Arrange the following in the order of property indicated for each set :
(i) `F_(2), Cl_(2) Br_(2),I_(2)`- increasing bond dissociation enthalpy.
HF,HCl, HBr, HI- increasing acid strength.
`NH_(3), PH_(3), AsH_(3), SbH_(3), BiH_(3)`- increasing base strength.

Answer

(i)Bond dissociation enthalpy should decrease as the bond distance increases from `F_(2)` to `I_(2)` due to the corresponding increase in the size of the atom as we move from `F` to `I`.However, the `F-F` bond dissociation enthalpy is smaller than that of `Cl-Cl` and even smaller than that of `Br-Br`.This is due to the reason that the `F` atom is very small and hence the three lone pairs of electrons on each `F` atom repel the bond pair holding the `F`-atom in `F_(2)` molecule. Thus, the bond dissociation enthalpy increases in the order : `I_(2)ltF_(2)ltBr_(2)ltCl_(2)`.
(ii)The relative acid strength of `HF , HCl HBr` and `Hl` depends upon their bond dissociation enthalpies. Since the bond dissociation enthalpy of `H-X` bond decreases from `H-F` to `H-I` as the size of atom increases from `F` to `l`.Therefore, acid strength increases in the opposite order, i.e., the acid strength increases in the order : `HFltHClltHBrltHI`.
(iii)Due to the presence of a lone pair of electrons on the central atom in `NH_(3), PH_(3), AsH_(3), SbH_(3)` and `BiH_(3)` all behave as Lewis bases. However, as we move from `NH_(3)` to `BiH_(3)`.the size of the atom increases.As a result, the lone pair of electrons occupies a larger volume. In other words, the electron density on the central atom decreases and hence the basic strength decreases as we move from `NH_(3)` to `BiH_(3)` Thus, the basic strength increases in the order : `BiH_(3)ltSbH_(3)ltAsH_(3)ltPH_(3)ltNH_(3)`