Chemical Affinity - Thermodynamics

Thermodynamics

The present IUPAC definition is that affinity A is the negative partial derivative of Gibbs free energy G with respect to extent of reaction ξ at constant pressure and temperature. That is,

It follows that affinity is positive for spontaneous reactions.

In 1923, the Belgian mathematician and physicist Théophile de Donder derived a relation between affinity and the Gibbs free energy of a chemical reaction. Through a series of derivations, de Donder showed that if we consider a mixture of chemical species with the possibility of chemical reaction, it can be proven that the following relation holds:

With the writings of Théophile de Donder as precedent, Ilya Prigogine and Defay in Chemical Thermodynamics (1954) defined chemical affinity in terms of the uncompensated heat of reaction Q' the reaction progress variable or reaction extent ξ; as the ratio of their infinitesimal increments:

This definition is useful for quantifying the factors responsible both for the state of equilibrium systems (where A = 0), and for changes of state of non-equilibrium systems (where A ≠ 0).