A thermochemical method is presented by which multiphase processes can be simulated with concurrent calculation of the Gibbs energy of the reactive mixture during a chemical change. Algorithmic constraints are set for the overall reaction kinetics when the Lagrange method of undetermined multipliers is used to minimize the Gibbs energy of the multicomponent system. Consequently, the chemical change is calculated as a series of successive "virtual" states, which follow the extent of the overall reaction. From the Gibbs energy of these intermediate states, other thermodynamic quantities for the changing chemical system can be deduced, and the entropy production of the process can be calculated. A particular process model was developed for two isothermal aqueous systems and for a nonisothermal high-temperature process. The entropy production in the reactive systems is presented, and the validity of the simulation models is assessed in terms of their time-dependent Gibbs energy and entropy profiles.