Thermal Dissociation

Also found in: Dictionary.
Related to Thermal Dissociation: heat of decomposition
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

Thermal Dissociation


a chemical reaction in which a substance decomposes reversibly when the temperature is raised.

A substance undergoing thermal dissociation may form several substances (2H2O 2H2 + O2, CaCO3 ⇄ CaO + CO2) or a single, simpler substance (N2O4 ⇄ 2NO2, Cl2 ⇄ 2C1). In thermal dissociation, equilibrium is attained in accordance with the law of mass action and may be characterized by either the equilibrium constant or the degree of dissociation, which is the ratio of the number of dissociated molecules to the total number of molecules. In most cases, thermal dissociation is accompanied by an absorption of heat; that is, the increment of enthalpy is greater than zero (ΔH > 0). According to Le Châtelier’s principle, therefore, heating intensifies thermal dissociation, and the extent to which the equilibrium shifts with temperature is determined by the absolute value of ΔH. Pressure inhibits the thermal dissociation process, which is accompanied by a change Δn in the number of moles of gases; this inhibition becomes stronger as the increase in Δn becomes larger. When Δn = 0, as in the reaction 2HI ⇄ H2 + I2, the degree of dissociation is independent of pressure.

The thermal dissociation pressure is uniquely determined by temperature in the case of solids that do not form solid solutions and are not in highly dispersed states. In order to carry out the thermal dissociation of such solids as oxides and crystal hydrates, it is important to know the temperature at which the dissociation pressure becomes equal to the external pressure, in particular, atmospheric pressure. Since the evolved gas may overcome the pressure of the ambient medium, the decomposition process is sharply accelerated when this temperature is reached.

The most useful of the many thermal dissociation processes are the homogeneous reactions in which H2O and CO2 are decomposed and certain hydrocarbons are dehydrogenated, and the heterogeneous reactions in which carbonates and sulfides are dissociated. These dissociation reactions are encountered in many heat engineering, chemical, and metallurgical processes, particularly the calcination of limestone, the production of cements, and processes occurring in blast furnaces.


Kireev, V. A. Kurs fizicheskoi khimii, 3rd ed. Moscow, 1975.
Karapet’iants, M. Kh. Khimicheskaia termodinamika, 3rd ed. Moscow, 1975.


The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.
References in periodicals archive ?
Primary thermal dissociation. Industrial and Engineering Chemistry, 26:454-57.
The thermal dissociation of ethane, propane, normal butane, and isobutane.
In this article, we demonstrated a strategy for the preparation of TPEU/MCPA6 blends using in situ anionic ring-opening polymerization and in situ compatibility through the thermal dissociation reaction of TPEU in a weak alkaline solution.
Several publications are devoted to investigation of the mechanism (chemistry) of thermal dissociation of [Na.sub.2]C[O.sub.3].
Generalization and critical analysis of literature data of [Na.sub.2]C[O.sub.3] thermal dissociation in case of evapo ration of double oxides are presented in the monography [7].
According to data of [7], temperature dependence of pressure of thermal dissociation products has the following form, Pa:
and heat of thermal dissociation reaction [DELTA][H.sup.0.sub.T] = = 192.28 kJ/mol.
Pobuzhsky Ferronickel Integrated Works Ltd., Dnepropetrovsk, Ukraine Thermal dissociation tension of some sulfides and oxides relating to slag systems of ferronickel production Dissociation reaction of sulfide 2 I eS [right arrow] 2Me Dissociation + [S.sub.2] reaction of oxide 2MeO [right arrow] 2Me + [O.sub.2] Sulfide [MATHEMATICAL Oxide [MATHEMATICAL EXPRESSION EXPRESSION NOT NOT REPRODUCIBLE REPRODUCIBLE IN ASCII.] IN ASCII.] CaS 5 * [10.sup.-42] CaO 2 * [10.sup.-51] MgS* 3 * [10.sup.-29] MgO 3 * [10.sup.-46] [Cr.sub.2][S.sup.*] 6 * [10.sup.-17] [Cr.sub.2] 3 * [10.sup.-26] [O.sub.3] FeS 5 * [10.sup.-6] FeO 1 * [10.sup.-16] Ni[S.sup.*] 3 * [10.sup.-2] NiO 2 * [10.sup.-11] CuS 1 * [10.sup.3] CuO 1 * 10 Note.