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

Prob 1. While Cu, Ag and Au are considered as transition elements but Zn, Cd and Hg are not considered as transition elements although all the mentioned elements have complete d-orbitals. Explain.

Sol. Although Cu, Ag and Au have their d – orbitals complete in the elemental state. They do have incomplete d orbitals in their compound state. So they are included in the transition elements.

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Zn, Cd and Ag have their d-orbitals complete in their elemental state as well as compound state. So they are not included in the transition elements.

Prob 2. (i) CrO₃ is an acid anhydride. Explain.

(ii) Between Na⁺ and Ag⁺ which is a stronger Lewis acid and why?

Sol. (i) , i.e CrO₃ is formed by loss of one H₂O molecule from chromic acid.

(ii) Between Na⁺ and Ag⁺, Ag⁺ is stronger Lewis acid. Because Ag⁺ has pseudo noble gas configuration which makes it more polarizing.

Prob 3. It is well known that alkali and alkaline earth metals displace hydrogen from dilute acids. But most of the transition elements do not behave so. Explain.

Sol. Alkali and alkaline earth metals have positive oxidation potential. But most of the transition elements have negative oxidation potentials. So they are not as good oxidizing agents as the alkali and alkaline earth metal are.

Prob 4. In the melting point curves of transition metals, one observes a dip in the curves at the end i.e. Cu, Ag & Au and Zn, Cd & Hg have lower melting points when compared to other transition metals. Explain.

Sol. In the last two groups of transition elements i.e. Cu, Ag, Au, Zn, Cd and Hg all the electrons are paired which can not take part in metallic bonding. As a result, metallic bond in these elements is weak resulting in the lower melting points of these metals.

Prob 5. 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 those 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 6. Explain the following:

(a) Scandium forms no coloured ions, yet it is regarded as a transition element.

(b) Transition elements have many irregularities in electronic configurations.

Sol. (a) Scandium in the ground state has one d electron. Hence it is regarded as transition element.

(b) In the transition elements, the (n – 1)d subshell and ns subshell have very small difference in energy. The incoming electron may enter into either ns or (n-1)d subshell. Hence they show irregularities in their electronic configurations.

Prob 7. Explain the following

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

(b) Cobalt (II) is stable in aqueous solution but in the presence of strong ligands, it is a easily oxidised to cobalt (III).

Sol. (a) chromium has 5 unpaired electrons in its d – orbitals which make its metallic bond very stronger. Whereas in mercury there are no unpaired d electrons, so its metallic bond is very weak.

(b) CO(III) has greater tendency to form complex than CO(II) hence in the presence of ligands CO(II) changes to CO(III).

Prob 8. Write down the products of the following reactions.

(a) CuSO₄ solution is treated with KI solution.

(b AgNO₃ solution is added to Na₂S₂O₃ solution.

Sol. (a) Free iodine is liberated along with the formation of a white precipitate of cupric iodide.

(b) A white precipitate of Ag₂S₂O₃ is obtained which turns yellow, brown and finally black on keeping.

Prob 9. Explain the following

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

(b) Blue colour of the CuSO₄ solution is discharged slowly when an iron rod is dipped into it.

Sol.(a) Conc. H₂SO₄ is a covalent compound. Hence does not contain H⁺ ions. Dilute H₂SO₄ contains H₃O⁺ which reacts with Zn and liberates H₂.

(b) Fe is more electropositive than Cu, hence it displaces copper form CuSO₄ solution.

Prob 10. An aqueous solution containing one mole of HgI₂ and two moles of NaI is orange in colour. On addition of excess NaI the solution becomes colourless. The orange colour reappears on subsequent addition of NaOCl. Explain with equations.

Sol.