CBSE 2015, Set 2

Question

(a) Account for the following :
(i) Zr and Hf have almost similar atomic radii.
(ii) Transition metals show variable oxidation states.
(iii) $Cu^+$ ion is unstable in aqueous solution.

(b) Complete the following equations :
(i) $2MnO_2 + 4KOH + O_2 \rightarrow$
(ii) $2Na_2CrO_4 + 2H^+ \rightarrow$

Edvaya · Accepted Answer

The correct answer is Subjective. **SOLUTION** (a) (i) Due to lanthanide contraction, Zr and Hf have almost similar atomic radii. It can be explained on the basis of shielding effect. The electrons present in inner shells, shield the outer electrons from nuclear charge, making them experience a low effective nuclear charge. The shielding effect exerted by the electrons decreases in the order s > p > d > f. The f subshell poorly shields the outer electrons from nuclear attraction, which results in more attractive pull of nucleus on outer electron. In case of post lanthanide elements like Hf, 4f subshell is filled and it is not very effective at shielding the outer shell electrons. Therefore, Zr and Hf have almost similar atomic radii. (ii) The transition metals have their valence electrons in (n-1)d and ns orbitals. Since, there is very little energy difference between these orbitals, both energy levels can be used for bond formation. Thus, transition elements exhibit variable oxidation states. (iii) $Cu^{2+}$ is more stable than $Cu^+$ in aqueous medium. $Cu^{2+}$ has high hydration energy that compensates for the energy required to remove one electron from $Cu^+$ to form $Cu^{2+}$. So in aqueous medium, $Cu^+$ gives $Cu^{2+}$ and Cu. $$2Cu^+ (aq) \rightarrow Cu^{2+} (aq) + Cu(s)$$ (b) (i) $2MnO_2 + 4KOH + O_2 \rightarrow 2K_2MnO_4 + 2H_2O$ (ii) $2Na_2CrO_4 + 2H^+ \rightarrow Na_2Cr_2O_7 + 2Na^+ + H_2O$

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Chemistry > Inorganic Chemistry > D And F Block Elements

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