the algebraic sum of the heat absorbed in a given chemical reaction and the external work done, less the work done against external pressure. If heat is liberated in the reaction or work is done by the system, the corresponding quantities appear in the sum with a minus sign.

At constant temperature and volume, the heat of reaction is equal to the change in internal energy of the reactants ΔU, and at constant temperature and pressure, it is equal to the change in enthalpy ΔH. The heat of reaction is usually expressed in kilojoules or kilocalories and is determined relative to the number of moles of the reactants that corresponds to the stoichiometry of the reaction. For individual types of chemical reactions, special terms, such as “heat of formation” and “heat of combustion” are used instead of the term “heat of reaction.”

The heat of reaction depends on the temperature and pressure (or volume); the temperature dependence is given by Kirchhoff’s equation. For the comparison of heats of reaction and the simplification of thermodynamic calculations, all values of heats of reaction are given for standard conditions (all the reactants are in standard states). Data on heats of reaction may be obtained directly through calorimetry, by studying the chemical equilibrium at different temperatures, or by calculation, for example, by calculation from the heats of formation of all the reactants. In the absence of basic data, heats of reaction can be determined through approximate methods of calculation based on relationships between heats of formation or heats of combustion and the chemical composition of compounds. Heats of reaction are important for theoretical chemistry and are necessary for calculating the equilibrium composition of mixtures, the yield of reaction products, and the specific thrust of reaction-engine fuels. Heats of reaction also play an important role in the solution of many other applied problems.