Thomsen–Berthelot principle

In

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 was the true measure of its affinity.

Limitations

The experimental objections to the Thomsen–Berthelot principle include incomplete dissociation, reversibility, and spontaneous endothermic processes.^[2] Such cases were dismissed by orthadox thermochemist as outliers not covered by the principle, or the experiments were manipulated to fit it through with somewhat contrived justifications was later disproved.^[2] In 1873, Thomsen acknowledged that his theory might not have universal or definitive credibility.^[3] Later, under newly created chemical thermodynamics framework, the principle was explained to only be valid as an idealization under extreme conditions (i.e., absolute zero).^[2] Thomsen openly admitted that his initial understanding was merely a close estimate of the reality, emphasizing that while chemical reactions typically release heat, this heat isn't always a trustworthy indicator of the strength of the bonds formed.^[4] On the other hand, Berthelot, was more resistant and continued to assert the validity of the principle until 1894.^[5] In 1882 the German scientist Hermann von Helmholtz proved that affinity was not given by the heat evolved in a chemical reaction but rather by the maximum work, or free energy, produced when the reaction was carried out reversibly.

References