Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

2 CONSISTENCY WITH EQNS. OF TYPE -d[A]/dt = k[A]"[B]' 369fact that the reaction rate does not reach instantaneously the value correspondingto the rate coefficient characteristic of the run as a whole. The simplest exampleof such an error is that which arises when the two reactants are mixed together at atemperature different from that at which the reaction is to be studied. Obviouslythere is a time lag between mixing and attainment of thermostat temperature sothat the observed value of the time taken for a to drop from unity to a particularvalue a' is not the same as the time which would have been required had the reactionmixture reached thermostat temperature instantaneously. Provided thatdeterminations of a are not commenced until thermal equilibrium has beenattained, such an effect can be represented as a systematic error in the reaction timeas shown in the schematic diagram of Fig. 2. This shows the type of plot foundwhen the reactants are mixed together at a lower temperature than that at whichthe reaction is studied kinetically. In the period of time indicated by Ato, thetemperature of the reaction mixture increases and the rate of disappearance of Acannot be characterized by a single value of the rate coefficient. Once the reactionhas reached the thermostat temperature, the plot of f(a) against t-to becomeslinear but its intercept at ?--to = 0 is clearly not f(ao); its slope, however, is equalto mk. Therefore, to find the rate coefficient, we use the equationf(a) = 2-mk(t-to)rather than eqn. (56)f(a) = f(ao)-mk(t-to)References p. 407~~0 t- toFig. 2. Schematic diagram of f(a) against t-t@.