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August 18, 2010 11:36 9in x 6in b985-ch06 Elementary Physical Chemistry
Chapter 6
Phase and Chemical Equilibria
Two important applications of thermodynamics are phase equilibria and
chemical equilibria.
6.1. Phase Equilibrium
We will treat here phase equilibria involving one-component systems and
two-component systems. No chemical reactions are assumed to occur. In
the next section, we will take up chemical reactions.
Figure 6.1 depicts a phase diagram of a one-component system, namely
aplotof P vs. T . There are three areas and three lines. The areas represent
the phases gas, liquid and solid. The lines represent regions of coexistence
of gas–liquid, liquid–solid and solid–gas phases. The point where the lines
meet is called the triple point.
6.1.1. The Phase Rule
The Phase Rule tells how many independent intensive variables (pressure,
temperature, mole fraction, etc.) there are that can be varied in each region.
These variables are normally referred to as degrees of freedom. The phase
rule reads
F = c − p +2, (6.1)
Here, F stands for the variance or degrees of freedom; c represents the
number of components and p the number of phases.
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