Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

4 EXPERIMENTAL METHODS FOR SLOW REACTIONSthe expression,ASTp, is the standard entropy change for the reaction in pressure units. If a changein the number of moles occurs upon reaction as in reaction (3), and A, is in concentrationunits, AS;b, will have to be converted into concentration units. Thet-BuO. + Me. + Me,CO (3 1relation between K(p, and K(,,, the equilibrium constants for reaction (3) in pressureand concentration units respectively, leads to the result,AS,",, = ASTp,-AnR In R T(F)where AS,",, is the standard entropy change in concentration units. In this case6,AS" may be determined from the expression,ASo = ZS"(Products) -Z:S"(Reactants)(HIHence the rate of the reverse reaction may be calculated from equations (B) and(G). Perhaps more important from the point of view of this chapter, this alsoprovides a check on the determined values for Ef and Af and hence the proposedmechanism. Laidler and Polanyi have reviewed empirical methods for predictingE, via various relationships between it and the heat of reaction'.AH" and AS" may be calculated from values of HF and So listed in variousor calculated from Atom or Group Additivity Rules8314.If the particular reaction studied is the unimolecular decomposition of a freeradical, such as (3), then the use of a "trap" will enable the effective concentrationof the radical to be measured. A radical trap will indicate the presence or absenceof a free radical reaction and may sometimes provide evidence for a partly orentirely molecular reaction. Rate data for free radical reactions are derived assumingthe occurrence of a steady state concentration of radicals. The time requiredto produce a steady state concentration of methyl radicals in the pyrolysisof AcH is shown for various temperatures in Fig. 1. Realistic values for ratecoefficients may be obtained only if the time of product formation is long comparedto the time to achieve the steady state concentrations of the radicals concerned. Thusdeductions from the results from the bromination of isobutanel 5, neopentaneI6,and toluene" have been criticised on the grounds that a steady state concentra-