Concept: Volatility and Intermolecular Forces

Volatility is inversely related to the strength of intermolecular forces. Stronger intermolecular forces lead to higher boiling points and lower volatility. Among hydrogen halides (HX), the primary intermolecular forces are dipole-dipole interactions and London dispersion forces. However, HF exhibits an additional, much stronger intermolecular force: hydrogen bonding.

Why (A) is correct:

Hydrogen fluoride (HF) is the only hydrogen halide that forms strong hydrogen bonds due to the high electronegativity of fluorine and the small size of both hydrogen and fluorine atoms. These strong hydrogen bonds require significantly more energy to overcome compared to the weaker dipole-dipole interactions and London dispersion forces present in HCl, HBr, and HI. Consequently, HF has an anomalously high boiling point (19.5 °C) compared to the other hydrogen halides, making it the least volatile among them.

Option Analysis:

• A) HF: Forms strong hydrogen bonds, leading to a high boiling point and low volatility. This is the correct answer.
• B) HCl: Exhibits dipole-dipole interactions and London dispersion forces. Its boiling point is -85 °C.
• C) HBr: Exhibits dipole-dipole interactions and stronger London dispersion forces than HCl due to larger size. Its boiling point is -66.8 °C.
• D) HI: Exhibits dipole-dipole interactions and the strongest London dispersion forces among HCl, HBr, and HI due to its largest size. Its boiling point is -35.3 °C.

While London dispersion forces increase from HCl to HI, leading to an increase in boiling points in that series, the dominant effect of hydrogen bonding in HF makes it significantly less volatile than all other hydrogen halides.

Correct Answer: (A)