chemical physics Chemical physics is a subdiscipline of chemistry and physics that investigates physicochemical phenomena using techniques from atomic and molecular physics and condensed matter physics; it is the branch of physics that studies chemical process ...

and

physical chemistry Physical chemistry is the study of macroscopic and microscopic phenomena in chemical systems in terms of the principles, practices, and concepts of physics such as motion, energy, force, time, thermodynamics, quantum chemistry, statistica ...

, chemical affinity is the electronic property by which dissimilar

chemical species A chemical species is a chemical substance or ensemble composed of chemically identical molecular entities that can explore the same set of molecular energy levels on a characteristic or delineated time scale. These energy levels determine the wa ...

are capable of forming

chemical compounds A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...

. Chemical affinity can also refer to the tendency of an

atom Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons. Every solid, liquid, gas, ...

or compound to combine by

chemical reaction A chemical reaction is a process that leads to the IUPAC nomenclature for organic transformations, chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the pos ...

with atoms or compounds of unlike composition.

History

Early theories

The idea of ''affinity'' is extremely old. Many attempts have been made at identifying its origins. The majority of such attempts, however, except in a general manner, end in futility since "affinities" lie at the basis of all

magic Magic or Magick most commonly refers to: * Magic (supernatural), beliefs and actions employed to influence supernatural beings and forces * Ceremonial magic, encompasses a wide variety of rituals of magic * Magical thinking, the belief that unrela ...

, thereby pre-dating

science Science is a systematic endeavor that Scientific method, builds and organizes knowledge in the form of Testability, testable explanations and predictions about the universe. Science may be as old as the human species, and some of the earli ...

Physical chemistry Physical chemistry is the study of macroscopic and microscopic phenomena in chemical systems in terms of the principles, practices, and concepts of physics such as motion, energy, force, time, thermodynamics, quantum chemistry, statistica ...

, however, was one of the first branches of science to study and formulate a "theory of affinity". The name ''affinitas'' was first used in the sense of chemical relation by German philosopher

Albertus Magnus Albertus Magnus (c. 1200 – 15 November 1280), also known as Saint Albert the Great or Albert of Cologne, was a German Dominican friar, philosopher, scientist, and bishop. Later canonised as a Catholic saint, he was known during his li ...

near the year 1250. Later, those as

Robert Boyle Robert Boyle (; 25 January 1627 – 31 December 1691) was an Anglo-Irish natural philosopher, chemist, physicist, alchemist and inventor. Boyle is largely regarded today as the first modern chemist, and therefore one of the founders of ...

John Mayow John Mayow FRS (1641–1679) was a chemist, physician, and physiologist who is remembered today for conducting early research into respiration and the nature of air. Mayow worked in a field that is sometimes called pneumatic chemistry. Lif ...

Johann Glauber Johann Rudolf Glauber (10 March 1604 – 16 March 1670) was a German-Dutch alchemy, alchemist and chemist. Some historians of science have described him as one of the first chemical engineers. His discovery of sodium sulfate in 1625 led to t ...

Isaac Newton Sir Isaac Newton (25 December 1642 – 20 March 1726/27) was an English mathematician, physicist, astronomer, alchemist, theologian, and author (described in his time as a " natural philosopher"), widely recognised as one of the grea ...

, and

Georg Stahl Georg Ernst Stahl (22 October 1659 – 24 May 1734) was a German chemist, physician and philosopher. He was a supporter of vitalism, and until the late 18th century his works on phlogiston were accepted as an explanation for chemical processes.K ...

put forward ideas on elective affinity in attempts to explain how

heat In thermodynamics, heat is defined as the form of energy crossing the boundary of a thermodynamic system by virtue of a temperature difference across the boundary. A thermodynamic system does not ''contain'' heat. Nevertheless, the term is ...

is evolved during combustion reactions. The term ''affinity'' has been used figuratively since c. 1600 in discussions of structural relationships in chemistry,

philology Philology () is the study of language in oral and written historical sources; it is the intersection of textual criticism, literary criticism, history, and linguistics (with especially strong ties to etymology). Philology is also defined as th ...

, etc., and reference to "natural attraction" is from 1616. "Chemical affinity", historically, has referred to the " force" that causes

s. as well as, more generally, and earlier, the ″tendency to combine″ of any pair of substances. The broad definition, used generally throughout history, is that chemical affinity is that whereby substances enter into or resist decomposition. The modern term chemical affinity is a somewhat modified variation of its eighteenth-century precursor "elective affinity" or elective attractions, a term that was used by the 18th century chemistry lecturer William Cullen. Whether Cullen coined the phrase is not clear, but his usage seems to predate most others, although it rapidly became widespread across Europe, and was used in particular by the Swedish chemist Torbern Olof Bergman throughout his book (1775). Affinity theories were used in one way or another by most chemists from around the middle of the 18th century into the 19th century to explain and organise the different combinations into which substances could enter and from which they could be retrieved.

Antoine Lavoisier Antoine-Laurent de Lavoisier ( , ; ; 26 August 17438 May 1794),
CNRS (

Traité Élémentaire de Chimie ''Traité élémentaire de chimie'' (''Elementary Treatise on Chemistry'') is a textbook written by Antoine Lavoisier published in 1789 and translated into English by Robert Kerr in 1790 under the title ''Elements of Chemistry in a New Systemati ...

(Elements of Chemistry)'', refers to Bergman's work and discusses the concept of elective affinities or attractions. According to chemistry historian Henry Leicester, the influential 1923 textbook ''Thermodynamics and the Free Energy of Chemical Reactions'' by Gilbert N. Lewis and

Merle Randall Merle Randall (January 29, 1888 – March 17, 1950) was an American physical chemist famous for his work with Gilbert N. Lewis, over a period of 25 years, in measuring reaction heat of chemical compounds and determining their corresponding free ...

led to the replacement of the term "affinity" by the term " free energy" in much of the English-speaking world. According to Prigogine, the term was introduced and developed by

Théophile de Donder Théophile Ernest de Donder (; 19 August 1872 – 11 May 1957) was a Belgian mathematician and physicist famous for his work (published in 1923) in developing correlations between the Newtonian concept of chemical affinity and the Gibbsian concep ...

Goethe Johann Wolfgang von Goethe (28 August 1749 – 22 March 1832) was a German poet, playwright, novelist, scientist, statesman, theatre director, and critic. His works include plays, poetry, literature, and aesthetic criticism, as well as tr ...

used the concept in his novel ''

Elective Affinities ''Elective Affinities'' (German: ''Die Wahlverwandtschaften''), also translated under the title ''Kindred by Choice'', is the third novel by Johann Wolfgang von Goethe, published in 1809. Situated around the city of Weimar, the book relates the ...

'' (1809).

Visual representations

The affinity concept was very closely linked to the visual representation of substances on a table. The first-ever ''affinity table'', which was based on

displacement reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the positions of electrons in the forming and breaking ...

s, was published in 1718 by the French chemist

Étienne François Geoffroy Étienne François Geoffroy (13 February 16726 January 1731) was a French physician and chemist, best known for his 1718 affinity tables. He first contemplated a career as an apothecary, but then decided to practice medicine. He is sometimes kn ...

. Geoffroy's name is best known in connection with these tables of "affinities" (''tables des rapports''), which were first presented to the French Academy of Sciences in 1718 and 1720, as shown below: During the 18th century many versions of the table were proposed with leading chemists like Torbern Bergman in Sweden and Joseph Black in Scotland adapting it to accommodate new chemical discoveries. All the tables were essentially lists, prepared by collating observations on the actions of substances one upon another, showing the varying degrees of affinity exhibited by analogous bodies for different reagents. Crucially, the table was the central graphic tool used to teach chemistry to students and its visual arrangement was often combined with other kinds diagrams. Joseph Black, for example, used the table in combination with chiastic and circlet diagrams to visualise the core principles of chemical affinity. Affinity tables were used throughout Europe until the early 19th century when they were displaced by affinity concepts introduced by

Claude Berthollet Claude Louis Berthollet (, 9 December 1748 – 6 November 1822) was a Savoyard-French chemist who became vice president of the French Senate in 1804. He is known for his scientific contributions to theory of chemical equilibria via the mecha ...

Modern conceptions

and

, chemical affinity is the electronic property by which dissimilar

are capable of forming

. Chemical affinity can also refer to the tendency of an

or compound to combine by

with atoms or compounds of unlike composition. In modern terms, we relate affinity 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 that, "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".

Ilya Prigogine Viscount Ilya Romanovich Prigogine (; russian: Илья́ Рома́нович Приго́жин; 28 May 2003) was a physical chemist and Nobel laureate noted for his work on dissipative structures, complex systems, and irreversibility. B ...

summarized the concept of affinity, saying, "All chemical reactions drive the system to a state of equilibrium in which the ''affinities'' of the reactions vanish."

Thermodynamics

The present

IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...

definition is that affinity ''A'' is the negative partial derivative of

Gibbs free energy In thermodynamics, the Gibbs free energy (or Gibbs energy; symbol G) is a thermodynamic potential that can be used to calculate the maximum amount of work that may be performed by a thermodynamically closed system at constant temperature and ...

''G'' with respect to

extent of reaction In physical chemistry and chemical engineering, extent of reaction is a quantity that measures the extent to which the reaction has proceeded. Often, it refers specifically to the value of the extent of reaction when equilibrium has been reached. It ...

''ξ'' at constant

pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...

and

temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measurement, measured with a thermometer. Thermometers are calibrated in various Conversion of units of temperature, temp ...

. That is, :

A = -\left(\frac\right)_.

It follows that affinity is positive for spontaneous reactions. In 1923, the Belgian mathematician and physicist

derived a relation between affinity and the Gibbs free energy of a

. Through a series of derivations, de Donder showed that if we consider a mixture of

with the possibility of chemical reaction, it can be proven that the following relation holds: :

A = -\Delta_rG. \,

With the writings of

as precedent,

and Defay in ''Chemical Thermodynamics'' (1954) defined chemical affinity as the rate of change of the uncompensated

of reaction ''Q as the reaction progress variable or reaction extent ''ξ'' grows infinitesimally: :

A  = \frac. \,

This definition is useful for quantifying the factors responsible both for the state of equilibrium systems (where ), and for changes of state of non-equilibrium systems (where ''A'' ≠ 0).

Notes

References

*{{EB1911, wstitle=Affinity, Chemical, volume=1, page=301

External links

William Whewell William Whewell ( ; 24 May 17946 March 1866) was an English polymath, scientist, Anglican priest, philosopher, theologian, and historian of science. He was Master of Trinity College, Cambridge. In his time as a student there, he achieved ...

"Establishment and Development of the Idea of Chemical Affinity""> "Establishment and Development of the Idea of Chemical Affinity"
''History of Scientific Ideas''. 2:15ff.

- 1920 Nobel Prize in Chemistry Presentation Speech by

Gerard De Geer Baron Gerard Jacob De Geer (20 November 1858 – 24 July 1943) was a Swedish geologist who made significant contributions to Quaternary geology, particularly geomorphology and geochronology. De Geer is best known for his work on varves. In 1890 D ...

Physical chemistry Jacobus Henricus van 't Hoff