In chemical thermodynamics, the Gibbs free energy is the portion of energy of an isothermal, isobaric system that can be converted into useful external work. The conception was introduced by American mathematical physicist Willard Gibbs in 1873 and called by him "available energy". [1] In chemistry, for simple reactions, the change in Gibbs free energy, between the initial stage and final stage of a reaction, for a system of reacting species, takes the form:

∆G = ∆H – T∆S

where ∆H is the change in enthalpy, T the temperature, and ∆S the change in entropy for the reaction. A spontaneous or energetically favored reaction will satisfy the following condition:

∆G < 0

To determine the feasibility or reaction possibility for a given reaction, the value of ∆G must be determined. To do this, precalculated values of enthalpy change and entropy (per species) are generally listed in standard thermodynamic tables, as shown below (left):


Prior to 1882, wherein German physicist Hermann von Helmholtz showed that free energy was a measure of the chemical "affinity" between the reactants, these reaction calculation tables took the form of affinity tables, the first of which was constructed in 1718 by French physician and chemist Étienne Geoffroy, as shown above (right).

Goethe's elective affinities

See main: Goethe's human chemistry

In 1809, German polymath Johann von Goethe used Swedish chemist Torbern Bergman's 1775 chemistry textbook A Dissertation on Elective Attractions and specifically its fifty-row, fifty-column affinity table, showing thousands of possible chemical reactions between the known chemical species, to write the famed novella Elective Affinities, a chemical treatise on the origin of love, in which the characters react according to their natural affinity preferences, producing or absorbing work, and forming or breaking bonds along the way. This was the start of the science of human chemistry. [2]

In 1995, American chemical engineer Libb Thims, unaware of Goethe's previous work, began to try to figure out the same type of logic for reactions between people using thermodynamic tables. In took seven years before the technical and conceptual issues behind the problem began to make any sort of sense.


References
1. Gibbs, Willard. (1873). "Graphical Methods in the Thermodynamics of Fluids", Transactions of the Connecticut Academy, II., pg. 309-342. April-May.
2. Thims, Libb. (2007). Human Chemistry (Volume Two), (preview), (ch. 10: "Goethe's Affinities", pgs. 371-422). Morrisville, NC: LuLu.