J.J. Luetich I. The Gibbs energy form of the Fundamental Equation for multi-phase multi-reaction systems within the physical approach (314K, Adobe 4.0 PDF) ABSTRACT. A general Gibbs energy representation of the fundamental equation for multi-phase systems in chemical equilibrium is presented. No difference is made between physical and chemical transitions, i.e. between phase and combination changes. This paper is the first member of a tetralogy conceived to give insight into the concept of microscopic reversibility.