Thermochemistry

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For other uses, see Chemical thermodynamics.

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]

Lavoisier and Laplace’s law (1780): the heat exchange accompanying a transformation is equal and opposite to the heat exchange accompanying the reverse transformation.
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.

1 History
2 See also
3 References
4 External links

[edit] History

In 1780 Antoine Lavoisier and Pierre-Simon Laplace laid the foundations of “thermochemistry” by showing that the heat evolved in a reaction is equal to the heat absorbed in the reverse reaction. They also investigated the specific heat and latent heat of a number of substances, and amounts of heat evolved in combustion. Similarly, in 1840 Swiss chemist Germain Hess formulated the principle that the evolution of heat in a reaction is the same whether the process is accomplished in one-step or in a number of stages. This known as Hess's law. With the advent of the mechanical theory of heat in the early 19th century, Hess’s law came to be viewed as a consequence of the law of conservation of energy.