Question

How does electronegativity affect boiling point?

Answer 1

Compounds that have hydrogen atoms bound to strongly electronegative atoms tend to have disproportionately high boiling points.

Explanation:

The cases in point are `H_2O` and `HF`. `H_2O` has a normal boiling point of `100` `""^@C`; `HF` has a normal boiling point of `19.5` `""^@C`. Because hydrogen is bound to a strongly electronegative element in each molecule, the heteroatom polarizes electron density towards itself such that the hydrogen acquires a positive charge, and the heteroatom, `O` or `F` acquires a partial negative charge.

This charge separation, this polarity, is an additional INTERMOLECULAR force that binds molecules together, and must be overcome before the molecules enter the gas phase. The boiling point of water far exceeds that of its Group VI congeners, hydrogen sulfide, and hydrogen selenide; which are are room temperature gases. `HCl`, and `HBr`, and `HI` are also all room temperature gases and for the lower Group members the degree of intermolecular hydrogen bonding is diminished (though their ionization upon aqueous solution allows their shipment as the aqueous acids).

Note also that ammonia (while this is a gas at RT) also has an unusually high boiling point, at `-33.3` `""^@C`, for precisely the same reasons.