Thermodynamic affinity in constrained free-energy systems

Affinity is the generic measure of the deviation of a state from stable equilibrium. Affinity, as introduced by de Donder, is a thermodynamic state property defined in terms of p, T, and system composition during the course of a chemical change. When incorporating reaction kinetic constraints to minimization of Gibbs energy of a multiphase system, affinity can be followed in terms of the extents of the constrained reactions. This property then becomes calculated in terms of the constraint potentials received as additional Lagrange multipliers in the minimization routine. Thus, received affinities are consistent with the respective values calculated from the chemical potentials of the reactants and products of the constrained reactions and their limiting behaviour corresponds to that defined for both stationary and stable equilibrium states. The intermediate affinities can be used in the respective reaction rate calculations, or as input parameters, to define the local chemical equilibrium set by known reaction kinetic constraints. Thus, they become a useful concept in modelling reactive processes.