Law of Mass Action

Law of Mass Action reveals the relationship between the direction and speed of chemical interactions, on the one hand, and the "active mass" of the reacting substances, on the other, the active mass, in general, being expressed by the concentration of the substances. For dissolved substances, concentration corresponds to their content in a unit volume and usually relates to molecular weights (molecular concentration); for gases at constant pressure, concentration is measured by partial pressure; for solid substances that only touch a liquid or gas with their surface, the concept of concentration is not applicable. The significance of the concentration of the participating substances for the course of chemical reactions was first indicated by Wenzel (1777), but the Law of Mass Action was stated in its modern and general form by Guldberg and Waage (1867), which is why it is often named after them. They established that, all other things being equal (certain substances with their chemical affinity, constant medium, constancy of temperature and other physical factors), the speed of chemical interaction is proportional to the concentration of the molecules participating in the reaction (active mass of the reacting substances). This law is strictly experimental, but it also follows from kinetic conceptions of matter, and from this point of view, relying on the concept of probability, it must be considered as a limiting law of a large number, i.e., in essence, it has the character of a statistical one. If one imagines chemical interaction as the result of the mutual collision of reacting molecules, then the probability of these collisions will be the greater, the greater the number of molecules in a unit volume, i.e., the higher the concentration of the substance. If molecules A, B, C... participate in the reaction A + B + C + ... -> A1 + B1 + C1 + ..., then the speed of the process should be proportional to the concentrations CA, CB, CC..., i.e., v = kCA CB CC..., where v denotes the speed of the process and k is some constant value (rate constant of the reaction), depending not only on external conditions (temperature, medium, etc.), but also on the properties of the substances themselves and on their chemical affinity; formally, the constant k expresses, under given conditions, the speed of the reaction at concentrations of substances equal to one. If some substances, for example A and B, enter the reaction not by one, but by several molecules, say n and m, forming p and q molecules of A1 and B1, then the equation of such a reaction can be written in the form: nA + mB + ... -> pA1 + qB1 + ... or A + A + A... B + B + B... --------,----------1- ^v--------v--------' -» P A1 + q B1 + ...; = nA

=mB from which v = kCA CA CA... xCB CB CB... = kC^ C^... and therefore, in its general form, the Law of Mass Action is expressed by the equation: v = k C^ C^ Cq..., i.e., the speed of the process is proportional to the concentrations of the active substances taken to the power of the number of molecules of each of these substances participating in the reaction. For example, for reactions 1) CH3COOCH3 + NaOH -> CH3COONa + CH3OH, v = kCCH3COOCH3 CNaOH, 2) N2 + 3H2 -> 2NH3, v = kCN2 CH3. The Law of Mass Action lies not only at the basis of chemical kinetics, i.e., the doctrine of chemical reactions, but also at the basis of chemical statics or the doctrine of chemical equilibrium. If the reaction: nA + mB + ... -> pA1 + qB1 + ... can proceed reversibly, i.e., in two opposite directions, then for the direction from left to right we have v1 = k1C^A C^B ... and for the direction from right to left: v2 = k2C^A C^B ... As the interaction proceeds from left to right, v1 decreases, and v2 increases. When v1 and v2 are equal, a mobile equilibrium of the two opposite reactions must be established, i.e., in both directions the speeds of the process under given external conditions will balance each other, it being indifferent whether the initial reaction takes place in one or the other direction. When v1 = v2, then k2 ft. " from which

_ф ---- =-c-=K. This expression will be the condition of chemical equilibrium, the ratio of the two rate constants, k1 and k2, combining into one so-called equilibrium constant - K. For example, for the reaction CH3COOCH3 + NaOH -> CH3COOH + CH3OH, K ^kether ^ksolvent. The equilibrium constant shows the ratio of the concentrations of the participating substances at the moment of equilibrium; for this example at ordinary temperature and at initial molecular concentrations equal to one, it is indifferent whether we start from the system-ether + water or-solvent + acid, equilibrium is established invariably when in the system there are ²/₃ mole of ether, ²/₃ mole of water, ¹/₃ mole of acid and ¹/₃ mole of solvent, from which K= *^!'^x '* =4 V.xV. In recent times, thanks to precise investigations, it has been found that the concept of concentration itself does not fully characterize the active participation of substances in a chemical process, and this is especially manifested in gas reactions. Not every collision of molecules leads to their interaction, and it is necessary to accept the existence of a special active state of certain molecules. In other words, the Law of Mass Action requires supplementation or correction for individual deviations in the properties of individual molecules. In its simplest formulation, this correction enters into the expression of active mass as some factor characterizing the properties of substances under certain conditions of their interaction. Such an experimental representation of the active state of individual molecules in solutions, especially dilute ones, at one time led Arrhenius and van't Hoff to the necessity of introducing a correction into the values of the concentrations of electrolytes participating in reactions and served as one of the starting points for the hypothesis of the existence of dissociated molecules and for the confirmation of the theory of electrolytic dissociation.