+$\displaystyle 2$) in MnO while in Mn2O7, Mn
is in higher Oxidation state (+$\displaystyle 7$).
(II)
Because of more number of unpaired electrons in iron than copper.
(III)
Greater stability of Mn in +$\displaystyle 2$ Oxidation state is due to 3d5 configuration whereas
Cr is already stable in +$\displaystyle 3$ oxidation state due to stable t2g3 configuration.
(ii)
2Na2CrO4 + 2H+ Na2Cr2O7 + 2Na+ + H2O
Na2 Cr2O7 + 2KCl K2Cr2O7 + 2NaCl
(i)
Because of their radioactive nature and ability to show wide range of oxidation
states.
(ii)
Because the sum of ionisation enthalpies (∆iH1+ ∆iH2) and sublimation enthalpies
are irregular across 3d series.
(iii)
(I)
Cr2O72-
(II)
Cerium / Ce
(iv)
(v)
Fe2+ , Ti3+
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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.