Repeat the previous problem for the opposite case where the liquid has a substantial negative mixing energy, so that its free energy curve dips below the gas’s free energy curve at a temperature higher than TB. Construct the phase diagram and show that this system also has an azeotrope.

Problem:

In constructing the phase diagram from the free energy graphs in Figure, I assumed that both the liquid and the gas are ideal mixtures. Suppose instead that the liquid has a substantial positive mixing energy, so that its free energy curve, while still concave-up, is much flatter. In this case a portion of the curve may still lie above the gas’s free energy curve at TA. Draw a qualitatively accurate phase diagram for such a system, showing how you obtained the phase diagram from the free energy graphs. Show that there is a particular composition at which this gas mixture will condense with no change in composition. This special composition is called an azeotrope.

Figure

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