CHAPTER 6 LECTURE NOTES
Degrees of Freedom:
The number of degrees of freedom (number of variables we can change
without affecting the &nature± of the system) is determined by the equation known
as the
phase rule
:
f
=
c
²
p
+ 2,
(6.2)
where
f
is the no. of degrees of freedom,
p
is the number of phases present and the
&2± represents the two variables temperature and pressure.
Applying this rule to a onecomponent phase diagram such as that of water,
we see that in the interior of the solid, liquid or vapor regions, where only one
phase is present, we get two degrees of freedom, since
f
= 1 ² 1 + 2 = 2.
This
means that we can change two variables, in this case temperature and pressure, and
still have a system consisting only of one phase.
When we have two phases in equilibrium, the number of degrees of freedom
decreases to 1:
f
= 1 ² 2 + 2 = 1.
This means that if we wish to change the
conditions and still retain the equilibrium between the two phases, we can only
change one of the variables independently.
The other variable is then determined
by the equilibrium condition.
When three phases are in equilibrium, as at the triple point, the number of
degrees of freedom drops to zero.
Therefore, the triple point is a unique point for
the system. In some cases, as in the case of sulfur, multiple triple points are
present (see Fig. 6.1).
Each of these are unique points with zero degree of
freedom.
Components:
Determining the number of components in a system is not a trivial task.
We
may make up a formula for this as follows:
c
=
n
²
e
²
o
,
where c is the number of components, n is the number of chemical species present,
e
is the number of equilibria between them, and
o
represents any other
relationships that may determine the relative amount of one species with respect to
another.
See Section 6.1 to see applications of these ideas.
This preview has intentionally blurred sections. Sign up to view the full version.
View Full Document
This is the end of the preview.
Sign up
to
access the rest of the document.
 Fall '09
 Physical chemistry, Thermodynamics, pH, wA, Nv

Click to edit the document details