Free energy - bound energy (diagram)
A depiction of the "free energy" component of the equation for the Helmholtz free energy.
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]

Overview
In 1942, American physical chemist Hugh Taylor stated the following about free energy: [10]

“The heat of a reaction is not the true thermodynamic criterion for chemical reaction. On the contrary, the change of free energy, the capacity of the system to do chemical, electrical or mechanical work, is the correct measure of the driving force of a reaction.”

In isothermal-isobaric systems, free energy is called "Gibbs free energy". In isothermal-isochoric systems, free energy is called "Helmholtz free energy".

Etymology
The term free energy, or freie energie in the original German, was introduced in German physicist Hermann Helmholtz' 1882 article "The Thermodynamics of Chemical Processes", a term he modeled or stylized on the earlier term "available energy" used in 1876 by American engineer Willard Gibbs. [6] Helmholtz conceived of "bound energy" (gebundene energie) and "free energy" in thermodynamic systems, and showed that the quantitative value of free energy in a reactive chemical system was a measure of the chemical "affinity" between the reactants. [4]

Helmholtz's 1882 was spurred into publication out of the growing incorrectness being pushed forward that the release of heat acts as the true driving force of a chemical reaction, i.e. the so-called Berthelot-Thomsen principle (1854-64). Helmholtz proved that, owing to the aspects of entropy, it is the free energy, not heat that is the measure affinities. This is the same article in which the term free energy was coined. The Berthelot-Thomsen principle, to note, still held sway in the minds of many, such as James Johnstone (1921), into the early 20th century. [11]

Human chemistry
In 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]

Calculation of
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]

Perutz | Schrodinger
In 1987 commentary on Erwin Schrödinger’s 1944 book What is Life? (see: Note to Chapter 6) 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) Helmholtz, Hermann. (1882). “On the Thermodynamics of Chemical Processes”, in: Physical Memoirs Selected and Translated from Foreign Sources, 1: 43-97. Physical Society of London, Taylor and Francis, 1888.
(c) 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.
(d) Muir, Matthew. (1884). A Treatise on the Principles of Chemistry (pg. 243). University Press.
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.
10. Taylor, Hugh, S. and Glasstone, Samuel. (1942). A Treatise on Physical Chemistry - Volume One: Atomistics and Thermodynamics. (pgs. 449-50). New York: D. Van Nostrand Co., Inc.
11. Johnstone, James. (1921). The Mechanism of Life in Relation to Modern Physical Theory (pg. ix). Longmans, Green & Co.

Videos
● ChemGuy. (2008). “Free Energy” (Ѻ), ChemGuy.com, Jan 4.

External links
● Free energy – Wikipedia.

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