The heat of reaction is the heat that is absorbed when a substance is formed from its elements. In common usage, the heat of reaction is used in place of the more precise term the enthalpy of reaction, which has the symbol ()H_rxn. The enthalpy of reaction is the heat absorbed when one mole of a specified substance completely reacts according to a specified chemical equation under three specific conditions: (1) the pressure remains constant, (2) the only possible work that occurs is expansion against the atmosphere (so-called P()V work) and (3) the temperature remains constant during the process.

In practice, a reaction is likely to result in an increase or decrease in temperature (reactions that occur with an evolution of heat are called exothermic and reactions that require heat are termed endothermic). In addition, it is generally easier experimentally to use a sealed container...