Water-Air Systems

Abstract

The fundamental equations seen in the previous chapter (Chapter III, Equation (26)) are valid for closed systems; no assumption regarding the internal structure of the system is implicit in them. Any of the four equivalent equations is a joint expression of the first and second principles, and therefore contains all that thermodynamics can say about closed systems, except as regards the third principle. Furthermore, while we deal with systems that, besides being closed, are homogeneous and of constant chemical composition*, it is not necessary to specify how the thermodynamic functions depend on the composition of the system, and in order to determine its state only two independent variables must be known (e.g., T and p); we do not need to take into account as another independent variable the total mass or the total number of moles, because this is assumed to be constant and, if we know the values of the extensive functions for the unit mass or the mole, generalization to any mass is done by simply multiplying by m or n (n: number of moles) (cf. Chapter I, Section 3).