COMMUNICATIONS TO THE EDrroit
Jan., 1962
191
the surface combine to give the dimple a significant over-all curvature.1° 4974 PALO DRIVE, TARZANA,
CALIFORNIA CHEMISTRY DEPARTMENT ~JNIVERSITY LOS
P. FRANKEL KAROL J. MYSEIS
STANLEY
O F SOUTHERN C A I J F O R N I A
ANGELES 7, C A I J F O R N I . 4 RECEIVED NOVEMBER 8, 1961
(10) D. C. Chappelear, J. Colloid Sci., 16, 186 (1961); 11. I’riilcon, Ixivate cotntriunication.
N
N
N
rJ
.ICTlV,ITIOS ENERGIES OF GAS-PIIASE OXIDATIOSS
Si?+: The over-all activation encrgics of oxidation reactions in the gas-phase sometimes can be used to calculate the activation energy of one of the elementary reactions occurring in the system. Ilowever, not only, of course, must the mechanism postulated be substantially correct, but the over-all activation energy must also be a true measure of the variation of the reaction rate with temperature. For the oxidation of propionaldehyde between 128 and 2%’ with a fixed aldehyde pressure of 50 mm. and oxygen pressures of 10, 15, 25 and 100 mm. the log maximum rate versus 1/T ‘IC. plots, using rates from pressure-time data, showed deviations from linearity a t the higher tcmperaturc (see the figure for typical results). The activation encrgics, calculated from the linear portions of the graphs, ranged from 9 kcal./mole a t “low” oxygen pressures to 16.5 kcal.jmole at “high” oxygen pressures. Under all conditions the rate became markedly less dependent on temperature between 180 and 226’. On the other hand the log maximum rate, measured from analytical data on the loss of oxygen in the system, versus reciprocal temperature plots were linear over the temperature range cmployed. The over-all activation energies were found to be larger than those derived from thc pressure-time curves, arid varied from approximately 12 kcal.jmole with 10 mm. to 21 kcdjmole with 100 mm. oxygen pressure. Similar results have been obtained for the oxidation of isobutene in the presence of hydrogen bromide. Over the temperature range 100-190’ thc log maximum rate (pressure-time data) vel sus 1/2’ O K . plot was distinctly curvcd (see figure), the over-all activation energy varying from 4.5 kca1.l mole a t low temperatures to 11.5 kcal./molc at high tcmperaturcs. Howevor, the Arrhenius plot for the maximum rates, measured from data on the loss of oxygen, again was linear, the activation energy being 15.8 kcal./mole. Tt, is intcwsting to note that for both systemq the over-all activation energy was higher when dctcrmined from data on oxygen loss than from pressure changes in the vessel. This is because the proportionality factor given by (dAP/dt)max/(dPoz/ dt),,,, while independent of reactant pressures at any one temperature, decreased with increase of temperature. The results stress the care ncccs(I) 8 0 , A CornlJe, 1 1 Siclause a n d du Petrole, 10, 786, 929 (1960).
XI. Letort, Ileuue Inst. Franc.
2.2
2.4
2.6
1000/T‘°K. Fig. 1.-Variation of the maximum rate of oxidation of propionaldehyde and of isobutene in the presence of hydrogen bromide with temperature. Open symbols refer to results from pressure-time data; closed symbols refer to r(1sults from analytical data on loss of ouygcn: A, iuobuteri~~ and oxygen pressurc, 140 mm., HBr pressure, 20 mm . 0, propionaldehydc preqsure, 50 mm., oxvgen pressurc, 100 rnm. ; 0,propionaldehyde pressure, 60 mm., oxygen pressurc, 15 mm.
sary in usiiig pressure-time data as criteria of oxid:ition rates. The decrease in the activation energy of propioiialdehyde oxidation as thc oxygen pressure is rcduced can be explained in terms of the incrcawd importance of the decomposition of the propionyl radical under oxygen deficient conditions. Atteution has been drawn prcviously to the probability that carbonyl radical decomposition is the major source of carbon monoxide in many low temperut i r e oxidations.2 DEPARTMENT OF INORGAKIC AND PHYSICAL CHEWISTRY wm UNIVERSITY LIVERPOOL GREATBRITAIN
P. FIURS’t‘ G. SKIRROW C:. F. €1. TIPPER B. P. WIIIM
I t e C E I V E D I)E(’EMREIi
12, 1961
(2) G. Skirrow a n d C. 1’. 11. Tipper, Severitli Sytn~iosiuution COIII-
bustion (l959), Buttcrworths. London,
1).
134.
AN IMPROVED METHOD OF TRANSPORT SUMBER MELISUREMEMEKT IN PURE
FUSED SALTS Sir: We have developed tt new method for measuring transport numbers in pure fused salts and havc found it to have important advantages over othcr methods now in use. We describe it here briefly in anticipation of a fuller account a t a later date.
