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 and 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 and 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.
192
COMMUNICATIONS TO THE EDITOR
Vol. 66
The method devised by Duke and Laity' prob- straight line at the beginning, thereafter shows a ably is the most reliable now in use for measure- decreasing slope, and finally reaches an asymptote ments on pure fused salts. A brief review of their when a steady state is reached between electrical technique provides a background against which to transport and hydraulic flow. compare our method. I n essence, Duke and Laity Our medium porosity plug has a leakage rate of employ a cell comprising two electrode compart- 1.8 g. of AgNO3 per hour per centimeter of hydroments connected by two tubes. The first tube static head-100 times greater leakage-rate than contains an ultra-fine porosity plug designed for for plugs used by previous investigators.3 Despite maximum hydraulic resistance and low electrical this, with currents of 25-50 ma. applied to our cell, resistance. The second connecting tube is a hori- the weight-time recordings are almost straight lines zontal capillary containing a gas bubble. This for times of one hour and more, Calculations show path, in the ideal case, is one of infinite electrical that, for these conditions, hydraulic leakage resistance and zero hydraulic resistance. When through the plug causes an error of only 3 parts in current is passed through the cell, the volume 1000 in the initial slope of the weight-time recordchange which accompanies the ion-transport proc- ing for a test of 30-minute duration. We use the ess and the electrode reaction is measured by ob- initial slope of the recording t o calculate the servation of the bubble motion in the second con- transport number of silver, and, in this calculation, necting tube. the error in the slope causes an error only half as Matter transport during conduction in a fused large (percentage-wise) in the transport number. salt creates a hydrostatic pressure difference beAt present, the sensitivity of our measurements tween the two electrode compartments with a re- is about 1 mg., and we expect t o improve on this sulting hydraulic back flow. The Duke and Laity by design alterations. This sensitivity correcell attempts to prevent this back flow by use of a sponds toavolume change of about 2.5 X low4ml. of plug of high hydraulic resistance and by use of the molten silver nitrate-a much better precision than bubble-tube, which is intended to reduce the hydro- can be obtained by the Duke and Laity technique. static head t o a small value and at the same time It is this greatly increased sensitivity which is reserve as an indicator of the volume change. Short- sponsible for many of the advantages of our method. comings of the method have been discussed by Since we require a much smaller amount of material Lorenz and Janz,2 who have shown that the bubble transport to obtain an accurate measurement, the indicator possesses neither infinite electrical re- hydrostatic head which develops is correspondingly sistance nor zero hydraulic resistance. Frictional small. Thus the need of a bubble indicator is forces a t the bubble interface can, in some cases, eliminated, with all of its attendant problems. permit sizable build-up of hydrostatic pressure and Likewise, the ultra-fine porosity plug is not necesconsequent hydraulic leakage through the porous sary. An additional advantage is that from the plug. A final criticism is that the excessive sur- change in slope of the weight-time curve we can disface-to-voIume ratio of the uItra-fine porous plugs, tinguish the flow of matter due to hydraulic backwhich must be used, may yield transport properties flow from that due to electro-migration in experiwhich are not the same as those of the bulk elec- ments of long duration or with plugs or capillaries trolyte. of low hydraulic resistance. The new method which we have developed With our present techniques we have definit,ely substitutes a measurement of the mass transport established a small temperature dependence for during electrolysis for the volume measurement the transport number of silver in silver nitrate, as of Duke and Laity. Our cell consists of two elec- these reproducible results show : trode compartments connected by a tube containing a medium-porosity fritted Pyrex plug (pore dia t 219O, T A =~ 0.781 rt: 0.006 ameter, 10-15 p ) . The distance between the a t 281.6', T A =~ 0.744 f 0.010 centers of the two compartments is equal to the length of the beam of an analytical recording bal- These are to be compared with 2 ' ~ = ~ 0.76 0.05 ance, and the cell is suspended below the pans of a t 225' and 2 7 5 O , obtained by Duke and Laity,4 the balance, The fine suspension wires also serve and TAg= 0.72 rt: 0.06 at 35Q0,reported by Duke t o carry the electrical current t o the silver elec- and Owens.s By improving the design of the aptrodes in each compartment. The cell is enclosed paratus, we expect a further increase in precision. in a furnace capable of maintaining temperatuse to This will permit more detailed investigation of the i1'. To this point, our measurements have been temperature dependence of the transport number, confined to an electrolyte of molten silver nitrate. as well as secondary effects such as the presence of When current is applied to the cell the balance impurities in the melt and the nature of the marecords the motion of its center of mass. At the terial of the porous plug. start of an experiment the hydrostatic pressure KRUMBSCHOOL OF MINES difference is zero and the change in mass results HENRY DEPARTMXNT OF MINERAL ENQINEERTNQ solely from the transport process. As matter COLUMBIA UNIVERSITY HERBERTH. KELLOQQ PAUL DUBY moves from one side of the cell to the other a NEWYORE27, N. Y. hydrostatic head is created, which, in turn, causes RECEIVED SEPTEMBER 5, 1961 hydraulic back flow. The result is a weightchange vs. time recording which is very close t o a (3) G. Harringtou and B. 8. Sundheim, ibid., 62, 1454 (1858). (4) R. W. Laity end F. R. Duke, J. Electrochem. Sac., 106, 97 (1) F. R. Duke and R. W. Laity, J. Am. Chem. Soc., 76, 4046
*
(1954); J . Phw. Chsm., 69$ 649 tl95S). (2) R. M. Loren5 and G. J. Janz, kbid. 61, 1683 (1957).
(1958). (5) F. R. Duke and B. Owens, ibid., 105, 548 (1958).
