Formalism and principles of thermodynamics

Thermodynamics, like the other exact sciences (mechanics, electromagnetism), is based on a limited number of postulates (or principles) discovered inductively from a wide variety of experimental observations.

From these postulates, thanks to the possibility of implementing a rigorous mathematical formalism, we deductively establish the expressions of scientific laws (hence the name "exact sciences").

The experimental and macroscopic origin of the laws of thermodynamics means that they are independent of any prior knowledge of the intimate structure of matter, and possess a general and rigorous character.

In this article, we apply the laws of thermodynamics to chemical systems, i.e. to quantities of matter liable to undergo transformations. Reality is complex, and its mathematical representation constitutes a model in which certain quantities, defined as properties of the system, are in fact abstract mathematical quantities which, because of their importance, are widely used and therefore become familiar.

If we consider a container holding a liquid, we can easily accept that its temperature can be uniform, but we know that the pressure at the bottom is greater than that at the surface. Experience (again!) shows that the properties of a liquid under usual conditions depend only very slightly on pressure, so we can legitimately neglect the influence of pressure. Under these conditions, the description and resulting calculations are simplified, but we must never forget the simplifying hypothesis and its limits of validity.

Thus, the basic thermodynamic system from which we will derive thermodynamic laws applicable to chemical systems is assumed to be at uniform pressure and temperature. Further simplifying assumptions are added, leading to the "uniform single-phase thermoelastic" system. This macroscopic model can be used to describe chemical systems (whether reactive or not).

Industrial systems are characterized by the fact that, very often, they operate as "open systems", i.e. with transfers of matter. The laws of thermodynamics are still applicable, and if the installation is in a stationary state, the relationships are remarkably simple compared to the complexity of the whole.