Question

How are the volatilities of the Group 16, and Group 17 hydrides rationalized?

Answer 1

Two factors are important here: (i) hydrogen bonding; and (ii) dispersion forces.

Explanation:

Of course, water possesses the greatest degree of hydrogen bonding. Oxygen as a 1st row element, is far more electronegative than sulfur and tellurium, and the `O-H` is far more polar as a result. The resultant intermolecular hydrogen bonding between adjacent molecules accounts for the exceptionally high boiling point of water, compared to the lower Group VI hydrides. This same differential boiling point can be seen of the Group VII hydrides. Compare the boiling point of `HF` versus `HCl` and `HBr`; hydrogen bonding accounts for the disparity.

For the other Group VI hydrides, dispersion forces are the dominant intermolecular force. Dispersion forces result from the transient polarization of the electron cloud. Thus bigger atoms, and bigger atoms in molecules, have a greater degree of dispersion forces, because there are more electrons, and more polarizable electron clouds. Thus after water, `H_2Te`, and `H_2Se`, have the highest boiling points.

That hydrogen sulfide has the lowest boiling point is harder to rationalize. The degree of hydrogen bonding for the `H_2S` molecule is probably marginal. Sulfur is far less electronegative than oxygen, so the polarity of the `H-S` bonds is dwarfed by the the polarity of the `H-O` bond. Thus, hydrogen sulfide, with the least degree of intermolecular interaction, has the lowest boiling point.

As an aside, you probably know that hydrogen sulfide smells pretty foul. It is also a fairly poisonous gas. When I was a grad student there was an inorganic lab next door that dealt with selenides and tellurides. Of course, a bit of sloppiness next door and the odour of hydrogen sulfide, hydrogen selenide, and hydrogen telluride would waft down the corridor, and sometimes cleared the floor, and sometimes the building. While `H_2S` smells foul, `H_2Se` and `H_2Te` smell worse than a dead dog; the smell was quite revolting. We found out later that these gases were more poisonous than hydrogen cyanide.