Concept: Intermolecular Forces and Boiling Point

Boiling point is directly related to the strength of intermolecular forces. Stronger intermolecular forces require more energy to overcome, leading to a higher boiling point. Hydrogen bonding is a particularly strong type of intermolecular force.

Why (C) is correct:

Fluorine is a highly electronegative atom. When hydrogen is bonded to a highly electronegative atom like fluorine (F), oxygen (O), or nitrogen (N), it forms a strong intermolecular force called a hydrogen bond. In HF, the hydrogen atom of one molecule forms a hydrogen bond with the highly electronegative fluorine atom of an adjacent HF molecule. This leads to the association of HF molecules into larger aggregates. Overcoming these strong hydrogen bonds requires a significant amount of energy, resulting in a comparatively high boiling point for HF compared to other hydrogen halides (HCl, HBr, HI) which primarily exhibit weaker dipole-dipole interactions and London dispersion forces.

Option Analysis:

• A) High reactivity of fluorine: While fluorine is highly reactive, its reactivity does not directly explain the high boiling point. Boiling point is related to intermolecular forces, not chemical reactivity.
• B) Small size of hydrogen atom: The small size of the hydrogen atom is a factor that allows it to participate in hydrogen bonding, but it's the formation of the bond itself, not just the size, that causes the high boiling point.
• D) High IE of fluorine: High ionization energy (IE) of fluorine indicates its reluctance to lose electrons, which is related to its electronegativity, but it's not the direct cause of the high boiling point. The high electronegativity leads to hydrogen bonding.

Correct Answer: (C)