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

Explanation

(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-atoms in `F_(2)` molecules. Thus, the bond dissociation enthalpy increases in the order : `I_(2) lt F_(2) lt Br_(2) lt Cl_(2)`
(ii) The relative acid strength of HF, HCl, HBr and HI 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 I. Therefore, the acid strength increases in the opposite order, i.e., the acid strength increases in the order : HF lt HCl lt HBr lt HI.
(iii) Due to the presence of a lene 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) lt SbH_(3) lt AsH_(3) lt PH_(3) lt NH_(3)`.