In thermodynamics, free energy is the measure of the portion of energy of a chemical system that can be converted into external work or the measure of a system's ability to do work. [1] In isothermal-isobaric systems, free energy is called "Gibbs free energy". In isothermal-isochoric systems, free energy is called "Helmholtz free energy". With human chemical reactions, which are constant temperature (isothermal), constant pressure (isobaric), surface chemistry reactions, it is the the Gibbs free energy is the quantity of importance. [2] Changes in the value of free energy can be used to determine if a reaction is thermodynamically favorable. [3]

History
In 1873, American engineer Willard Gibbs outlined the logic of "available energy". [6] In 1882, similar to Gibbs, German physicist Hermann von Helmholtz, who conceived of "bound energy" and "free energy" in thermodynamic systems, showed that the quantitative value of free energy in a reactive chemical system was a measure of the chemical "affinity" between the reactants. [4]

Free energy calculations
The first systematic study of all the thermodynamic date necessary for the calculation of the free energy changes in a group of important reactions was published in Germany by Fritz Haber in his 1905 Thermodynamics of Technical Gas Phase Reactions. [7]

Others following Haber include: German physical chemist Walther Nernst, Danish physical chemist Johannes Bronsted, and Americans Arthur Noyes, Merle Randall and Gilbert Lewis. [8]

Some argue that American physicist John Kirkwood, a protégé of Noyes, laid the foundations for the standard method for estimating free energy differences, namely perturbation theory and thermodynamic integration, by building on the chemical affinity and extent of reaction work of Theophile De Donder. Others to have furthered this approach include: Robert Zwanzig, Lev Landau, and Benjamin Widom. [9]

Life
In 1987 commentary on Erwin Schrödinger’s 1944 book What is Life? and his ideas on life and "negative entropy", Austrian-born English molecular biologist Max Perutz argued that we live on free energy and that there is no need to postulate the conception of negative entropy. [5]

References
1. Daintith, John. (2005). Oxford Dictionary of Physics. Oxford: Oxford University Press.
2. Thims, Libb. (2007). Human Chemistry (Volume Two), (preview), (ch. 11: "Affinity and Free Energy", pgs. 422-468). Morrisville, NC: LuLu.
3. Clark, John O.E. (2004). The Essential Dictionary of Science. New York: Barnes & Noble Books.
4. (a) Helmholtz, Hermann. (1882). “Die Thermodynamik Chemischer Vorgänge (The Thermodynamics of Chemical Operations”, SB: 22-39, in Wissenschaftliche Abhandlungen von Hermann von Helmholtz. 3 vols. Leipzig: J.A. barth, 1882-95. 2:958-78.
(b) Young, Paul T. (1936). Motivation of Behavior – the Fundamental Determinants of Human and Animal Activity, (ch. 2: “The Energetics of Activity”, pg. 68) New York: Wiley.
5. Perutz, Max. (1987). “Erwin Schrödinger's What Is Life? and molecular biology”, pp. 234–251 in Schrödinger: Centenary Celebration of a Polymath, edited by C. W. Kilmister. Cambridge University Press, Cambridge.
6. Gibbs, J. Willard. (1873). "A Method of Geometrical Representation of the Thermodynamic Properties of Substances by Means of Surfaces" (pgs. 49-50), Transactions of the Connecticut Academy, II. pp.382-404, Dec.
7. Haber, Fritz. (1905). Thermodynamics of Technical Gas Reactions, (Translator’s Preface, 1907, pg. vii). Longmans, Green, and Co.
8. Lewis, Gilbert N. and Randall, Merle. (1923). Thermodynamics and the Free Energy of Chemical Substances, McGraw-Hill Book Co., Inc.
9. Chipot, Christphe and Pohorille, Andrew. (2007). Free Energy Calculations: Theory and Applications in Chemistry and Biology (1.1.1: Pioneers of Free Energy Calculations, pgs. 1-2). Springer.