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Inchemical physics andphysical chemistry,chemical affinity is the electronic property by which dissimilarchemical species are capable of formingchemical compounds.[1] Chemical affinity can also refer to the tendency of anatom or compound to combine bychemical reaction with atoms or compounds of unlike composition.
The idea ofaffinity is extremely old. Many attempts have been made at identifying its origins.[2] The majority of such attempts, however, except in a general manner, end in futility since "affinities" lie at the basis of allmagic, thereby pre-datingscience.[3]Physical chemistry, however, was one of the first branches of science to study and formulate a "theory of affinity". The nameaffinitas was first used in the sense of chemical relation by German philosopherAlbertus Magnus near the year 1250. Later, those asRobert Boyle,John Mayow,Johann Glauber,Isaac Newton, andGeorg Stahl put forward ideas on elective affinity in attempts to explain howheat is evolved duringcombustion reactions.[4]
The termaffinity has been used figuratively since c. 1600 in discussions of structural relationships in chemistry,philology, etc., and reference to "natural attraction" is from 1616. "Chemical affinity", historically, has referred to the "force" that causeschemical reactions.[5] 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.[2]
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 lecturerWilliam Cullen.[6] 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 chemistTorbern Olof Bergman throughout his bookDe attractionibus electivis (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.[7][8]Antoine Lavoisier, in his famed 1789Traité Élémentaire de Chimie (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 textbookThermodynamics and the Free Energy of Chemical Reactions byGilbert N. Lewis andMerle Randall led to the replacement of the term "affinity" by the term "free energy" in much of the English-speaking world.
According to Prigogine,[9] the term was introduced and developed byThéophile de Donder.[10]
Johann Wolfgang von Goethe used the concept in his novelElective Affinities (1809).
The affinity concept was very closely linked to the visual representation of substances on a table. The first-everaffinity table, which was based ondisplacement reactions, was published in 1718 by the French chemistÉtienne François Geoffroy. Geoffroy's name is best known in connection with these tables of "affinities" (tables des rapports), which were first presented to theFrench Academy of Sciences in 1718 and 1720.
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 differentreagents.
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.[11] Affinity tables were used throughout Europe until the early 19th century when they were displaced by affinity concepts introduced byClaude Berthollet.
Inchemical physics andphysical chemistry, chemical affinity is the electronic property by which dissimilarchemical species are capable of formingchemical compounds.[1] Chemical affinity can also refer to the tendency of anatom or compound to combine bychemical reaction 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 bookChemistry 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 summarized the concept of affinity, saying, "All chemical reactions drive the system to a state ofequilibrium in which theaffinities of the reactions vanish."
The presentIUPAC definition is that affinityA is the negativepartial derivative ofGibbs free energyG with respect toextent of reactionξ at constantpressure andtemperature.[12] That is,
It follows that affinity is positive forspontaneous reactions.
In 1923, the Belgian mathematician and physicistThéophile de Donder derived a relation between affinity and the Gibbs free energy of achemical reaction. Through a series of derivations, de Donder showed that if we consider a mixture ofchemical species with the possibility of chemical reaction, it can be proven that the following relation holds:
With the writings ofThéophile de Donder as precedent,Ilya Prigogine and Defay inChemical Thermodynamics (1954) defined chemical affinity as the rate of change of the uncompensatedheat of reactionQ' as thereaction progress variable or reaction extentξ grows infinitesimally:
This definition is useful for quantifying the factors responsible both for the state of equilibrium systems (whereA = 0), and for changes of state of non-equilibrium systems (whereA ≠ 0).
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This article incorporates text from a publication now in thepublic domain: Chisholm, Hugh, ed. (1911). "Affinity, Chemical".Encyclopædia Britannica. Vol. 1 (11th ed.). Cambridge University Press. p. 301.