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.
Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

Get My Free Ebook


Post a comment