Chemical affinity

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Chemical affinity results from electronic properties by which dissimilar substances are capable of forming chemical compounds. Specifically, the term refers to the tendency of an atom or compound to combine by chemical reaction with atoms or compounds of unlike composition.

History

Physical chemistry was one of the first branches of science to study the "theory of affinity". The name affinitas was first used in the sense of chemical relation by German philosopher Albertus Magnus near the year 1250. Later, those as Robert Boyle, John Mayow, Johann Glauber, Isaac Newton, and Georg Stahl put forward ideas on elective affinity in attempts to explain how heat is evolved during combustion reactions.^[1]

The modern term chemical affinity is a somewhat modified variation of its eighteenth-century precursor "elective affinity" or elective attractions, a coinage of the Swedish chemist Torbern Olof Bergman from his book De attractionibus electivis (1775). Antoine Lavoisier, in his famed 1790 Elements of Chemistry, refers to Bergmann’s work and discusses the concept of elective affinities or attractions. The term generally relate to the phenomenon whereby certain atoms or molecules have the tendency to aggregate or bond. For example, in the 1919 book Chemistry of Human Life physician George W. Carey states: “Health depends on a proper amount of iron phosphate Fe₃(PO₄)₂ in the blood, for the molecules of this salt have chemical affinity for oxygen and carry it to all parts of the organism.” In this antiquated context, chemical affinity is sometimes found synonymous with the term "magnetic attraction". Many writings, up until about 1925, also refer to a “law of chemical affinity”.

Thermodynamics

In 1923, the Belgian mathematician and physicist Théophile de Donder derived a relation between affinity [A\,] and the Gibbs free energy [G\,] 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 proved that the following relation holds:

[ A = -\Delta G_r \,]

With the writings of Théophile de Donder as precedent, Prigogine and Defay in Chemical Thermodynamics (1954) defined chemical affinity (denoted by [A\,]) as a function of the increments in uncompensated heat of reaction and reaction progress variable (denoted by [d Q'\, ] and [d \xi\, ], respectively):

[A = d Q' / d \xi \, ].       (1).

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 \ne 0\, ]).

Related

In 1809, based on the work of Bergmann, German scientist and philosopher Johann Wolfgang von Goethe published the novella Elective Affinities which extended the chemical term “elective affinities” through storyline to human relationships, both intimate and political.

See also

• Chemistry
• Chemical reaction
• Chemical bond
• Electronegativity
• Electron affinity
• Étienne François Geoffroy - Geoffroy's 1718 Affinity Table
• Valency

References

1. Partington, J.R. (1937). A Short History of Chemistry. New York: Dover Publications, Inc. ISBN 0486659771

External links

• [Chemical Affinity and Absolute Zero] - 1920 Nobel Prize in Chemistry Presentation Speech by Gerard de Geer

 

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