Molecular Interactions Governing Vapor Pressure

Enthalpy and Entropy Contributions to the Free Energy of Vaporization 

Now we can see how a chemical s structure causes it to have a particular vapor pressure. This is possible because, as a first approximation, the free energy of vaporization, A G, mostly differs from compound to compound due to differences in those substances enthalpies of vaporization, Avap//,-. These enthalpies reflect the sum of intermolecular attractions that act to hold those liquid molecules together. Thus, we can expect that substances that exhibit high vapor pressures have structures that do not enable the molecules to have strong intermolecular attractions. Conversely, molecules with low vapor pressures must have structures that cause the molecules to be substantially attracted to one another. 

Moreover, this relation between chemical structure and vapor pressure also holds because enthalpies and entropies of vaporization are directly related, in general. Recall that the entropy of vaporization reflects the difference of a molecule s freedom in the gas phase versus the liquid phase (A pS = Si% - SiL). At ambient pressures, we may assume that differences in Avap5) between different compounds are primarily due to differences in molecular freedom in the liquid phase. (The freedom of the molecules in the gas phase is not that different between compounds). Hence, not surprisingly, molecules that exhibit stronger intermolecular attractions 

In general, we see that the enthalpic contribution is larger than the entropic one, but also that these contributions co-vary. This is true for a very diverse group of compounds at a given temperature (25°C), including apolar, monopolar, and bipolar compounds. Hence, if we view the forces between the molecules (the glue ) to be reflected primarily in the enthalpy term, then p h is a direct measure of these forces in the pure liquid. 

Trouton s Rule of Constant Entropy of Vaporization at the Boiling Point 

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Let us now try to derive a model that allows us to express quantitatively the molecular interactions that govern the liquid vapor pressure. We do this, not primarily with the goal of developing a predictive tool for estimating p L, but to... 

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