CH353 - Physical Chemistry I
Spring 2013, Unique 52575

Lecture Summary, 20 March 2013

Nonideal Solutions:  The requirements that must be fulfilled for a solution to be "idea" are quite restrictive, and there are very few completely ideal solutions.  When nonideal solutions are plotted on a composition diagram, the deviations from Raoult's law behavior is instructive for determining qualitatively how the two molecules are interacting.  Two examples that we discussed in class are shown in Figures 24.7-24.8 of your book, in which the system is dominated by repulsive or attractive forces.  From performing our thought experiment about what happens as one component of the solution approaches a situation in which it is mainly surrounded by itself, we modified Raoult's law a bit:

Pi --> xiP* as xi --> 1

In other words, Raoult's law becomes a more accurate description of a nonideal solution as the solution composition approaches 100% of species i.  We then used the Gibbs-Duhem equation to derive a new expression:

Pi --> xiKi  as xi --> 0

Where Ki is a constant of integration, in units of pressure, called the Henry's law constant.  This constant is an experimentally determined number that depends both on the identity of species i and the identity of the molecule that i is mixed with.  We are really more interested in understanding these model systems, and so we usually only care about the magnitude of Ki versus P*.