In thermodynamics, modern chemical thermodynamics, as compared to theoretical chemical thermodynamics (1873-1905) and quantitative chemical thermodynamics (1906-1922), according to American chemistry historian William Jensen’s classification scheme (see: etymology), refers to the era amid and following the 1905 to 1912 activity and fugacity work (ΡΊ) and subsequent 1923 publication of American physical chemist Gilbert LewisThermodynamics and the Free Energy of Chemical Substances. [1]

American chemical thermodynamicists J. Bevan Ott and Juliana Goates, to note, argue that chemical thermodynamics was not “modern” until the publication of English physical chemist Edward Guggenheim’s 1933 Modern Thermodynamics by the Methods of Willard Gibbs: [2]

“Lewis, Randall and Guggenheim must be considered as the founders of modern chemical thermodynamics because of the major contributions of these two books in unifying the applications of thermodynamics to chemistry.”

Here, to clarify, Merle Randall was more like a laboratory assistant, doing free energy laboratory measurements, and the person to whom Lewis dictated his 1923 textbook to, not “founder” as Ott and Goates see things.

References
1. Jensen, William B. (2005). “The Quantification of 20th-Century Chemical Thermodynamics: a Tribute to Thermodynamics and the Free Energy of Chemical Substances” (pdf), Lecture given at Boston Society Symposium on Classic 20th-Century Chemistry Textbooks, 230th meeting of the ACS, Washington, DC, Aug 29.
2. Boerio-Goates, Juliana, and Ott, J., Bevan. (2000). Chemical Thermodynamics: Principles and Applications (pg. 1-2). New York: Elsevier Academic Press.

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