YaxA I Rt J

For pure component A, the terms on the left hand side of the above equation are: yA = 1, xA = 1, and pA = psat,A, the vapor pressure. Therefore, the right hand side of the above equation is the vapor pressure of pure A, and the formula can be written as:

pA - YAxApsat,A

A similar equation applies to component B:

Equations (8.5) and (8.6) are the general relations between the concentrations of a component in the condensed phase and the equilibrium partial pressures in the gas phase. The latter depend on the composition of the condensed phase and on the deviations of the components from ideality, as represented by the activity coefficients. These two quantities are not independent, however; if yA is known as a function of composition, yB follows from application of the Gibbs-Gibbs-Dihem equation (see Eq (7.31)).

The activity coefficients can be greater or less than unity, depending on the strength of the bonds between A and B molecules compared to the mean of the A-A and B-B bond strengths. The curves in Fig. 8.2 illustrate these two cases; activity coefficients greater than unity give positive deviations from ideal behavior and activity coefficients less than unity result in negative deviations. This figure illustrates the type of measurement (i.e., partial pressure as a function of solution composition) that, in conjunction with Eq (8.5), provides experimental values of activity coefficients of components in solution.

Fig. 8.2 Equilibrium partial pressures of component A over A-B solutions that exhibit positive and negative deviations from ideality. The temperature is fixed.
Getting Started With Solar

Getting Started With Solar

Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.

Get My Free Ebook


Post a comment