Chemical Equilibrium

CHEMICAL EQUILIBRIUM. When a series of substances A, B,..., capable of reacting with one another come into contact, a reaction begins, proceeding in the direction of formation of new products A_x, B_y,... Simultaneously with the forward reaction, the reverse reaction also begins, and its rate increases as the quantities (concentrations) of the forming substances A_x, B_y... increase. Conversely, the rate of the forward reaction decreases due to the decrease in quantities (concentrations) of the initial substances A, B,... If all substances, both reacting and forming, remain in the system, then inevitably a moment will come when the rate of the reverse reaction equals the rate of the forward reaction—this state of the system is called chemical equilibrium. However, at chemical equilibrium, the system is not at rest; in it, at least two processes continuously flow in mutually opposite directions, and thus the result of one process is destroyed by the result of the other. A picture of apparent rest is created. The state of chemical equilibrium is determined by the law of mass action (see) (by the equation of the reaction isotherm) c_xc_y... _______ = K c_ac_b... The magnitude K (equilibrium constant) determines the final state of the system at a given temperature and pressure. A change in temperature entails a change in the equilibrium constant according to the following equation of the reaction isochore: lnK = -Q/RT + const, where Q is the heat effect of the reaction, taken as positive in the case of exothermic reactions and negative in endothermic reactions; R is the gas constant; T is the absolute temperature. From this equation it is evident that in the case of zero value of Q, the equilibrium constant does not depend on temperature—a case observed in many biochemical reactions, as for example in the hydrolysis of proteins, fats, and carbohydrates, where Q is close to zero. The isochore equation is a consequence of a more general law, according to which, if we externally affect a system in physicochemical equilibrium, processes will arise in the system that seek to weaken this effect (the principle of Le Chatelier, Gibbs, van't Hoff). Chemical equilibrium is disrupted—as soon as one of the reacting substances leaves the sphere of reaction (precipitates, volatilizes as a gas, enters into reaction with other substances, etc.) and is restored again when the previous value for the equilibrium constant is achieved. In chemical processes occurring in a living organism, we have a complex system of mobile equilibria of interconnected reactions. Equilibrium is constantly disrupted, since the products of one reaction interact with the products of other reactions, but at the same time equilibrium is restored again due to the further course of the original reaction. Thus, liver glycogen is in equilibrium with glucose dissolved in the blood; the latter, for example, entering muscle cells, is partially converted into hexosephosphoric acid, which in turn is in equilibrium with the labile form of glucose, which further decomposes, passing through a series of intermediate products that are in turn in a state of equilibrium with each other as well as with substances formed as a result of side reactions. This entire complex system of mobile equilibria is constantly disrupted due to the removal of end products from the sphere of reaction and is constantly restored again due to the further decomposition of glycogen. Many systems remain in a state of apparent rest without reaching a state of equilibrium, due to the presence of factors that retard one reaction or another—a state named by the proposal of Duhem false equilibrium. False equilibrium can be disrupted in the presence of a suitable catalyst. In a more general case, the system consists of several substances and several phases. The doctrine of the equilibrium of multiphase systems is considered in thermodynamics. The phase rule of Gibbs plays a great importance here—see Homo-, heterogeneous systems, as well as Reversible reactions.

Author signatures: ["A. Кузин"]