Question

The variation of the boiling points of the hydrogen halides is in the order $HF > HI > HBr > HCl$. What explains the higher boiling point of hydrogen fluoride ?

• A. The bond energy of HF molecules is greater than in other hydrogen halides
• B. The effect of nuclear shielding is much reduced in fluorine which polarises the HF molecule
• C. The electronegativity of fluorine is much higher than for other elements in the group.
• D. There is strong hydrogen bonding between HF molecules

Answer 1

The Correct Option is D

The question asks about the reason for the higher boiling point of hydrogen fluoride (HF) compared to other hydrogen halides such as HI, HBr, and HCl. To understand this, let's examine the concepts involved:

1. Understanding Boiling Points: Boiling point is the temperature at which a liquid turns into vapor. It is influenced by intermolecular forces. Stronger intermolecular forces lead to higher boiling points.
2. Intermolecular Forces in Hydrogen Halides: These include van der Waals forces and hydrogen bonding. In hydrogen halides, the presence of hydrogen bonding can significantly influence boiling points.
3. Hydrogen Bonding in HF: HF has strong hydrogen bonding due to the high electronegativity of fluorine. This is because hydrogen bonding occurs when hydrogen is bonded to highly electronegative elements like fluorine, which results in a strong dipole-dipole attraction between HF molecules. This is not as significant in other hydrogen halides due to their lower electronegativity.
4. Comparison with Other Options:
   • The bond energy and nuclear shielding stated in other options do not directly affect the boiling point in the same way as intermolecular hydrogen bonding.
   • While fluorine’s electronegativity is high and contributes to the dipole moment, it is the specific hydrogen bonding that most significantly raises HF's boiling point.
5. Conclusion: Among the provided options, the most accurate explanation for the higher boiling point of HF compared to other hydrogen halides is the presence of strong hydrogen bonding between HF molecules. This enhanced intermolecular attraction raises the energy required for HF to transition from liquid to gas.