Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

192 EXPERIMENT A L MET H 0 D S F 0 R HE T E R 0 GENE 0 US RE ACT I0 N Sthe following equationwhere V is the mean velocity of molecules in the gas phase. s can therefore bedetermined from the slope of the density us. time curve, provided desorption (FF)is negligible. Where this is not true, the rate coefficients may still be found bymeasuring dN/dt at different initial values of N. A plot of the net rate of adsorption(NSF--FF) against N for a constant amount adsorbed (see Fig. 4) gives a line ofslope S, and an intercept FF . On heating the filament, the pressure ideally shouldreturn to the starting pressure.(ii) Flow systemIn a flow system, the pressure is controlled by the rate of gas admission to thecell (FA) and the rates at which it is adsorbed by the surface and removed bypumping (NS,). The mass balance equation is thereforeIf FF and S, are known, the rate of adsorption (NSF-FF) can be deduced fromthe pressure us. time curve as for the closed system.At the beginning of the experiment, FA and S, are adjusted to give the desiredstarting concentration No (with the iilament hot so that no adsorption occurs).When the steady state is achievedFA = NOSEThe filament is then allowed to cool and the pressure drops due to adsorptionuntil a new steady state is established as shown in Fig. 5. The net flow of gas intothe system, (No-N)S,, is now balanced by the net loss of gas to the sample(FF-NSF). The number of molecules adsorbed at time t is given by[n(tl>-n(t~>IA = V(No-NJ+QiwheretlQi = SES (No-N)dttoi.e. the amount adsorbed is equal to the difference in the amounts of gas in thesystem at time to and t, plus the net inflow Qi (see Fig. 5). If the filament is flashedto a high temperature between time t, and t2, the amount desorbed is given by[n(ti>-n(tJlAQo = sET(N-No)dt11= V(N2-N1>+Qo