Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

22 EXPERIMENTAL METHOD§ FOR SLOW REACTIONSmagnets or by using an induction motor, or by pure diffusion. For a cylindricalvessel, diffusion times may be calculated from the expression"where t is the time in seconds, I is the length of the mixing vessel in cm and Dl,2is the diffusion coefficient for species (1) and (2) in cm2.sec-'. D1,2 may be calculatedfrom Chapman's equation''where i?1,2 is the relative mean velocity of the two species, n,is the total number ofmolecules per cc and cr1,2 is the mean collision diameter of the two species. Theimportance of allowing sufficient time for mixing is emphasised by the tabulatedvalues in Table 2. t is calculated on the basis that I is 25 cm. Since Dl, varies inverselyas the total pressure, t will be increased by a factor of 10 at 500 torr. Thisgives the range oft for normal experimental pressures.TABLE 2MIXING TIMES FOR VARIOUS GASES TOGETHER WITH THERELEVANT PARAMETERS IN EQUATION (X), AT A TOTALPRESSURE OF 54 torrGases u1.a 10-4C1,s D1,* at 70" t(A) (cm. sec-') (cma. sec-I) (set)CO,+HBr 3.15 5.04 3.14 149C02 + Br, 4.2 a*b 4.58 1.61 29 1dtBP+HBr 7.0 3.61 0.44 1067dtBP + HCI 7.0 4.97 0.63 745dtBPfSF, 8.9 b* C 3.14 0.31 1510el, is calculated from the expression Cl, = (8kT/np)+, where k is the Boltzmann constant, Tis the absolute temperature and p is the reduced mass of the two species.dtBP = di-tert.-butyl peroxide.a Handbook of Chemistry and Physics, Chemical Rubber Publishing Co., 45th edn., 1964-1965,F88.* Estimate of u1, based on measurement of the appropriate Catalin model.G. H. CADY, Aduances in Znorg. and Radiochem.. 2 (1960) 105.(e) DilatometryThe analogue of measurement of pressure change at constant volume for a gasphase reaction is the measurement of volume change at constant pressure for areaction in the liquid phase by dilatometrysl". This is used extensively in polymerisationstudies.