# Validity of Trouton's Rule

I would like to know under what circumstances is Trouton's rule obeyed by liquids and why certain systems i.e. substances may deviate from Trouton's rule.

By the way I know what Trouton's rule is and I have a vague idea that sometimes Trouton's rule is not obeyed due to the presence of hydrogen bonding in liquids such as water but I would like to know more about other reasons why certain situations the rule is invalid.

The rule says that for many liquids the entropy of vaporization is almost the same, around ${\rm 85\,J K^{-1}}$.

The (partial) success of the rule is due to the fact that the entropy of a gas is so much larger than that of any liquid. Therefore, the entropy of the initial state (e.g. the liquid) usually makes no big difference. However, if the liquid presents hydrogen bonding or any other kind of high ordered structure, its entropy will be particularly low and the entropy gain during vaporization will be larger than usual, too.

It's true that the rule states that entropy of vapourization is $85 – 88~\mathrm{J/K}$.However, the rule hardly works for high ordered substances exhibiting hydrogen bonding.

Other factors like the enthalpy of vaporization for a long chained organic molecule {strength of Van der Waals forces} may also play some significance role.

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