Why does HI have a higher boiling point than HBr, and HCl?

1 Answer
anor277
Dec 12, 2016

The hydrogen iodide molecule is larger and more polarizable than hydrogen chloride..............

Explanation:

Given its polarizability, the diffuse electron cloud of the HI molecule allows more effective (i.e. more attractive) intermolecular interaction. Given this enhanced intermolecular interaction, HI should have a higher boiling point/melting point than HCl. You will have to look up the physical constants, [here is a start.](https://www.quora.com/Out-of-HF-HCl-HBr-and-HI-which-has-the-lowest-and-highest-boiling-point-and-why)

But hang on, HF, has a boiling point of 19.5 ∘C, which is the highest of the hydrogen halides. Doesn't this contradict what I have said? What's going on?

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