Page 72 - Chemical equilibria Volume 4

48 Chemical Equilibria
NOTE 2.3.– Obviously, this theorem only makes sense if the Gibbs variance
is at least equal to 2.
2.5. Thermodynamically-equivalent systems

Consider one closed chemical system A. It is composed, to begin with, of
a mixture of a certain number of components in known quantities. This
system evolves spontaneously at constant pressure and temperature. In
general, with the exception of oscillating systems, this system approaches a
state of equilibrium. In order for this to happen, a number of reactions occur
which, at any given moment, are each characterized by their fractional extent
ξ . At equilibrium, each reaction reaches a limit extent or equilibrium extent
i
e
ξ i () . Now we construct another system, B, mixing the same components as
before, with the quantities identical to those obtained when the reactions in
system A have each attained a given fractional extent ξ . This system B
i
will, under the same conditions of temperature and pressure, obviously attain
the same state of equilibrium as the previous system. We say that the two
systems A and B are thermodynamically equivalent. We can even state that
the second system, B, is closer to equilibrium than system A.

The concept of thermodynamically-equivalent systems is used, in
particular, when studying sets of several reactions, using the predominant-
reaction method. The method is very frequently used for calculating the state
of equilibrium of ionic reactions in an aqueous solution – e.g. calculating the
pH of a solution.

2.6. Stability of equilibria

The concept of stability of a state of equilibrium is a relative notion. That
stability is understood in relation to a specific transformation. For example,
if we take oxygenated water at standard temperature and pressure (STP), we
know that this substance is unstable in terms of water but is stable in terms
of the hydrogen–oxygen mixture.