Amount of heat that must be added or removed during a chemical reaction to keep all substances involved at the same temperature.
If it is positive (heat must be added), the reaction is endothermic; if it is negative (heat is given off), the reaction is exothermic. Accurate heat of reaction values are needed for proper design of equipment used in chemical processes; they are usually estimated from compiled tables of thermodynamics data (heats of formation and heats of combustion of many known materials). The activation energy is unrelated to the heat of reaction.