Which has the highest heat of vaporisation and why?

Options

A. `HF`
B. `HCl`
C. `HBr`
D. `HI`

Answer

To determine which compound has the highest heat of vaporization among HF, HCl, HBr, and HI, we can follow these steps:

### Step 1: Understand Heat of Vaporization
Heat of vaporization is defined as the amount of heat energy required to convert one mole of a liquid into its vapor at its boiling point.

### Step 2: Analyze the Compounds
The compounds in question are HF (hydrogen fluoride), HCl (hydrogen chloride), HBr (hydrogen bromide), and HI (hydrogen iodide).

### Step 3: Identify Intermolecular Forces
- **HF**: Exhibits strong hydrogen bonding due to the presence of fluorine, which is highly electronegative.
- **HCl, HBr, HI**: These compounds primarily exhibit dipole-dipole interactions. While they do have some polar characteristics, they do not have hydrogen bonding.

### Step 4: Compare the Strength of Intermolecular Forces
- **Hydrogen Bonding**: HF has strong hydrogen bonds, which require a significant amount of energy to break.
- **Dipole-Dipole Interactions**: HCl, HBr, and HI have weaker dipole-dipole interactions compared to the hydrogen bonds in HF.

### Step 5: Conclusion
Since HF has strong hydrogen bonds, it requires more energy to convert it from liquid to vapor compared to HCl, HBr, and HI, which only have dipole-dipole interactions. Therefore, HF has the highest heat of vaporization among the four compounds.

### Final Answer
**HF (hydrogen fluoride) has the highest heat of vaporization due to the presence of strong hydrogen bonding.**

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