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.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

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.

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.
References in periodicals archive ?
the curing rate is equal to the rate of heat released as a fraction of the total heat of reaction. The rate constant [k.sub.1] depends upon the kinetics of the reaction without autocatalysis due to the product formed; while rate constant [k.sub.2] depends upon the autocatalysis effects of the product formed during the reactions.
The reaction rate is further configured to be independent of the oxygen concentration and the heat of reaction is artificially set to zero.
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.
Table 2 presents the mean residual heat of reaction ([DELTA][H.sub.R]), isothermal heat of reaction ([DELTA][H.sub.Io]), total heat of curing ([DELTA][H.sub.T]), and isothermal conversion ([[alpha].sub.I]).
where [H.sub.R] is the heat of reaction for the epoxy, and [partial derivative] [alpha]/ [partial derivative] [alpha]t is the curing rate of the epoxy which can be obtained from the following relation(2), (3)
Other constants: A pre-exponential factor, E activation energy, R gas constant, [DELTA]Hr heat of reaction, K heat transfer coefficient, S heat transfer surface.
The heat of reaction ([DELTA][H.sup.*.sub.rxn]) was calculated from the difference between the heat for formation of the products and the reactants:
The instrument can be used for measuring the thermal properties and thermal transitions of coatings or components, such as heat capacity, glass transition temperature ([T.sub.g]), crystallization temperature, melting point ([T.sub.m]) heat of fusion, and heat of reaction. DSC also can be used to monitor polymerization reactions and crosslinking processes and to characterize the unblocking temperature of blocked catalysts and blocked crosslinkers.
These scans allowed determining the heat of reaction [H.sub.residual] generated by the curing of the remaining epoxy resin not polymerized in the furnace.
It does this by using part of the exothermic heat of reaction while the other part is used for useful work.