In thermodynamics and physical chemistry, thermochemistry is the study of the heat evolved or absorbed in chemical reactions. Thermochemistry, generally, is concerned with the heat exchange accompanying transformations, such as mixing, phase transitions, chemical reactions, etc., which includes calculations of such quantities as the heat capacity, heat of combustion, heat of formation, etc. The laws of thermochemistry rest on two statements:^[1]

1. Lavoisier and Laplace’s law (1782): the heat exchange accompanying a transformation is equal and opposite to the heat exchange accompanying the reverse transformation.
2. Hess’s law (1840): the heat exchange accompanying a transformation is the same whether the process occurs in one or several steps

Both laws preceded the first law of thermodynamics (1850); it can be shown, however, that they are a direct consequence of it.

They also investigated specific heat and latent heat.