CH353 - Physical Chemistry I
Spring 2015, Unique 51170

Lecture Summary, 24 March  2015


Binary Systems: So far we have only discussed equilibrium in systems with one component.  The next thing we need to do on our path to a general expression for chemical equilibrium is figure out what happens when two or more components are mixed.  We still are not dealing with chemical reactions, just mixing:

   pure, unmixed 1 and 2 --> homogeneous mixture of 1 and 2

The particular binary system that we are investigating are liquid-liquid mixtures.  To figure out what is going on in the liquid, we monitor the pressure of the vapor in the head space above the liquid.  Molecules in this vapor are in equilibrium with the liquid, and so by referencing to the vapor phase we can figure out what is going on in the liquid phase.  An empirical observation about the partial pressure of one component of a mixture related to the partial pressure of that component in its pure form is codified in Raoult's law:

   Pi = xiP*

where the "*" denotes the pure form of the substance. This can be substituted into our expression for the chemical potential of each compoenent of a mixture:

   (mu)l = (mu)*l + RT ln xi

where xi is the mole fraction of component i in the solution and 0 <= xi <= 1.

The chemical potential of the mixture is therefore lower than of the pure liquid.