The heat of formation is the heat that is absorbed when a substance is formed from its elements. In common usage, the heat of formation is used in place of the more precise term the enthalpy of formation, which has the symbol ()H_f. The enthalpy of formation of a substance is the heat of reaction for the combination of elements in their standard (normal) physical states to form one mole of the substance, but only 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 formation 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...