SOLVED PROBLEMS

Board Level Questions

Problem 1:The chemical reactivity of lanthanides resemble to which other elements of the periodic table?

Solution: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.

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

Solution: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. 

Problem 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.

Solution:
(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.

          Zn + 2H₂SO₄ ————→ ZnSO₄ + SO₂ + H₂O

 Solution: ZnO > CdO > HgO
 

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

Solution: 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 O^2- or S^2- to the vacant orbital of Cu⁺ ion.