How would you account for the following?

(i) NF₃ is an exothermic compound but NCl₃ is not.

(ii) The acidic strength of compounds increases in the order:

PH₃ < H₂S < HCl
(iii) SF₆ is kinetically inert.

Answer

(i) As we move down the group 17, the size of the atom increases from fluorine to chlorine. The larger difference in the size of N and Cl results in the weakness of strength of N - Cl bond. On the other hand, the difference in size of N and F is small; consequently the N -F bond is quite strong. As a result, NF3 is an exothermic compound.

(ii) In a period, the electronegativity decreases in the order Cl > S > P. As a result, the loss of H⁺ ions decreases. Thus, the acidic strength of the hydrides decreases in the following order. HCl > H₂S > PH₃

(iii) The kinetic inertness of SF₆ can be explained on the basis of its structure.

As seen from the above structure, the six fluorides (F^-) atoms protect the sulphur atom from attack by the regents to such an extent that even thermodynamically most favourable reactions like hydrolysis do not occur.