Page 172 - Chemical equilibria Volume 4

148 Chemical Equilibria
– the descriptive form, if all the conditions were fixed, presents the values
of the enthalpy and entropy of the system, its specific heat capacity at
constant volume and pressure (the Gibbs energy of the system must be null,
or in practice, extremely slight), the list of phases and their compositions,
etc.; or
– a graphical output, if one of the constraints was only to remain within a
specific range.

In fact, there are two main families of computation methods:
– the equilibrium-constant-based method;
– the method of minimization of the Gibbs energy, which is most
commonly used for automated calculations.

4.10.3. Calculation method based on the equilibrium constants

By writing the law of mass action for each of the independent
transformations and the conservation of species, we obtain analytical
expressions (sometimes making certain approximations to simplify the
calculations) for the molar fractions (or partial pressures) of each of the
components as a function of the equilibrium variables, which may include
molar fractions (or partial pressures of certain components) and equilibrium
constants.

It must be acknowledged that the method quickly becomes highly tricky,
and is rarely possible without a certain degree of approximation. Simple
software tools are able to perform a quick calculation, although it may not be
hugely accurate.

A variant of the method is particularly widely used for quickly
performing the calculations for ionic solutions: the predominant reaction
method, which is based on the concept of thermodynamically-equivalent
systems. The method is usually described when studying these ionic
reactions, for which it is very often employed.

Certain automated calculation software packages use the equilibrium-
constants method.