# Heat of Reaction

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## heat of reaction

[′hēt əv rē′ak·shən]
(physical chemistry)
The negative of the change in enthalpy accompanying a chemical reaction at constant pressure.
The negative of the change in internal energy accompanying a chemical reaction at constant volume.

## Heat of Reaction

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.

References in periodicals archive ?
Downstream, the temperature declines slowly because the heat of reaction is decreased and the effect of heat transfer from the cooling barrel is visibly displayed.
The heat of reaction of the CCH and lignin CCH samples was observed by using DSC.
The heat of reaction estimated from area under the curve was 230 J/g.
Reaction Calorimetry: Reaction calorimetry can be used to determine the heat of reaction under isothermal (constant sample temperature) or isoperibolic (constant temperature of the surroundings) conditions and identify the effects of changes in feed rate, temperatures, and concentrations on the instant-by-instant behavior of a reaction system.
Further it is necessary to know also the heat of reaction [DELTA]Hr, system thermal capacity C and the temperature tangent direction dT/dt.
In-Situ Monitoring of the Curing of Epoxy Resins by DSC 629 A scan performed directly on a sample of the mixture epoxy resin/curing agent with no heat treatment allowed to determine the heat of reaction Htotal generated by the complete polymerization process.
It does this by using part of the exothermic heat of reaction while the other part is used for useful work.
The exothermic heat of reaction was measured for both softwood and hardwood cooks in a laboratory digester.
Excess water is evaporated, absorbing and carrying the heat of reaction out of the reactor with the hydrogen.
The equipment and techniques used for DSC analysis of residual heat of reaction were the same as those previously reported (Vick et al.
The simulations showed a defining feature of combustion: how the heat of reaction affects the exchange of energy in a turbulent system.
This utilizes the internal heat of reaction to form a high-purity compound with low energy input requirement.

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