Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

2 CONSISTENCY WITH EQNS. OF TYPE -d[A]/dt = k[A]"[B]* 393Other workers have considered the problems discussed in this section-forexample, see references 6-9. A useful discussion of the statistical background isgiven by Guest',.2.2 REACTIONS WITH UNKNOWN STOICHIOMETRYIf we study the kinetics of a reaction between A and B and do not know thestoichiometry, it should be clear from our previous discussion that very littlereal progress can be made in analysing the data. Suppose for the sake of argumentthat we are unable to analyse our reaction mixture for the reactant B but are ableto follow the disappearance of A. Even in the case of the simplest possible type ofrate expression, namely the one under discussiona knowledge of the concentration of B is required in general for the evaluationof the rate coefficient. If the reaction rate is characterized by one of the morecomplicated expressions, the instantaneous value of the concentration of B is verylikely to be involved and so we have insufficient information to make any generalstatement regarding the form of the differential rate expression.However, there are two situations in which the previous calculations can befruitfully undertaken without requiring a knowledge of the concentration of B.These are when the order with respect to B is zero or when the disappearance of Acan be studied in the presence of a considerable excess of B. If the preliminaryexamination of the data suggests that the order of the reaction with respect to Bis zero, then the validity of the rate expressioncan be demonstrated by showing that the rate coefficient k is independent of [A],and [B],. Where the disappearance of A can be conveniently followed in thepresence of a large excess of B, we Grst have to estimate the order with respectto B at the same time as we estimate the order with respect to A (see p. 360); wethen compute k' in the equationand obtain a value for the rate coefficient, k, from the expression k' = k[B]t. IfReferences p. 407