Question

Arrange the following in the order of property indicated for each set:

(i) F₂, Cl2, Br₂, I₂ - increasing bond dissociation enthalpy.
(ii) HF, HCl, HBr, HI - increasing acid strength.
(iii) NH_3, PH₃, AsH₃, SbH₃, BiH₃ − increasing base strength.

Answer 1

(i) Bond dissociation energy usually decreases on moving down a group as the atomic size increases. However, the bond dissociation energy of F₂ is lower than that of Cl₂ and Br₂. This is due to the small atomic size of fluorine. Thus, the increasing order for bond dissociation energy among halogens is as follows:
I₂ < F₂ < Br₂ < Cl₂
(ii) HF < HCl < HBr < HI
The bond dissociation energy of H-X molecules where X = F, Cl, Br, I, decreases with an increase in the atomic size. Since H-I bond is the weakest, HI is the strongest acid.
(iii) BiH₃ ≤ SbH₃ < AsH₃ < PH₃ < NH₃
On moving from nitrogen to bismuth, the size of the atom increases while the electron density on the atom decreases. Thus, the basic strength decreases.