Page 164 - Chemical equilibria Volume 4

140 Chemical Equilibria
4.7. Determination of the equilibrium constants on the basis of
spectral data and statistical thermodynamics
In Appendix 2 (section A2.5), we recapped the relations that exist
between the equilibrium constant for different types of reaction, the
molecular partition functions and the difference between the fundamental
energies at the temperature of 0 K of the substances involved in the reaction.
These partition functions and this energy difference can be calculated on the
basis of the vibration frequencies of the molecules, their moments of inertia
and their masses. Those data can be accessed on the basis of the absorption
spectra, essentially in the infrared. Thus, it is possible to calculate the
equilibrium constants a priori.
We can show that the relations obtained are valid if the polycomponent
phases exhibit perfect behavior in the sense of the solutions. In the case of
gases, those relations are applicable to real gases, provided the mixture of
gases is perfect in the sense of the solution. Thus, the relations are applicable
when the Lewis approximation is satisfactory (see relation [3.5]). In the case
of solutions with non-perfect behavior, to the difference between the
fundamental energies we must add the energy (or enthalpy) of mixing, which
can be obtained either on the basis of the measurements or by modeling the
interactions between the molecules in the solution.

4.8. Thermodynamic tables and databanks

There are a wide variety of thermodynamic tables. Some are very broad,
and others cover a particular family of products. These tables can be
classified into:
– two families relative to the date when they were established;

– two families relative to their degree of accuracy and finesse.

Depending on the dates at which they were established, recent tables are
set apart from older ones by the units employed and the choice of the
pressure in the standard state.

The recent tables are established in the international system (SI), and
choose the Joule as the unit of energy and 1 bar as the pressure of the
standard state.