<sod>  The present [[IUPAC]] definition of chemical [[affinity]] is: '''Negative partial derivative of Gibbs energy with respect to extent of reaction at constant pressure and temperature. It is positive for spontaneous reactions'''.<ref> [http://www.IUPAC.org IUPAC Green Book and Gold Book in .pdf]</ref>
 
 == History ==
"Chemical affinity", historically, refers to the "[[electromagnetic force|force]]" that causes [[chemical reaction]]s.<ref>[[Thomas Thomson]]. (1831). ''A System of Chemistry'', vol. 1. p.31 (chemical affinity is described as an "unknown force"). 7th ed., 2 vols.</ref>  A broad definition, used generally throughout history, is that chemical affinity is that whereby substances enter into or resist decomposition.<ref name="Levere" >{{cite book | last = Levere | first = Trevor, H. | title = Affinity and Matter – Elements of Chemical Philosophy 1800-1865 | publisher = Gordon and Breach Science Publishers | year = 1971 | id = ISBN 2881245838}}</ref>  In current use, it
 
 The following statement, made by [[Ilya Prigogine]], summarizes the concept of affinity:
 
 <eod> <sod> The present [[IUPAC]] definition of chemical [[affinity]] is: '''Negative partial derivative of Gibbs energy with respect to extent of reaction at constant pressure and temperature. It is positive for spontaneous reactions'''.<ref> [http://www.IUPAC.org IUPAC Green Book and Gold Book in .pdf]</ref>
 
 == History ==
"Chemical affinity", historically, refers to the "[[electromagnetic force|force]]" that causes [[chemical reaction]]s.<ref>[[Thomas Thomson]]. (1831). ''A System of Chemistry'', vol. 1. p.31 (chemical affinity is described as an "unknown force"). 7th ed., 2 vols.</ref>  A broad definition, used generally throughout history, is that chemical affinity is that whereby substances enter into or resist decomposition.<ref name="Levere" >{{cite book | last = Levere | first = Trevor, H. | title = Affinity and Matter – Elements of Chemical Philosophy 1800-1865 | publisher = Gordon and Breach Science Publishers | year = 1971 | isbn = 2881245838}}</ref>  In current use, it
 
 The following statement, made by [[Ilya Prigogine]], summarizes the concept of affinity:
 
 <eod>