Question

Give reasons for the following:

(i) CCl₄ is immiscible in water, whereas SiCl₄ is easily hydrolysed.

(ii) Carbon has a strong tendency for catenation compared to silicon.

Answer 1

(i) CCl₄ is a covalent compound while H₂O is a polar compound. Therefore, it is insoluble in water. Alternatively, CCl₄ is insoluble in water because carbon does not have orbitals to accommodate the electrons donated by oxygen atom of water molecules. As a result, there is no interaction between CCl₄ and water molecules and hence CCl₄ is insoluble in water. On the other hand, SiCl₄ has d-orbitals to accommodate the lone pair of electrons donated by oxygen atom of water molecules. As a result, there is a strong interaction between SiCl₄ and water molecules. Consequently, SiCl₄ undergoes hydrolysis by water to form silicic acid.

(b)  The bond dissociation energy decreases rapidly as the atomic size increases. Since the atomic size of carbon is much smaller (77 pm) as compared to that of silicon (118 pm), therefore, carbon-carbon bond dissociation energy is much higher (348 kJ mol^-1) than that of silicon-silicon bond (297 kJ mol^-1). Hence, because C - C bonds are much stronger as compared to Si-Si bonds, carbon has a much higher tendency for catenation than silicon.