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a branch of physical chemistry and a specific area of chemical thermodynamics concerned with the measurement and calculation of the heat effects of reactions, the heats of such phase transitions as vaporization, and the heats of other processes; also, the study of the heat capacities, enthalpies, and entropies of substances and physicochemical systems and the temperature dependence of these quantities.
The experimental side of thermochemistry is known as calo-rimetry, which entails the development of techniques for determining the properties listed above. Thermochemical measurements are performed with calorimeters.
The importance of studying heat effects and heat capacities was first pointed out by M. V. Lomonosov in the years 1752–54. The first thermochemical measurements were carried out by J. Black, A. Lavoisier, and P. Laplace in the second half of the 18th century. Calorimetric measuring techniques were refined in the 19th century through the work of such scientists as G. I. Gess (G. H. Hess), P. Berthelot, H. P. Thomsen, and V. F. Luginin. In the early 20th century, progress in thermochemistry was marked, on the one hand, by increases in the accuracy and temperature range of experiments and, on the other, by the establishment of links between the energy effects of processes, the structure of particles (atoms, molecules, ions), and the position of elements in D. I. Mendeleev’s periodic system. In addition, the number of substances that were investigated increased. In the mid–20th century, theories of thermochemistry came to be based on statistical concepts and concepts from quantum chemistry.
The difficulty and, sometimes, impossibility of directly measuring the heat effects of many processes often necessitates determinations through an indirect method, which is based on the fundamental law of thermochemistry—Hess’s law. These calculations employ standard heats of formation of various substances. Standard heats of combustion are used for the reactions of organic compounds. Heats of formation of chemical reactions may be recomputed at other temperatures using Kirchhoff’s equation. When the data required for a calculation are lacking, it is often necessary to resort to approximate relationships. These relationships make possible a determination of various energy properties of processes and substances on the basis of composition and structure, as well as by analogy with previously studied substances and processes.
Data from thermochemical studies, together with experimentally established relationships, are used to compute the heat balance of technological processes, investigate the heat value of fuels, calculate chemical equilibria, and determine the correlations between the energy properties of substances and the substances’ composition, structure, stability, and reactivity. In conjunction with other thermodynamic properties, thermochemical data make it possible to choose the optimal conditions for the production of chemicals.
Broad advances have been made in the thermochemistry of solutions, specifically, in determining heat capacities, heats of solution, mixing, and vaporization, and the dependence of these quantities on temperature and concentration. When these quantities are known, the properties of individual components may be found, and the heats of solvation and the heat effects of other processes may be calculated. This information is important as a basis for theories on the nature and structure of solutions. Thermochemical methods are employed in, for example, colloid chemistry and studies of biological processes.
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M. KH. KARAPET’IANTS