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When two molecules A and B react irreversibly with formation of a third substance C, the velocityv of the reaction, according to the classical law of mass action, ought to be directly proportional to the product of the concentrations Ca and Cb of A and B at the time of observation, that isv =k Ca Cb , wherek is a constant. Although this equation has been successfully applied to some reactions there are many expectations, particularly reactions involving ions, where the irregularity is indicated by a progressive change ink with the initial concentration. Corrections have therefore had to be made in the above equation to account for the deviations from the classical mass action law. It has been suggested that the concentration terms should be replaced by thermo-dynamic activities, for the equilibrium constant K of the reaction A + B ↔ C is given exactly by K =a c /a a a b , wherea a ,a b , anda c represent the activities of A, B, and C. Mere substitution of activities for concentrations in the velocity equation does not, however, explain the kinetic phenomena observed. The views that up to the present have met with most success have been put forward by Brönsted. He supposes that probably all reactions involve the primary formation from the reactants of a fugitive complex which is in thermo-dynamic equilibrium with them. The rate of the reaction is determined by the rate of formation of this complex X which then breaks down spontaneously into the final product, at a rate proportional to the concentration of X. Brönsted finally deduces the following equation for the velocity of reaction:v =ka a a b /f x =k Ca Cb f a f b /f x =k Ca Cb F, where thef terms represent activity coefficients. The reaction kinetic factor F is in a great measure dependent on the ionic concentration of the solution, and in general, it changes with the course of the reaction. The change on X, in a reaction involving ions, is the algebraic sum of the charges on A and B, and its activity coefficient depends in the same way as that of a substance on the medium. Brönsted's hypothesis has been tested in a great many reactions involving are expected to undergo a considerable change, owing to medium changes, either due to continued reaction, or to the addition of various electrolytes.

The rate of transformation in aqueous solution of methylammonium cyanate into methylurea