CBSE Class 12 Chemistry Notes: d and f Block Elements – Important Questions for Boards

Prob 1.  The chemical reactivity of lanthanides resemble to which other elements of the periodic table?

Sol.  The chemical reactivity of the starting lanthanides resemble calcium due to similar first and second ionization energy. But latter lanthanides resemble Al due to ability of showing +3 oxidation state and similarity in I.E.

Prob 2.    Enthalpies of atomization of transition elements are higher than those of alkali and alkaline earth metals. Explain.

Sol. The number of unpaired electrons in transition elements are more when compared to these in alkali and alkaline earth metals. As a result, the metallic bonds in transition metals are stronger and enthalpies of atomization are higher than those of alkali and alkaline earth metals.

Prob 3. Explain the following:

(a) Chromium is a typical metal while mercury is a liquid metal.

 (b) Zinc readily liberates H2 from cold dil. H2SO4 but not form cold conc. H2SO4.

Sol. (a) Chromium has five unpaired electrons in its d-orbitals which make its metallic bond very strong, whereas in mercury there is no unpaired d-electrons so its metallic bond is very weak, hence it is a liquid.

(b) Since, conc. H2SO4 act as an oxidizing agent hence does not evolve H2 when it reacts with zinc.


Prob 4. Compare thermal stability of ZnO, CdO and HgO.

 Sol.  ZnO > CdO > HgO

Prob 5.    Cu+ ion has 3d104s0 configuration and colourless but Cu2O is red and Cu2S is black. Explain.

 Sol. Cu+ ion has 3d104s0 configuration, i.e. it has no unpaired electron hence there is no d-d transition possible and it is colourless. But Cu2O and Cu2S are coloured due to charge transfer of electrons from O2- or S2- to the vacant orbital of Cu+ ion.

 

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