Hydrogen fluoride (HF) has the highest boiling point among hydrogen halides (HF, HCl, HBr, HI) due to the presence of strong intermolecular hydrogen bonding.

Let's analyze the boiling points and the intermolecular forces:

• HF: 19.5°C
• HCl: -85°C
• HBr: -67°C
• HI: -35°C

Step 1: Understanding Intermolecular Forces
The boiling point of a substance is a measure of the energy required to overcome the intermolecular forces holding the molecules together in the liquid state. Stronger intermolecular forces lead to a higher boiling point.

Step 2: Comparing the Forces in Hydrogen Halides
For the hydrogen halides (HX), the primary intermolecular forces are:

1. Hydrogen Bonding: This is a special, very strong type of dipole-dipole interaction. It occurs when hydrogen is covalently bonded to a highly electronegative atom (F, O, N). Fluorine (F) is the most electronegative element, making the H-F bond highly polar. This allows HF molecules to form very strong hydrogen bonds with each other.
2. van der Waals' Interactions (London Dispersion Forces): These are weak forces that increase with increasing molecular size and mass. As we move down the group from HCl to HI, the molecular size and mass increase, which is why the boiling points increase from HCl to HI. However, these forces are much weaker than hydrogen bonding.

Step 3: Why the Other Options are Incorrect

• Lowest Dissociation Enthalpy: The H-F bond has the highest bond dissociation enthalpy due to its short bond length and high polarity. This high bond strength is related to the covalent bond within an HF molecule, not the intermolecular forces between molecules. Intermolecular forces determine the boiling point.
• Strongest van der Waals' Interactions: Van der Waals forces are weakest in HF because it is the smallest molecule. They are strongest in HI, which has the most electrons. Yet, HF's boiling point is highest, proving a stronger force (hydrogen bonding) is at work.
• Lowest Ionic Character: The H-F bond actually has the highest percent ionic character (around 43%) among the H-X bonds due to the greatest electronegativity difference. This high polarity is what enables the strong hydrogen bonding.

Final Answer: HF has the highest boiling point because it has the strongest hydrogen bonding.

Related Topics & Formulae

Hydrogen Bonding: A strong dipole-dipole attraction between a hydrogen atom (which is bonded to a highly electronegative atom like F, O, or N) and a lone pair of electrons on another electronegative atom. It is represented as X-H···Y, where X and Y are F, O, or N.

Trend in Boiling Points: The anomalous boiling point of HF is a classic example of how intermolecular forces override the expected trends based on molecular mass. ${\mathrm{BP}}_{HF}>{\mathrm{BP}}_{HI}>{\mathrm{BP}}_{HBr}>{\mathrm{BP}}_{HCl}$