Page 65 - Chemical equilibria Volume 4

To the above three equilibria, we need to add the following two equilibria
of phase transfer: Properties of States of Physico-Chemical Equilibrium 41
NH (aqu = NH (gas )
)
3
3
+
−
Cl + Ag = AgCl (solid )
Thus, the dimension of our vector space is 5. There can be no
transformation within the gaseous phase and the solid phase, because these
phases have only one component.
Crystalline solids – in the case of crystalline solids, we can no longer identify
components and chemical species (atoms, ions or molecules), because the same
species can occupy different sites in the crystalline lattice, which thus constitutes
two different components. In addition, to take account of some of the properties
of solids, we also need to take account of certain entities which appear to be
irregularities in the arrangement of the species in space. These irregularities
constitute point defects. We then define what is known as structure elements,
and it is these which play the role of components. We are then led to the method
used for ionic solutions.

2.3. Phase laws

Phase laws are used to determine the number of independent variables in
a system at equilibrium.

There are two phase laws:
– Gibbs’ phase rule, which pertains to the external intensive values and
the composition variables. This law is applicable both to open and to closed
systems;
– Duhem’s law, which relates to all the intensive and extensive variables,
and is applicable only to closed systems.

2.3.1. Reminder of Gibbs’ phase rule

Gibb’s phase rule states that the variance of a system with p external
intensive physical variables, containing c independent components and φ
phases, is given by:
−
v =+ p ϕ [2.39]
c
G