Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

1 INTRODUCTION 347rate of the reaction must include terms in [PI and/or [Q] and hence eqn. (1) cannotpossibly represent the data. One other property of the simple rate expressionsgeneralized by eqn. (1) is that the maximum rate occurs when the concentrationof the reactants is at a maximum, that is at zero time. Consequently, if the rate ofthe reaction increases to a maximum value over a period of time, it is certain thatan expression such as (2), (3) or (4) will be required.Thus if a preliminary examination of the concentration-time data shows anyone of the following three features:(1) a variable stoichiometry;(2) an initial rate which is affected by the presence of product;or (3) an initial rate which is less than the rate observed at a later stage in thereaction,then the methods employed to process the data are those appropriate to expressionsof the type (2), (3) or (4). Alternatively if we observe:(1) an invariant stoichiometry;(2) an initial rate unaffected by the presence of product;and (3) the maximum rate at the beginning of the reaction,then we attempt to process the data on the basis of eqn. (1).However, it is often the case that not all the concentrations which are requiredfor the calculation of the reaction stoichiometry are available. It may be thatexperimentally it is only practicable to measure the concentration of, say, thereactant A at a series of times subsequent to the reaction being started. In thissituation, our only help comes from the initial rate. As before, if the initial rateof disappearance of A is affected by the presence of product and/or is less thanthe rate observed somewhat later in the reaction, we know immediately that themore complicated expressions are required. The converse observations, however,do not necessarily imply that the rate can be represented by the simple equation,(1). In those cases where the preliminary examination of the data indicates thatthe maximum rate occurs at zero time and is unaffected by the presence of product,the data are examined on the basis of eqn. (1) first; if inconsistent rate coefficientsare found together with non-simple orders, the data are then re-examined on thebasis of the more complicated expressions. This same procedure is adopted whenthe data consist of a series of concentrations and times but where, for experimentalreasons, the early concentration-time values are either unobtainable or sufficientlyunreliable as to preclude any reasonable estimates of the initial rate being made;the data are examined on the basis of the simple expression first and in the eventof inconsistencies re-examined on the basis of the more complicated expressions.It should be clear that improvements in the experimental technique designed toreduce the uncertainties in the initial rates can more than repay the effort involved.The same general point can be made but with greater force on the type of experimentin which a single physical property characteristic of the reaction systemas a whole is measured at a series of times (e.g. pressure-time measurements inReferences p. 407