[xylofunk]

Hi there!

I'm a little confused by this question on Heat of Vaporization:

Which one of the following would have the highest molar heat of vaporization?

a. CH₃NH₂
b. CH₄
c. C₂H₆
d. SiH₄
e. H₂S

Online I see that many people mark the answer as CH₃NH₂, but H₂S has the highest molecular weight out of the five choices.  Is there a point where intermolecular forces overwhelms molecular weight or vice versa when considering heat of vaporization? How do I determine which is more important in general?

Thanks so much!

[Corribus]

Molecular weight is only a surrogate for intermolecular forces.  That is, what matters is intermolecular forces, and there is a rough correlation between molecular weight and strength of intermolecular forces... because heavier molecules tend to have stronger van der Waals and London dispersion interactions. There are plenty of exceptions, however, where smaller molecules can have stronger intermolecular forces (and thus heat of vaporization) than larger once.  Butane, for existence, is a gas a room temperature while water is a liquid, even though butane is ... what, three times as heavy?

Use molecular weight as a guide, not a rule.

[xylofunk]

Thanks for your response!