Question -
Answer -
(i) F_2, Cl_2, Br_2, I_2 - increasing bond dissociation enthalpy.
(ii) HF, HCl, HBr, HI - increasing acid strength.
(iii) NH_3, PH_3, AsH_3, SbH_3, BiH_3 − increasing base strength.
Answer
(i) Bonddissociation energy usually decreases on moving down a group as the atomic sizeincreases. However, the bond dissociation energy of F2 is lower than that ofCl2 and Br2. This is due to the small atomic size of fluorine. Thus, theincreasing order for bond dissociation energy among halogens is as follows:
I2 < F2 2 < Cl2
(ii) HF< HCl < HBr < HI
The bond dissociation energy ofH-X molecules where X = F, Cl, Br, I, decreases with an increase in the atomicsize. Since H-I bond is the weakest, HI is the strongest acid.
(iii) BiH3 ≤ SbH3 3 < PH3 < NH3
On moving from nitrogen tobismuth, the size of the atom increases while the electron density on the atomdecreases. Thus, the basic strength decreases.