Heat of Formation

heat of formation [′hēt əv fȯr′mā·shən]

(physical chemistry)

The increase in enthalpy resulting from the formation of 1 mole of a substance from its elements at constant pressure.

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Heat of Formation 

the heat of reaction resulting from the formation of a substance from its elements. Various types of heat of formation are distinguished, including the following: the heat of formation from free atoms; the heat of formation from simple substances corresponding to the most stable state of the elements at a given pressure and temperature; the heat of solvation, that is, the heat of formation of solvation spheres around ions when substances interact with a solvent; and the heat of crystallization, that is, the heat of formation of crystals from the atoms, molecules, or ions that make up the crystal lattices. The heats of formation most often used are the heat of formation from simple substances and the heat of formation from free atoms (or the opposite in sign heat of decomposition of molecules into their constituent atoms). These quantities are generally given for substances in standard states.

Heats of formation can be determined in the following ways: through direct calorimetric measurement; by using the equations describing the constant-pressure and constant-volume temperature dependence of the equilibrium constant of the formation reaction; by calculation, on the basis of Hess’s law, from the heat of a reaction in which the given substance participates (when the heats of formation of the other reactants and reaction products are known); by calculation from the Gibbs free energy and entropy of all the reactants; by using the Gibbs-Helmholtz equation and the variation in the electromotive force produced by a galvanic cell at different temperatures; and through calculations based on numerous regularities exhibited by the heats of formation of various substances.

Reliable experimental data on heats of formation are available for approximately 5,000 compounds. By using known values of heats of formation, heats of reaction can be determined for tens of thousands of reactions without carrying out experiments. Together with other data of chemical thermodynamics, data on heat of formation provide the basis for the calculation of changes in the Gibbs free energy; knowledge of these changes permits assessment of the comparative stability of various chemical compounds.

The heats of formation of a large number of substances can be estimated with a high degree of accuracy from expressions relating the heat of formation of a substance to its structure. These expressions have been obtained through analysis of a vast amount of experimental data on the basis of the classical theory of the structure of chemical compounds and on the basis of the quantum mechanics of molecules (seeQUANTUM CHEMISTRY). The expressions make use of the periodicity of the properties of compounds of the same type formed from elements in groups and periods of D. I. Mendeleev’s periodic system. The expressions also make use of the approximate constancy of the structure and properties of individual structural units of the molecules in homologous series.

REFERENCES

Termicheskie konstanty veshchestv. Edited by V. P. Glushko. Moscow, 1965–74.
Karapet’iants, M. Kh., and M. L. Karapet’iants. Osnovnye termodinamicheskie konstanty neorganicheskikh i organicheskikh veshchestv. Moscow, 1968.
Cox, J. D., and G. Pilcher. Thermochemistry of Organic and Organometattic compounds. London-New York, 1970.

M. E. ERLYKINA