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.

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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