Ncert Exercises & Solutions

The correct order of the boiling points of \(\mathrm{H}_2 \mathrm{O}, \mathrm{HF}, \text{ and} \ \mathrm{NH}_3\) is:

1. HF > H₂O > NH₃

2. H₂O > HF > NH₃

3. NH₃> HF > H₂O

4. NH₃> H₂O > HF

HINT: Boiling point depends on the extent of hydrogen bonding.

Explanation
Number of hydrogen bonds per molecule (most important factor):

H₂O: Each molecule has 2 H atoms (donors) and 2 lone pairs on O (acceptors) → can form up to 4 hydrogen bonds per molecule (average ~2–4 in liquid state, forming a 3D network). This extensive network requires the most energy to break → highest boiling point.
HF: Each molecule has 1 H atom (donor) and 3 lone pairs on F (acceptors) → can form an average of 2 hydrogen bonds per molecule (one as donor, one as acceptor in zigzag chains). Fewer bonds than water → lower boiling point than H₂O.
NH₃: Each molecule has 3 H atoms (donors) and 1 lone pair on N (acceptor) → can form an average of only 1 hydrogen bond per molecule (limited by the single acceptor site). Weakest hydrogen bonding network → lowest boiling point.