Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

396 TREATMENT OF EXPERIMENTAL DATAcombinations of the form shown in eqn. (86) with the forms shown in the othertwo equations. The various functions of the reactant and product concentrationsare in most cases analogous to the simple form discussed earlier, e.g.f ([reactants], [products]) = [Ay[B]b[P]p[Q]qThe exponents a, b, p, q are usually integers or half-integers in the range 0 to 2though, unlike our simple formulation, negative values are allowed. Many examplesof these forms are given in Chapter 1, Vol. 2, but, for our purposes, three exampleswill suffice:(1) in aqueous alkaline solution, methyl iodide disappears at a rate given by- dCCH311 = k,[CH,I] + k,[CH,I][OH-];dt(2) in the gas phase, hydrogen iodide decomposes into hydrogen and iodine at arate--= dCH1ldtk,[HII2 - k2[H2][12];(3) in the gas phase, hydrogen and bromine react together to produce hydrogenbromide at a rateThe problem with which we are concerned here is the procedure which is employedto recognize the various forms.Before developing these criteria, we suggest that it is unnecessary to concentrateany attention on expressions of the type generalized by eqn. (86) at the beginningof the investigation. The reason is that eqn. (86) is formally equivalent to eithereqn. (84) or eqn. (85). The procedure is to attempt to represent the data by linearcombinations of terms such asas suggested in eqns. (84) and (85); if it is found that the data require a largenumber of such terms, it is then worthwhile attempting to fit the data to anexpression of the type (86) which may be more economical. In fact, where noguidance as to the likely form of the rate expression can be obtained from the