CH353 - Physical Chemistry I
Spring 2015, Unique 51170

Lecture Summary, 31 March  2015


Composition Diagrams:  Binary systems can be described in composition diagrams, which are phase diagrams that represent the phase of the system (liquid, vapor, or liquid + vapor) at any composition (i.e. mole fraction) measured as a function of pressure or temperature.  Composition diagrams are presented as the mole fraction of one of the species on the x axis and pressure or temperature on the y axis.  Unlike the simple 1 component phase diagrams we were working with earlier in the course, the overall shape and phase regions of a composition diagram can look very different depending on the variables that are selected for the x and y axis.  Chapter 24-4 in your book shows a series of composition diagrams that all give the same kind of information but look very different because of different selections for the axis.  When trying to interpret a composition diagram, the first thing you always need to do is make sure you understand the axis. 

Although the shape of the curves of composition diagrams are empirically derived, they show that the vapor above the head space of a liquid mixture will be enriched in the more volatile component, as we would expect.  We can use a combination of Raoult's law and Dalton's law to determine the exact composition of both liquid and vapor phases. 

Nonideal Solutions:  The requirements that must be fulfilled for a solution to be "idea" arequite restrictive, and there are very few completely ideal solutions.  When nonideal solutions are plotted on a compositiondiagram, 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 orattractive forces.  From performing our thought experiment about what happens as one component of the solution approaches a situation inwhich 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 anonideal 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 theHenry'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 understandingthese model systems, and so we usually only care about the magnitude of Ki versus P*.