Which of the following substances is expected to have the highest boiling point at standard pressure?
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Correct answer
B. HF
Principle or equation
Boiling point depends on the strength of intermolecular forces. Hydrogen bonding is a particularly strong intermolecular force that occurs when hydrogen is bonded to highly electronegative atoms like F, O, or N. Among the given hydrogen halides, HF can form hydrogen bonds, while HCl, HBr, and CH4 cannot.
Why this answer is correct
HF has hydrogen bonding due to the high electronegativity of fluorine and the small size of the F atom, leading to strong dipole-dipole interactions. HCl, HBr, and CH4 have weaker dispersion forces and dipole-dipole interactions (for HCl and HBr) but no hydrogen bonding. Therefore, HF has the highest boiling point.
Example
Water (H2O) has a higher boiling point than H2S because water forms hydrogen bonds, while H2S does not.
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