CORRESPONDENCE THE KINETIC DERIVATION OF THE MASS ACTION EXPRESSION the stages of the transformati&. This is proportional to the chance that any given particle may pass through all the stages within unit time, which in turn is proDEARSm: portional to the product of the chances for the inI t is admitted that chemical reactions of higher order dividual stages. When we work this out we find that than the third practically never occur. Nevertheless, the ultimate reaction rate is precisely that given by the in deriving the mass action expres~ion~or eqnilibrium mass action expression, A".Bb.CC..., whereas the constant, by equating two opposing reaction rates, reaction rate in the ordinary sense is a much larger chemical textbooks appear to assume that every re- quantity, which measures the reaction rate of each and action is accomplished by the mechanism indicated by every one of the steps in which the given transformation its ordinary chemical equation, even though this may is accomplished. By equating two such expressions seem to imply a reaction of a very high order. The we are therefore merely subscribing to the proposition apparent inconsistency resides in a little obscurity in that when we have individually tagged or identified a what we mean by reaction rate. Ordinarily reaction large number of molecules then the number of such rate means the actual rate a t which the transformation molecules a t eqnilibrium that succeed in passing all the proceeds in some specified step in the series of successive successive stages of a transformation within unit time steps by which the forward or reverse reaction is ac- is equal to the number that succeed in passing through complished. At equilibrium the rates for the succes- all the reverse stages within unit time. If this were sive steps are all equal, provided we do not have what not true, then with passage of time we would have a might be termed a branched transformation; hence change in the distribution of the tagged or identified the reaction rate for any stage is that for the complete molecules among the successive reaction stages. set of transformations in the given direction. Yours sincerely, By contrast we may define what might be termed the H. G. DEMING ultimate reaction rate as being a measure of the number of molecules, individually identified a t the start, which succeed within one unit of time in passing through all
To the Editor
