I. The Gibbs Energy Form of the Fundamental Equation for Multi-Phase Multi-Reaction Systems within the Physical Approach

A general Gibbs energy representation of the fundamental equation for multi-phase multi-reaction systems is presented. The integral expression of the Gibbs energy comprises of as many terms as variables which have been chosen to describe the composition. The simplest set of composition variables is the one used in the physical approach to equilibrium problems. Within this frame, the differential form of the Gibbs energy for closed systems consists of two terms, like the one corresponding to systems with invariable composition. The central point of the argument is the following property: at given conditions, the stable equilibrium state of any system is the particular state for which the potential of "element" is independent of the state of the "atoms" taken to carry out the variation that, keeping unchanged the remaining variables, produces the variation ∂. To prove this, the interpretation of the employed composition variables is facilitated by using the complete Legendre transformation of , homogeneous function of , instead of the traditional, mathematically equivalent method of Lagrange multipliers. No difference is made between physical and chemical aggregation, i.e. between phase and combination changes. A modification of the expression is also discussed