Driving force

In chemistry, driving force of a chemical reaction or "force of reaction" is the "chemical affinity" or affinity A, which is measured by negative change in the free energy (Gibbs free energy -ΔG or Helmholtz free energy -ΔH) on going from the reactants to the products. [1]

In 1922, English inorganic chemist Joseph Mellor stated the relationship as such: [7]

Chemical affinity can be regarded as the driving force of a chemical reaction.”

This term 'driving force', in a colloquial sense, often goes by the shortened term "drive". A mathematical expression for driving force is found in the equation for the affinity of reaction. [6]

In affinity chemistry, the elusive quantity 'affinity' (or chemical affinity), a term dating to at least the 13th century (Magnus, 1250), has long been known as the 'driving force' of chemical reactions and processes. In 1882 and 1884, respectively, German physician and physicist Hermann Helmholtz and Dutch physical chemist Jacobus van’t Hoff, independently, determined that free energy change was the true measure of affinity or the driving force of chemical reactions. [4]

In 1910, Joseph Klein, American mechanical engineer, stated that the “change of entropy (ΔS) constitutes the driving motive in all natural events”; and that “it has therefore a reach and universality which even transcends that of the first law or principle of the conservation of energy.” [2]

In 1923, Gilbert Lewis, in his Thermodynamics and the Free Energy of Chemical Substances, specifically the section "The Driving Force of a Chemical Reaction and a New Test for Equilibrium", has since become the most referenced thermodynamics textbook of all-time. [3]

American physical chemist Ron Salzman explains the concept of 'driving force' by stating that for chemical reactive systems at constant T and p, the Gibbs free energy seeks a minimum; subsequently, one can use ΔG to tell whether or not a reaction will proceed spontaneously as written: [5]

If ΔG > 0 then the reaction will not go as written (the reverse reaction will go)
if ΔG < 0 then the reaction will go as written.

The quantity ΔG, according to Salzman, is a measure of the "driving force" of a chemical reaction; conveying the idea that ΔG tells whether or not a given reaction will really run spontaneously. The two components of the reaction "driving force", according to Salzman, are:

ΔH is the drive toward stability. When ΔH < 0 the products are more stable than the reactants (and vice versa).
ΔS is the drive toward disorder. When ΔS > 0 the products are more disordered than the reactants.

In the expression for Gibbs free energy change:


the negative sign in front of the ‘TΔS’ product shows that a positive ΔS makes a negative contribution to ΔG which tends to drive the reaction in the forward direction. One should note, according to Salzman, that increasing T increases the influence of ΔS on the reaction "driving force"; also, that for a chemical reaction, ΔH and ΔS are independent of each other; that is, one cannot be calculated from the other. Reactions can also have situations where both are positive, both are negative, or one is positive and the other negative. It should be noticed, according to Salzman, that if ΔH and ΔS have the same sign, it means crudely that they are energetically working against each other. A good rule, according to Salzman, is that one can make the entropy win by increasing the temperature or you can make the enthalpy win by decreasing the temperature.

The above "driving force" logic scales up absolutely (i.e. there is no unbridgeable gap) to the sociological level, i.e. to the human-human reaction level or human chemical reaction level. As summarized by American physical chemist Thomas Wallace, from his 2009 appendix section "The Fundamentals of Thermodynamics Applied to Socioeconomics": [8]

"The thermodynamic parameter free energy:


[where] enthalpy H and entropy S represent the variables of heat content and probability, respectively, represents the fundamental driving force in nature and determines whether physical and chemical processes conducted by nature and society will take place."

This, according to the standard model, is what is called the correct point of view. This view, however, is in direct opposition to thinkers, such as Indian chemical engineer DMR Sekhar, who, in an attempt to maintain belief in free will, consciousness, choice, etc., promote theories such as self-drive (see: self-motion), which amount, in effect to perpetual motion, or specifically perpetual motion of the living kind, and are classified as ontic opening theories. [9]

See also
Affinity of reaction

1. (a) Keii, Tominaga. (2004). Heterogeneous Kinetics: Theory of Ziegler-Natta-Kaminsky (Volume 77 of Springer series in Chemical Physics) (2.3: Chemical Affinity: Thermodynamic Force of Reaction, pgs. 12-15). Springer.
Driving force (affinity) of a reaction – IUPAC Gold Book.
2. Klein, Joseph Frederic. (1910). Physical Significance of Entropy or of the Second Law”, (pg. 2). New York: D. Van Nostrand Co.
3. Lewis, Gilbert N. and Randall, Merle. (1923). Thermodynamics and the Free Energy of Chemical Substances (pg. 159-61). New York: McGraw-Hill Book Co., Inc.
4. (a) van’t Hoff, Jacobus H. (1884). Études de Dynamique chimique ("Studies in Chemical Dynamics"), Amsterdam: F. Muller & Co.
(b) Helmholtz, H. v. "Die Thermodynamic Chemischer Vorgange," (The Thermodynamics of Chemical Operations) SB, pg. 23, pg. 22-29, in Wissenschaftlich Abhandlundgen von Hermann von Helmholtz. 3 vols. Leipzig: J.A. Barth, 1882-95.
5. Salzman, W. Ron. (2004). “Gibbs free energy and Reactions”, Physical Chemistry, University of Arizona.
6. Kondepudi, Dilip and Prigogine, Ilya. (1998). Modern Thermodynamics – from Heat Engines to Dissipative Structures, (section: “Chemical Potential and Affinity: the Driving Force of Chemical Reactions”, pgs. 103-13). New York: John Wiley & Sons.
7. Mellor, Joseph W. (1922). A Comprehensive Treatise on Inorganic and Theoretical Chemistry, Volume One (Section: Chemical Affinity, pgs. 291-93). Longmans.
8. Wallace, Thomas P. (2009). Wealth, Energy, and Human Values: the Dynamics of Decaying Civilizations from Ancient Greece to America (Appendix: "The Fundamentals of Thermodynamics Applied to Socioeconomics", pgs. 469-90). AuthorHouse.
9. No attack (2012) - Hmolpedia threads.

Further reading
● Kondepudi, Dilip and Prigogine, Ilya. (1998). Modern Thermodynamics – from Heat Engines to Dissipative Structures, (section: “Chemical Potential and Affinity: the Driving Force of Chemical Reactions”, pgs. 103-13). New York: John Wiley & Sons.
● Cavanagh, John and Akke, Mikael. (2000). “May the Driving Force be with You – Whatever it is”, Nature Structural & Molecular Biology, 7: 11-13, Jan 01.
● Pross, Addy. (2003). “The Driving Force for Life’s Emergence: Kinetic and Thermodynamic Considerations”, Journal of Theoretical Biology, 220: 393-406.

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