Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

1 INTRODUCTION 345so that eqn. (1) reduces to---d[A1dt- k[Ay[BlbThe quantities a and b are termed the orders with respect to A and B while thefunction k (T, [catalyst], P etc.), usually contracted to k, is termed the rate coeficient.There are, however, many reactions in which the expression for theinstantaneous rate of disappearance of A is more complicated, e.g.oror-= -d[A1dt-= -dCA1dt---d[A]dtc ki f,([reactants], [products])ic ki f,([reactants], [products])i- k;f;([reactants], [products])i- kf([reactants], [products])1 -t. k’f ‘([reactants], [products])These four expressions for the reaction rate have one feature in common, namelythat the influence of the concentrations of reactants and products on the reactionrate is separated from the influence of the other reaction parameters. The reasonfor proceeding in this way is that these rate expressions suggest possible mechanismsfor the reaction in question, a feature which will be dealt with later on, in Volume 2.Although the rate of the reaction is the parameter in kinetic studies whichprovides the link between the experimental investigation and the theoretical interpretation,it is seldom measured directly. In the usual closed or static experimentalsystem, the standard procedure is to follow the change with time of the concentrationsof reactants and productst in two distinct series of experiments. In thefirst series, the initial concentrations of the reactants and products are varied withthe other reaction variables held constant, the object being to discover the exactrelationship between rate and concentration. In the second series, the experimentsare repeated at different values of the other reaction variables so that the dependenceof the various rate coefficients on temperature, pressure, ionic strength etc.,can be found. It is with the methods of examining concentration-time dataobtained in closed systems in order to deduce these relationships that we shall beconcerned in this chapter. However, before embarking on a description of theset Throughout this chapter, the term “product” refers to any substance which is not a reactant;thus, any reference to products must be taken to include intermediates.References p. 407