Physical & Theoretical Chemistry - Thermochemistry | Chemistry Net

Physical & Theoretical Chemistry - Thermochemistry




Thermochemistry is the branch of chemistry concerned with the heat effects that accompany chemical reactions - heat produced by or required for a chemical reaction. In thermochemistry, chemical reactions are divided into two categories:

  • Exothermic (Qreaction < 0) - heat is produced by the reacting system
  • Endothermic (Qreaction > 0) - heat is absorbed by the reacting system

Thermochemistry deals with determining quantities of heat produced or absorbed by a reaction both by measurement and by calculation. It rests on Lavoisier and Laplace's law and on Hess's law:

Lavoisier and Laplace's law: The energy change accompanying any reaction is equal and opposite to energy change accompanying the reverse process

Hess' law: The energy change accompanying any reaction is the same whether the process occurs in one or in several steps

These statements preceded the first law of thermodynamics (1845) and helped in its formulation.


  1. P. Atkins, J de Paula,  “Physical Chemistry: Thermodynamics, Structure and Change”, 10th Edition, W. H. Freeman, 2014
  2. D. A. McQuarrie, J. D. Simon,“Physical Chemistry: A Molecular Approach”, 1st Edition, University Science Books, 1997
  3. K. J. Laidler, J.H. Meiser, B.C. Sanctuary, “Physical Chemistry”, 4th Edition, Brooks Cole, 2002

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