Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

3 CONSISTENCY WITH COMPLICATED RATE EXPRESSIONS 395point is that concentration-time data which cannot be represented by a simpleexpression may be equally well described by quite different types of equation.For example, the two expressionsand- dB1 = kl[A][B] - kz[A]’dtare equivalent when kl [B] >> k, [A] as can be, seen by expanding(I+ B ) - land neglecting terms involving [AI2/[Bl2 and above. Consequently data which fitthe one may be equally well represented by the other over a limited range of concentrationsand so it may not be possible to make a choice between the two on thebasis of concentration-time data alone. Since these two expressions for the rateof the reaction correspond to quite different mechanisms, it follows that it is notpossible in every case to make a choice between alternative mechanisms on thebasis of concentration-time data alone; additional criteria (and possibly furtherexperiments) are necessary in order to select one mechanism rather than another.These difficulties are, of course, well known to experienced kineticists; however,provided that the dangers of utilizing preconceived views of the reaction mechanismare realized, it is perfectly legitimate to use these as a guide to the type of rate expressionto be investigated.At this stage, it is useful to repeat the examples of complicated rate expressionsgiven earlier in generalised form [eqns. (2), (3) and (4)],oror--- drA1 - C k,f,([reactants], [products])dt--- d[A1 - C k,f,([reactants], [products])dt-C kif/([reactants], [products])---dtd[A] - kf ([reactants], [poducts])1 k’f’([reactants], [products])In principle we can have even more complicated expressions by taking linearReferences p. 407