In chemistry, thermo-chemistry is the study of the function of heat and temperature involved in chemical transformation. In modern physical chemistry, thermochemistry is defined as a subject concerned with heats of chemical reactions, heats of formations, etc. [1]

History
English chemistry historian James Partington attributes the foundation of thermochemistry to the construction of the ice-calorimeter in the winter of 1782-83, by French chemist Antoine Lavoisier and mathematician Pierre-Simon Laplace, used to determine the heat evolved in various chemical changes; calculations which were based on Scottish physicist Joseph Black’s prior discovery of latent heat. [3]

The conjunctional term “thermo-chemistry” dates to at least 1840. In fact, German physical chemist Wilhelm Ostwald cites Swiss-born Russian chemist Germain Hess, with his 1840 article “Constant Heat Sums” (Constanz der Warmesummen), which established Hess’ law, as being the founder of thermo-chemistry. [2]

References
1. Daintith, John. (2004). Oxford Dictionary of Chemistry. Oxford University Press.
2. Meyer, Ernst von. (1898). A History of Chemistry from the Earliest Times to the Present Day. (pg. 507). The Macmillan Company.
3. Partington, J.R. (1957). A Short History of Chemistry. MacMillan and Co.

EoHT symbol