Choose the correct statements in respect of hydrides of group 15. (A) Reducing power increases down the group (B) Basic nature increases down the group (C) Stability decreases down the group (D) Boiling point decreases regularly down the group.
Show Hint
The anomalous boiling point of $\text{NH}_3$ (and $\text{H}_2\text{O}, \text{HF}$) is a classic example of the effect of intermolecular hydrogen bonding.
(A) Reducing power: As we move down group 15 ($\text{NH}_3 \to \text{BiH}_3$), the E-H bond length increases and bond dissociation energy decreases. It becomes easier to release hydrogen. Reducing power increases. Correct.
(B) Basic nature: Depends on the availability of the lone pair. As size of the central atom increases, electron density decreases. Basic nature decreases down the group ($\text{NH}_3>\text{PH}_3>\dots$). Incorrect.
(C) Thermal stability: Decreases down the group due to weakening E-H bond. Correct.
(D) Boiling point: Expected to increase with molar mass. However, $\text{NH}_3$ has a higher boiling point than $\text{PH}_3$ and $\text{AsH}_3$ due to hydrogen bonding. The trend is $\text{PH}_3<\text{AsH}_3<\text{NH}_3<\text{SbH}_3<\text{BiH}_3$. It does not decrease regularly. Incorrect.
Correct statements: A and C.
