Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

400 TREATMENT OF EXPERIMENTAL DATAFinally we come to the case where a reaction apparently exhibits invariantstoichiometry, an initial rate which is not exceeded at a later stage in the reactionand which is unaffected by the presence of products, and yet whose rate cannotbe expressed according to the simple expression--- d[A1 - k[ATIBIbdtwith a constant value of the rate coefficient, k. Assuming that we can discountpurely physical effects arising from increases in such factors as the viscosity orionic strength of the reaction system which may occur on the formation of theproducts, the reason almost certainly lies in the existence of minor parallel reactionswhich do not seriously influence the reaction stoichiometry. In other words,such problems are best dealt with by treating the data on the basis of eqn. (87),thus--- d[A1 - k,[AIai[Blbi+ k2[A1(12[BlbZdtA summary of these general considerations is given in Appendix 3 in the formof a flow sheet. It is not suggested that the questions and answers cover all possiblesituations. Furthermore, the majority of the questions will not be capable of beinganswered with complete confidence in any experimental investigation. For example,to the question “Are all reactants and products identified at all stages of thereaction?”, only a qualified “Yes” is possible; there may well be products formed,intermediate between the reactants and final products, which are present in verysmall concentrations and which are undetectable by the analytical methodsemployed. It follows that Appendix 3 is no more than a guide to the identificationof reaction type. Finally, a word about the term, “simple reaction”; in the contextof Appendix 3 and in this article, we use this term to describe a reaction, the rateof which is given by eqn. (l), oiz.--- d[A1 - k[AT[BIbdtA simpIe reaction in the sense used here is, therefore, not necessarily an elementaryreaction.We shall not attempt a detailed account of the methods of processing theexperimental data whe3 the reaction is complicated since there are so manypossible differential expressions; examples are given in Chapter 1, Vol. 2. Thegeneral procedure is as follows.On the basis of the previous discussion and guided by Appendix 3, we select