Chemical Reaction

CHEMICAL REACTION, a chemical change in a substance occurring under the influence of external factors or due to internal causes, in contrast to a physical reaction, in which the substance remains unchanged. The entire diversity of chemical reactions can be reduced to three basic types: 1) decomposition or disintegration reactions, when several simpler substances are obtained from a complex substance; 2) combination or synthesis reactions, when a smaller number of more complex substances are formed from several substances; and 3) exchange decomposition, displacement, or rearrangement reactions, when the number of reacting substances equals the number of substances obtained, and the change consists of the mutual displacement of parts of a molecule from one substance to another. A chemical reaction can be expressed in the form of a chemical equation showing which substances and in what quantity react, and how many and which substances are formed as a result of the reaction. Depending on how many substances enter into interaction, monomolecular, bimolecular, and higher-order reactions are distinguished (see Chemical Kinetics). A chemical reaction occurs instantaneously if ions are reacting; the reaction proceeds more slowly in the case of the interaction of undissociated molecules in a homogeneous medium and slows down even more when the reacting substances are in different phases (heterogeneous reaction). In the latter case, which plays a large role in biochemical processes, we have a number of new factors (adsorption, surface energy, diffusion rate, etc.) influencing the course of the reaction. The majority of chemical transformations encountered in a living organism represent the sum of a whole series of simple reactions occurring simultaneously. The summary equation of such a complex reaction can be broken down into several equations expressing individual stages of the process; however, often a chain of chemical reactions is so complex that we can speak only of their summary effect—the physiological reaction of the organism. Those chemical reactions as a result of which substances with characteristic properties are obtained, for example, colored, poorly soluble, having a specific odor, and the like, find wide application for analysis. Such are the numerous reactions of analytical chemistry, reactions used in clinical investigations, in histochemistry, in forensic medicine, etc.

Many chemical reactions are accompanied by a large release of heat (exothermic reactions) and serve as an important source of energy (e.g., reactions of carbohydrate oxidation in the organism); other chemical reactions, on the contrary, require the input of energy from the outside (endothermic reactions); the substances formed in this process contain the absorbed energy in the form of chemical potential energy (reactions of carbohydrate synthesis by a plant); finally, a large number of reactions are accompanied by very small changes in the amount of energy contained in the reacting substances (this includes numerous reactions of hydrolysis of carbohydrates, proteins, fats, etc.). See also Reversible reactions.