Question

Which hydride amongst the following has the least boiling point?

• A. $NH_3$
• B. $PH_3$
• C. $AsH_3$
• D. $BF_3, PF_5$

Answer 1

Answer: (B)

$PH_3$

Answer 2

(a) $NH _{3} \rightarrow$ Inspite of small size of $N$ -atoms. It has strong H-bonds due to high electronegativity of nitrogen. Thus has high boiling point.

(b) $PH _{3} \rightarrow$ Due to the small size of $P$ -atoms, it has least van der Walls force and will not form intermolecular H-bonds.

(c) $AsH _{3} \rightarrow$ It has larger As-atoms which provides more van der Walls force.

(d) $BF _{3} \cdot PF _{5} \rightarrow$ It has also larger molecular size than that of $PH _{3}$ and $AsH _{3}$, thus has more van der Walls force.

(e) $BiH _{3} \rightarrow$ It has larger size than that of $PH _{3}$, thus offers more van der Walls force. Thus $PH _{3}$ has least boiling point.