Question

Account for the following :
(i) `NH_(3)` is a stronger base than `PH_(3)`
(ii) Sulphur has a greater tendency for catenation than oxygen.
(iii) Bond dissociation energy of `F_(2)` is less than that of `Cl_(2)`.
(OR)
Explain the following situations :
(i) In the structure of `HNO_(3)` moleucle, the `N-O` bond (121 pm) is shorter than the ` N-OH` bond (140 pm).
(ii) `SF_(4)` is easily hydrolysed whereas `SF_(6)` is not easily hydrolysed.
(iii) `XeF_(2)` has a straight linear structure and not a bent angular structure.

Answer 1

(i) Due to the presence of lone pair of electrons on the central atom both `NH_(3)` and `PH_(3)` are Lewis Bases . When `NH_(3)` or `PH_(3)` accepts a proton , an additional N-N or P-H bond is formed .
`H_(3)N : + H^(+) to NH_(4)^(+) , H_(3)P : +H^(+) to PH^(+)`
Due to smaller size of N than P , N-H bond thus formed is much stronger than P-H bond . As a result `NH_(3)` has more tendency than `PH_(3)` to accept a proton. Therefore `NH_(3)` is stronger base than `PH_(3)`.
(ii) Sulphur has a stronger tendency for catenation than oxygen because of stronger S-S bonds as compared to O-O bonds . Due to small size of oxygen , lone pairs of e`bars` on oxygen atomrepel the bond pair of O-O bond to greater extend have the O-O bond is weaker than S-S bond .
(iii) Due to smaller size , the lone pairs of electrons on the F-atom repel the bond pair of the F-F bond as compared the larger size of Cl atoms , the lone pairs of Cl do not repel the bond pair of Cl-Cl bond . Hence F-F bond energy is lower than that of Cl-Cl bond energy.