Lewis/Randall rule and Henry s law

The Gibbs/ Duhem equation provides a relation between the Lewis/Randall rule and Henry s law. Substituting dGt from Eq. (11.28) for dAft in Eq. (11.8) gives, for a binary solution at constant T and P,... 

Both the Lewis-Randall rule and Henry s law represent limiting behaviors where the fugadty of component maybe calculated using simple relationships. A system that behaves according to the Lewis/Randall rule is called ideal in the Lewis-Randall sense (or simply, ideal solution), and a system that obeys Henry s law is called ideal in the sense of Henry s law. And in both cases, activity coefficients are introduced to account from deviations from the ideal behavior. 

On a molecular level, we define a solution as ideal when the intermolecular interactions are the same between all components of the mixture. Thus, in using the ideal solution as a reference state, we will be comparing the behavior of real mixtures to that where all the species in the mixture interact with equal magnitude. However, there is more than one possible interaction upon which to define We will now explore two common choices of that we use for reference states The Lewis/Randall rule and Henry s law. 

In the intermediate concentration ranges in Figure 7.5, we see that the fugacity of species a in the liquid phase is between the two limiting cases given by the Lewis/ Randall rule and Henry s law. We expect this behavior because, at intermediate concentrations, molecule a sees some other a molecules (characteristic of the Lewis/Randall rule) and some b molecules (characteristic of Henry s law). Thus, the fugacity of species... 

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See also in sourсe #XX -- [ ]

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