THERMOCHEMISTRY

From Big Medical Encyclopedia

THERMOCHEMISTRY (Greek therme warmth + chemistry) — the section of physical chemistry devoted to quantitative studying of heat effects of chemical processes and establishment of their patterns. Thermochemical data are necessary, e.g., during the calculation of the energy balance of chemical and biochemical processes; broad practical application of a microcalorimetry led to creation of the recent scientific trend — thermobiology. The analysis of thermograms gave the chance to analyze thin changes in the course of various processes happening in live organisms. By means of a microcalorimetry measure action of antibiotics on thermogenesis of bacteria (e.g., for control of efficiency of bacteriostatic properties of these substances), analyze influence on thermogenesis of animal anesthetics (see) and temperature exceeding the body temperature, etc. Big role use of methods T. played in a research of heateducational function of pulmonary fabric. T. makes an essential contribution to the solution of a number of theoretical questions. Results of thermochemical researches often give the chance to explain stability of one connections and instability of others and to define on this basis the preferential direction of a course of chemical reactions, including and in a human body and animals.

For the first time reasons about need of studying of the heat effects accompanying chemical processes were stated still by M. V. Lomonosov, A. Lavoisier, P. S. Laplace in 18 century. Besides to the period also the early pilot studies in the area T belong. The largest thermochemists of the end of 19 century and the beginnings of 20 century were Bertlo (R. E. M of Berthelot) and Tomsen (N. P. J. Thomsen). In Russia the first thermochemical laboratory was founded by V. F. Lugini-ny, to-ry was a founder of school of the Russian thermochemists.

By the main method T., is allowing to measure amount of the heat generated or absorbed during various chemical or biochemical processes calorimetry (see). Devices for measurement of heat effects carry the name of calorimeters. In addition to a calorimetry, thermochemical characteristics can be received also as a result of chemical balance, and also on the basis of the electrochemical and spectroscopic data characterizing the studied process; however, as a rule, these sizes are less exact, than received at calorimetric researches.

According to the first law thermodynamics (see) warmth generally is not characteristic size. However if only such processes are considered, in to-rykh only a type of work is work of expansion (compression) of system and, besides, process is carried out at the constant volume or pressure, then follows from the first law of thermodynamics that Q v = dU and Q P = dH, i.e. heat effect (warmth) of the process (reaction) which is taking place at the constant volume, Q v , heat effect of the process proceeding with a constant pressure, Q is equal to change of an internal energy of the dU system, and P it is equal to enthalpy change (heat content) of dH in this process.

In thermochemical researches usually study processes (reactions) happening with a constant pressure or constant volume. That the measured sizes dH or dU could be carried to a certain process, it is necessary to establish precisely what initial and final conditions of system were: its qualitative and quantitative structure, temperature, pressure, aggregate and phase states of the logging-in substances. It often demands carrying out in parallel with thermochemical measurements of big chemical analysis work. For the description of processes use the so-called thermochemical equations, with to-rymi it is possible to operate algebraically. The thermochemical equations as well as the usual equations of chemical reactions, sense of formulas of reactants and stoichiometric coefficients in them the same, as in the usual equations of chemical reactions register (see. Stekhiometriya ). At a formula of each substance specify by indexes a condition of this substance in thermochemical equations, and after the equation (usually through a semicolon) write down change in this process of a certain thermodynamic function — an enthalpy (dH) or an internal energy of system (dU) or specify amount of the marked-out or absorbed warmth. E.g., 0,5O 2 (gas) + N 2 (gas) = = N 2 O (zhidk)  ; dH = 285,8 kJ.

Not always it is possible to measure heat effect of specific process directly (reactions;. Heat effects of other, simpler reactions, the sum measure in these cases to-rykh corresponds to result of the considered reaction, and calculate heat effect of the interesting process on the basis of the law of constancy of the sums of warmth — the law of constant heat summation, to-ry is formulated as follows: heat effect of reaction depends only on a reference and final state of systems and does not depend on intermediate stages of process. The law of constant heat summation has almost important investigation, according to Krom heat effect of chemical reaction is equal to the sum of formation heats of final substances minus the sum of formation heats of mother substances.

The main method of a thermochemical research of organic compounds is definition of warmth of burning of connection in a calorimetric bomb. Along with definition of warmth of burning definition of warmth of various chemical reactions often meets participation of gaseous reagents. Such researches are conducted usually also in calorimetric bombs.

The isolated position is held by T. solutions (see). Refer definition of warmth of dissolution, dilution, neutralization, warmth of chemical reactions in solutions to it, etc. Designs of the calorimeters used for these definitions substantially differ from calorimeters for definition of warmth of burning (see. Calorimetry ).



Bibliography: Benson S. Thermochemical kinetics, the lane with English, M., 1971, bibliogr.; Glinka N. L. General chemistry, page, 159, M, 198.3; Kalvee. iprata. A microcalorimetry, Use in physical chemistry and biology, the lane with fr., M., 1963; Mortimer K. Warmth of reactions and durability of bonds, the lane with English, M., 1964; Thermochemistry, under the editorship of V. A. Sokolov, h, 1 — 2, M., 1964 — 1966, bibliogr.; Spink C. H. a. Wadsol. Thermochemistry of solutions of biochemical model compounds, J. chem. Thermodyn., v. 7, p. 561, 1975.


A. F. Vorobyov.

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