Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

Chapter 5The Treatment of Experimental DataD. MARGERISON1. IntroductionThe majority of kinetic investigations are carried out with the object of gaininginsight into the mechanism by which chemical change occurs. Very often, theinvestigation may be solely concerned to establish the series of steps by which thereactants are converted into the products-the reaction mechanism, as it is called.In other cases where the reaction mechanism is known, studies of the kineticsof the reaction may be employed to throw light on the details of the interactionof the reactant molecules. These objectives demand that the experimental data besummarized in equations of the same form as those which appear in theoreticaltreatments of reaction kinetics. It is the purpose of this article to explain theprocedures employed but we shall not consider the whole range of reaction types.In fact, we shall confine our attention to homogeneous reactions occurring in thegaseous and liquid phases though it should be noted that some of the methodsdiscussed can be applied to reactions in which a surface is involved.The key parameter throughout all discussions of reaction kinetics is the instantaneousvalue of the reaction rate. This may be defined in as many ways as thereare reactants or products; for example, in the case of a reaction between tworeactants A and B to give two products P and Q, there are four alternative definitionsof the reaction rate~ -d[A] -d[B] d[P] -or-dLQ1 -dt ’ dt ’ dt dtwhere [A], [B], [PI and [Q] represent the instantaneous values of the molarconcentrations of the various species at time t. It is clear that each rate can befound by constructing the tangent to the appropriate concentration-time curve atthe particular time under consideration. It should be noted that the alternativedefinitions of the reaction rate are not usually equal in value though all are positive.Consequently, in any discussion involving the term reaction rate, it is necessaryto define precisely which of the various alternatives is used. However, in the author’sopinion, there is a strong case for confining discussion to the rate of change of oneor other of the reactant concentrations whenever this is possible since, from aReferences p. 407