Question

Which hydride amongst the following has the least boiling point?

• A. NH$_3$
• B. PH$_3$
• C. AsH$_3$
• D. SbH$_3$
• E. BiH$_3$

Hint:

In hydride groups ($15, 16, 17$), the first member usually has an anomalously high boiling point due to hydrogen bonding, making the second member (like PH$_3$ or H$_2$S) the one with the lowest boiling point.

Answer 1

The Correct Option is B

Concept: The boiling point of hydrides depends on molecular weight (Van der Waals forces) and the presence of intermolecular hydrogen bonding. Generally, boiling point increases down a group with increasing size, unless hydrogen bonding is present.

Step 1: {Evaluate the effect of Hydrogen bonding.}
Ammonia (NH$_3$) contains highly electronegative Nitrogen, allowing it to form strong intermolecular hydrogen bonds. This gives it a significantly higher boiling point than its heavier neighbor, PH$_3$.

Step 2: {Analyze the trend for other Group 15 hydrides.}
From PH$_3$ to BiH$_3$, the molecular size and mass increase. This leads to stronger Van der Waals (London dispersion) forces, which progressively increases the boiling point: $$\text{PH}_3 < \text{AsH}_3 < \text{SbH}_3 < \text{BiH}_3$$

Step 3: {Compare NH$_3$ and PH$_3$.}
While NH$_3$ has a high boiling point due to hydrogen bonding, PH$_3$ lacks hydrogen bonding and has the lowest molecular weight among the remaining hydrides. Consequently, PH$_3$ has the lowest boiling point in the series.