From Big Medical Encyclopedia

CHEMICAL REACTIONS — the transformations of chemical elements or their connections (reagents) which are followed by formation of new connections (products).

Unlike nuclear reactions (see) chemical changes cannot lead to change of qualitative structure of the elements participating in reaction. Result of X. the river is either education, or a cleavage; according to it allocate reactions of connection (education) and decomposition. Depending on type of the particles which are taking part in reaction distinguish molecular, ionic, radical and other chemical changes. As motive power of these reactions in isothermal conditions serves the difference of free energies of reagents and products. X. the river is directed to decrease in free energy of the reacting system (see Ender-gonichesky reactions). At the same time absorption or calorification (endothermic and exothermic reactions) is possible. Schematically X. rubles represent in the form of the equations of reaction with the indication of quantitative ratios between the reacting molecules and products (see Stekhiometriya), and also the directions of reaction.

X. the river, products to-rykh can react, reproducing initial substances, are called reversible; in stationary conditions they come to an end with establishment of balance, at Krom the speed of forward reaction is equalized with a speed of the return. Nek-ry reagents have an opportunity to undergo transformations in various directions that leads to simultaneous course of simultaneous or concurrent reactions. Often mother substances form end products not at once, and as a result of the consecutive reactions which are characterized by existence of modes of formation of intermediate products capable to react with each other or with mother substances (see Kinetics of biological processes). When in such reaction sequence there is an intermediate product in the form of the free radical (see Radicals free) leading to resuming of all number of transformations, X. rubles call chain (see. Chain reactions). Stages consecutive X. the rubles limiting the speed of formation of an end product call limiting. Set of transformations of substances in the course of X. the river defines her mechanism. Many X. rubles are carried out only under the influence of external energy — thermal, electric (electrochemical alterations), light (photochemical reactions), etc. Nek-ry by X. rubles in itself can be an energy source, napr, electric in a galvanic cell or thermal during the burning. Speed is X. the river, as a rule, depends on concentration of reagents (see. The operating masses the law), temperatures, pressure, existence in the reacting system of catalysts (see) and other factors. Dependence of speed of X. the river from concentration of reagents defines order of reaction. X. the river has a zero order if speed does not depend on concentration of reactants. In case of proportionality of speed of X. river of concentration only of one of substances reaction has the I order. Also more difficult dependences and according to reaction II,

III and fractional orders are known. As reagents and to the nature of their interaction distinguish exchange reactions> of hydrolysis (see), neutralizations (see Neutralizations a method), redoxes (see Redoxreactions), complex formations (see Complexons), etc. In false organisms the huge number various X proceeds. the river (biochemical reactions), idiosyncrasy to-rykh is dependence on existence in system biol. catalysts of enzymes (see).

Bibliogrbenson S. V. Fundamentals of chemical kinetics, the lane with English, M., 1964; O. S Hares. General chemistry, M., 198.3; M and t e. and Pang and to about R., the Course of theoretical fundamentals of inorganic chemistry, lane with fr., M., 1975.

V. A. Pekkel.