Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

348 TREATMENT OF EXPERIMENTAL DATAgaseous reactions) and in which the stoichiometry of the reaction is assumed inorder to deduce the concentration of a particular reactant or product from themeasured property. With such limited information on a reaction, it is difficult toreach valid conclusions as to the form taken by the rate expression.2. Testing the data for consistency with equations of the type-d[A]/dt = k[A]"[BlbEquations of this type are the simplest to deal with. The quantities a and bare termed the orders of the reaction with respect to A and B as mentioned previouslywhile their sum, (a+b), is termed the total order of the reaction. The equation canbe generalized without difficulty to cover the case in which the concentrations ofmore than two reactants enter the rate equation. It must be emphasized that thevarious orders cannot be equated to or deduced from the stoichiometric coefficientsbut must be obtained from the kinetic data by the methods to be explained.In attempting to fit our concentration-time data to equations of this type, wehave to find rate coeflcients and orders which remain constant over the timescale of any particular experiment and which do not alter throughout a series ofexperiments in which the initial concentrations of the reactants are varied underotherwise constant conditions. Furthermore, if we take the view that the mainpurpose of processing the data is to cast the results into a form capable of interpretation,the values which a or b can take are limited to 0,0.5, 1, 1.5 and 2 withminor exceptions to include such simple fractions as a etc.; it is probably true tosay that very often these special cases can be anticipated from the prior knowledgeavailable to the investigator and so little difficulty is experienced in such situations.If values for the orders a and b other than small integers or simple fractions arefound, it can be concluded that a rate expression more complicated than-d[A1-=dtk[AY[Blbis necessary to represent the data adequately. For example, if it were found that theexpressionrepresented a certain set of concentration-time data, it is likely that the datawould equally well fit the expression-- -dCA1 - k[A] + k'[A]*dt