Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

398 TREATMENT OF EXPERIMENTAL DATA(a) a period in which the chosen stoichiometric ratio varies with time;(b) a second stage in which the chosen stoichiometric ratio remains constant.This second stage appears when there is ody one reaction determining therate,the other reactions which caused the complications having effectively died outowing to the disappearance of one of the reactants. Thus, in our example, thepreliminary experiments would indicate that the reaction was of the formA+B -+P+Q+Rwhere the stoichiometric ratios vA/vB and vJvP varied with time; however, bystudying these ratios over a range of conditions, we should observe that vJvpeventually reached a constant value of unity at the same time as the concentrationof B became negligible; with this information, we should deduce that the productP is formed by a reaction not involving B; similarly the timevariation of vA/vQ andvA/vR would enable us to recognize that these products were formed from thereaction of A and B. A close study of the time dependence of the stoichiometriccoefficients can thus be of value in separating a complicated reaction into itsconstituent parts. An equivalent separation of the constituent reactions makingup certain types of complex reactions can sometimes be achieved by varying thetemperature; that is to say, a reaction exhibiting variable stoichiometric ratios atone temperature may show constant stoichiometric ratios at another owing to theeffective suppression of all but one of the reactions of the sequence. Clearly in suchcases where the conditions can be adjusted to make the stoichiometric ratios timeinvariant, the kinetics of the reaction in this region can be examined using eqn. (1).These results can then serve as the basis for a more complete interpretation of thereaction kinetics. It should be appreciated, however, that the discovery of conditionsin which the stoichiometric ratios remain time invariant for such a reaction is theexception rather than the rule.Perhaps the most important single preliminary observation is the influence ofthe products on the initial rate. If the products have no influence, we have animmediate simplification of eqns. (84) or (85) and a limited number of possibilitiesfor our rate expression. In this case, the reaction is most likely to be a set ofparallelreactions with a rate given by eqn. (84), e.g.---d[A1 - k,[A~1[B]b1+kz[A~[B]b2..dtor a chain reaction (see Chapter 2, Vol. 2) in which the product does not participate.Both of these reaction types wouid normally exhibit variable stoichiometry, although,in the case of a chain reactioi, this feature might be limited to an initialperiod of short duration. In fact, the only way of distinguishing parallel reactionsfrom this group of chain reactions without using special tests (see Chapter 2, Vol. 2).