Page 93 - Chemical equilibria Volume 4

single solid phase as for zones of demixing of the solid solution which
exhibit a platform. Molecular Chemical Equilibria 69
3.2. Graphical representations of equilibria – pole diagrams

A certain number of graphical representations are used to visualize the
results of a chemical equilibrium. Some of these graphs superpose several
equilibria, and others also add the results of physical equilibria of state
change. In this section, we examine one series of such diagrams; others are
introduced when we look at particular systems such as equilibria in aqueous
1
solutions.
The pole diagram is a helpful tool to represent the progression of a
reaction and compare reactions within the same family. It is generally
plotted in convention (I), (pure-substance reference).

3.2.1. Principle of the pole diagram

The pole diagram (see Figure 3.2) is a representation of the variations in
(I)
the affinity as a function of the quantity RlnQ for a reaction at a certain
temperature T.

In Figure 3.2, P denotes the point with coordinates ( Δ s , Δ h− r 0 ), at
0
r
standard entropy and enthalpy – i.e. in reference (I) – associated with the
reaction under study. These values may be positive or negative (in the case
0
0
of Figure 3.2, we have chosen negative values for Δ s and Δ h ). P is
r
r
called the pole of the reaction. It characterizes the physical states (states of
aggregation) of the components at the chosen temperature T. The straight
line which, for the reaction at hand at temperature T, represents the
(I)
variations in affinity as a function of RlnQ , passes through that point P. it
also passes through the point I, which represents the real experimental
conditions at the initial state of the reaction. The ordinate of that point I is
(I)
(I)
the initial affinity, its abscissa value is RlnQ , and Q is the initial value
0
0
(I)
of Q . Point B on the line, on the axis of the affinities, represents the

1 See Volume 6 of this Set of books, Electrolytes and Electrochemical Thermodynamics.