d and f Block Elements — CBSE Class 12 · 2019 Board Question with Solution

Give reasons for the following:
(i) Transition elements and their compounds act as catalysts.
(ii) The E° value for Mn²⁺/Mn is negative whereas for Cu²⁺/Cu it is positive.
(iii) Actinoids show irregularities in their electronic configurations.

Solution

(i) They adopt multiple oxidation states, forming reactive intermediates that lower the activation energy, and their surfaces adsorb reactants through vacant d-orbitals — both effects speed reactions (Fe in Haber's process, Ni in hydrogenation).
(ii) E°(M

^{2 +}

/M) balances atomisation + ionisation against hydration. For Mn the ionisation cost is modest and the product Mn

^{2 +}

(stable 3d

^{5}

, strongly hydrated) forms readily — E° is negative (−1.18 V). For Cu the high enthalpy of atomisation plus IE

_{1}

+IE

_{2}

exceed the hydration energy of Cu

^{2 +}

— E° ends up positive (+0.34 V).
(iii) The 5f and 6d orbitals lie so close in energy that electrons distribute irregularly between them (with f

^{0}

^{7}

^{14}

stability also competing), so actinoid configurations do not follow a smooth pattern.

CBSE Class 12 Board · 2019 · Subjective

CBSE Class 12 · 2019 · d and f Block ElementsShort Answer

Solution

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