Thomsen-Berthelot principle
Encyclopedia
In thermochemistry
Thermochemistry
Thermochemistry is the study of the energy and heat associated with chemical reactions and/or physical transformations. A reaction may release or absorb energy, and a phase change may do the same, such as in melting and boiling. Thermochemistry focuses on these energy changes, particularly on the...

, the Thomsen–Berthelot principle is a hypothesis in the history of chemistry
History of chemistry
By 1000 BC, ancient civilizations used technologies that would eventually form the basis of the various branches of chemistry. Examples include extracting metals from ores, making pottery and glazes, fermenting beer and wine, making pigments for cosmetics and painting, extracting chemicals from...

 which argued that all chemical changes
Chemical process
In a "scientific" sense, a chemical process is a method or means of somehow changing one or more chemicals or chemical compounds. Such a chemical process can occur by itself or be caused by somebody. Such a chemical process commonly involves a chemical reaction of some sort...

 are accompanied by the production of heat
Heat
In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...

 and that processes which occur will be ones in which the most heat is produced. This principle was formulated in slightly different versions by the Danish chemist Julius Thomsen in 1854 and by the French chemist Marcellin Berthelot
Marcellin Berthelot
Marcelin Pierre Eugène Berthelot was a French chemist and politician noted for the Thomsen-Berthelot principle of thermochemistry. He synthesized many organic compounds from inorganic substances and disproved the theory of vitalism. He is considered as one of the greatest chemists of all time.He...

 in 1864. This early postulate in classical thermochemistry became the controversial foundation of a research program that would last three decades.

This principle came to be associated with what was called the thermal theory of affinity, which postulated that the heat evolved in a chemical reaction
Chemical reaction
A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...

 was the true measure of its affinity
Chemical affinity
In chemical physics and physical chemistry, chemical affinity is the electronic property by which dissimilar chemical species are capable of forming chemical compounds...

. This hypothesis was later disproved, however, when in 1882 the German scientist Hermann von Helmholtz
Hermann von Helmholtz
Hermann Ludwig Ferdinand von Helmholtz was a German physician and physicist who made significant contributions to several widely varied areas of modern science...

 proved that affinity was not given by the heat evolved in a chemical reaction but rather by the maximum work, or free energy
Thermodynamic free energy
The thermodynamic free energy is the amount of work that a thermodynamic system can perform. The concept is useful in the thermodynamics of chemical or thermal processes in engineering and science. The free energy is the internal energy of a system less the amount of energy that cannot be used to...

, produced when the reaction was carried out reversibly
Reversibility
Reversibility can refer to:* Reversible dynamics, a mathematical dynamical system, or physical laws of motion, for which time-reversed dynamics are well defined* Reversible diffusion, an example of a reversible stochastic process...

.
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