This early postulate in classical thermochemistry became the controversial foundation of a research program that would last three decades.
The experimental objections to the Thomsen–Berthelot principle include incomplete dissociation, reversibility, and spontaneous endothermic processes.
[2] Such cases were dismissed by orthodox thermochemist as outliers not covered by the principle, or the experiments were manipulated to fit it through with somewhat contrived justifications was later disproved.
[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.