Concept:The boiling point of a molecular substance is directly proportional to the strength of its intermolecular forces (IMFs). Hydrogen bonding is the strongest IMF present in these hydrides.
Formula:$$ \text{B.P.} \propto \text{Strength of IMF} $$
Solution:- \( \text{H}_2\text{O} \) has extensive hydrogen bonding (2 H-bonds per molecule) and a high boiling point of \( 100^\circ\text{C} \).
- \( \text{HF} \) has very strong hydrogen bonding due to high electronegativity, but forms fewer network bonds per molecule than water. Its boiling point is \( 19.5^\circ\text{C} \).
- \( \text{NH}_3 \) has weaker hydrogen bonding compared to HF, boiling at \( -33^\circ\text{C} \).
- \( \text{HCl} \) does not form strong hydrogen bonds (only dipole-dipole forces) and has the lowest boiling point at \( -85^\circ\text{C} \).
Why other options are incorrect:Other options incorrectly place HF above \( \text{H}_2\text{O} \) or switch the order of \( \text{NH}_3 \) and HCl, demonstrating a misunderstanding of the cumulative network effect of water's hydrogen bonds versus the single strongest bond in HF.
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