DOYLEJ ~ I ~ I T T O S
742
introducing a greater amount of strain energy. Summary.-1. A manometer has been designed for t htl nitvisurcmcnt of p r e s w w of vapors which :ire slon.l,v reacti1.e toward, or diw)lvt\ in, the manonirtric liquid. 2 . The iublimation pressure aiid vapor pressure of SnRr4 has been measured from 7 to 41" and the results found to be appreciably lower than the literature values. The heat5 of vaporization, sublimation and fusion are 12.65 =t 0.14, 15.13 * 0.17 and 2.48 i 0.21 kcal., mole, respectively. 3. The sublimation pressure of TiBr, was m e a w r d from 1 1 to 36" and found to be appreciably loirer than the literature values. The heat of sublimation is 16.1; 0.15 kcal.,'mole. 4. The suhliniation pressures of solid solutions of PnT3r4 in SnTi have tieen measured over :L
*
lTo1. 64
range of temperature. Marked negative deviations from ideality were evident and interpreted as a tendency to form mised halide5 from the coniponents even in the solid stale, for which there had previously been evidence only in the liquid state. This was further substantiated hy the finding of volatile iodide in the solutions C T ('11 though Si& itself is non-volatile at these temperatures. 5 . The solid-liquid equilibria in the system dnBr4-Sn14-CC14 m r e investigated at 13'. Although the hysteni showed n Roozebi)om Type I1 behavior there was some evidencc at the extreme iodide side of the diagram of compound formation hetween SnBr4aiid Sn14. 6. The sublimation pressure^ of solid wlutions of SnBr4 in TiRr4 were measured at 22". The results were erratic, but they defiiritely iildicated positive deviationg from ideality.
SHOCK \TAiT'ES IS CIIEAIICL41, KISETI("S: FT'RTHER STT'DIES IS THE RLiTE OF DISSO(:I,ITION OF BROMIXE BY DOYLE BRITTOX School of Chemist)IJ, r'nizwsitt/ of Minnesota, dlinnenpolzs, JIinnPsoto Received n'osember 18, I969
The rat,e of dissociation of molecular bromine has been determined in the presence of argon, helium, nitrogen, carbon monoxide, oxygen and carbon dioxide in the temperature range 1300-1900"K. in a shock tube. The general features of t,he results agree with previous work on iodine dissociat,ion. JVhen the dissociat~iontook place in the presence of s,, CO or Or the vibrational relaxation of the latter gases occurred a t about the same rate as the dissociation of the brominr. and coniplicated the interpretation. There was no evidence for any specific chemica,l effect, with carbon monoxide. Bromine atoms and molecules both appeared to be about t,en t o twenty times more effective than argon as third bodies.
Introduction It is possible to cause the dissociation of halogeii molecules according to the reaction kn
gas plus studies with helium, nitrogen, oxygen, carhon monoxidt: and carbon dioxide. The rate constants indicated above nre more esplicitly defined by
x>+ > r e x + x + \I kp,
by passing a shock wave through a mixture of the halogen and a suitable inert gas. By varying the qtrength of the shock v-ave it is possible to vary the temperature and the rate of the reaction. The rate of dissociation of molecular iodine in the presence of qeveral inert gases has been measured by Davidson and co-n-orkerc.' ? The rate of dissociation of bromine has also been studied before by this means,3,4 but only in the presence of argon. Since flash photolysis experimentssaab a t , or just above, room temperature haT,e led to the determination of this dissociation rate in the preqence of argon, nitrogen, oxygen arid carbon dioxide, it was of interest to extend the shock tube studies to include all of these gases. This has been done, and the work reported here incbludes a further qtudy on argon as an inert (1) D. Aritton, S . Tlavidson and Cr. Schott, Disc.Farada:! Soc., 17, 58 (1954). (2) D. Hritton, ?i. I)avidson, W.Gehxnan a n d G . Schott, J . Chem. Phys., 26, 804 (1956). (3) D. Aritton a n d N. Davidson, ibid., 26, 810 (1956). (4) €3. B. Palmer a n d D. F. Hornin, ibid.,26, 9 8 (1957). trong, J. C. W. Chicn, P. I?. Graf a n d J. E. T4'illartl, {bid., 2 6 , 1287 (1957): (b) IT. G. G i v r n s . .Jr.. a n 0 .J. E. JTillnrd, .I. .Am. Ciiem. S o c . . 81. 4773 (1959).
Strictly speaking, this equation should include separate terms for each gas present sinc*e X-D and k~ depend upon 11. In particular, the possibility that X2 and X serve in this role ghould be considered. Both Br2and Br are somewhat more efficient as third bodies than the other gases used. Palmer and Hornig4 concluded that Br2 was about twice as efficient as argon, while the results obtained in this present work suggest that it might be ten times more efficient. I n view of the uncertainty, the rate constant? here are reported for the mixture and no correction has been made for the Br?. This point is discussed further in the section 011 result!, under the heading Bromine. Experimentally k D is determined hut it is often more convenient to discuss k ~ .These two constants are related bv the equilibrium collrtallt, for the dissocxiatioll 01 Br2 which has been calculated oyer the temperature range of interest from spectroscopic tla.ta.6 In all of the rate expressions moles/liter was the voncentration unit and seconds the time unit, Tht: inert gas (6) W. 13. E v a n s , T. I+. Miinson and n. D. l\'ngrli:in. .I. l?carnrcia ;\'nil.
Bur. Standards. 55, 147 (1llA.i).
present geiierally will be referred t’o as the third body, again thinking of the recombination reaction.
Experimental The Shock Tube. The esperimental apparat,us was similar t o thoso n-hirh Ir:tvr been described previously.’~4 The high pressiircs or driving section of the shork tube consisted of a 240-rm. length of 10-cm. i.d. aluminum pipe. The low pressure section consisted of a 240-cm. length of 10-cm. i.d. aluminum pipe followed by a 240-cm. length of IO-em. i.d. Pyrex pipe. The two sections were separated by cellulose acetate membranes from 3 to 15 mils in thickness. The driving sectinn W:LF connected to a manifold which allowed for evacuatir.g t,hc t ube, filling it with hydrogen (which was the custom:z~,ydriving g ~ s or ) nit,rogen, and measuring the pressure. The low p’wsure section was connected t o a vacuum line in which reaction mixtures could be made up and stored. The pumping out of the low pressure system could be done through the L”CIIIIII~ line n-it11 a mercury diffusion pump, or tlircrtlj- with a mechanical pump. With the diffusion pump tho mtire loir pressiire section coiild be evacuated t o a pressiire of 0.1 f i . Thi, pressure in the low pressure section :tt eriment was of the order of atm. run it mas found very convenient to get) ],id of the spent reaction mixt,urc and driving gas by flushing the tube out, through a stopcock a t the don-n-st,ream end of thr: low presiiurc section with nitrogen admitted at the upst,re:tm end of the high pressure section. This was much more rapid t h m evacuat,ion and just as satisfactory othernisi,. After the flushing, t,he tube was opened and the debris from the broken membrane cleaned out. Detection and Recording.-The detection system consisted of foul. stations about 400 em. downstream from the membrane. The first and third, 40 em. apart, were used t o measure the velority of t,he shock m-ave and t o trigger the oscilloscope -;races. The second and fourth stations, each 10 em. from the proceding trigger stat’ion, were the actual observation c,tations. The light sources for all of the stations were General Electric i1763 6.1-v. inst,rument lamps. For the trigger stations the lamps were run a t 7.5 v. from an a x . transformer. For the obsemxtion st,ations the lamps were run at, 8.0 v . from storage batteries. The optical arrangement a t each station is shmvn i n Fig. I .7 The use of mirrors gave a n effwtive opticil path length of 30 em. For the trigger stations Corning =5538 glass filt,ers xere used t’o wlert t h a t m,ve length region \There bromine absorbed strongly. A t the obserrat .on stations Bausch and Lomb 500 mp secondorder interfeimrc. filters were used in conjunction n i t h Corning $3385 glass cut-off filters t o give light that, m as approximately monochromatic. For detectors 1P-28 photomultipliers were ii:jcd. The described arrangement gave a photociirrent of 100 pamp. when the phototubes were operated a t 60-70 volt,s 1:cr stage. T h e signnls from the trigger stations went’ to an amplifier hyratron. Thp t,hyratron signal triggered the veep antl t,he timer. The timer, a Model 1 EPCT and Timer made by the Berkeley Division of Ikclim:i,n Tnstrunients, measured t,o the nearest microsecond the time it, took the shock wave to pass b e t w e n T h e signals from the observation on a Tektronix 535 oscilloscope ,‘54Cdual trace plug-in unit ,E and v (Ire rerordrd photogr:tphicallg with a Dumont type 2620 rroid 1,antI i*:irnc.riL. The writing rate of the oscilloscope chosen s ) that t h r observations a t the first, observation ion (rccoitled on thc sweep of t,he A t,race of the dual trace unit) rwre ct’ncludrd bcfore observations were begun a t the aec’ond otiscr.,.:rtion station (recorded on the B trac-e). The ii,:il rocircliniite on the oscillosciope represented the intensity of t h r light Figrial and \vas calibrated with flat traces riiii at i l v i i iitcy, I-tion-n pliotocurrents. The horizontal ~
Fig. 1.-The
experimental arrangement at each olmrvation Ptation.
coordinat,e on t,hc oscilloscopo represented the t,ime. It was foiind that, the snclep spcrd as intlicatrtl by thc. selector ,witch was iiniform and accurat,e to bt,tter th:tn 1 yo so that, no calibration markers were necessary. The emission vas followed by setting the piiot,onii~ltiplier nt, I80 pamp. photocurrent, (this was the v a l w ivhii:h an original setting of 200 pamp. would drift, down t,oj and thcn shutting off the light. The sensit,ivity of tho oscilloscope was generally increased hy a factor of 2 or 4 sinw the (mission wvas rat,her small rchtive to the absorption. Chemicals.-Thc Br?~ ~ Mallinckrodt 1 s ainalytic:al reagent. .2 bulb-to-bulb distillat,ion n‘as performed in the mciiuni line and the middle fraction taken. l r g o n from nfntheson Co., Inc., stated t,o be !N.OC, pnre was used without, furt,her purification. Helium from t,he Puritan Co. of unstated purity mas passed through a tetratrap9 a t liqiiid nitrogen temperature before use. Airco dry nitrogen of unstated purity n-as treated in the same fashion as the helium. Commercial grade carbon monoxide from Mat,lieson stmatedto lie 95.0c’, pure \vas purified by passing it through two tetratraps a t D r y Ice temperature to remove any impurities condensahle a t t,his temperat>ure (in particular H20 antl Fe(C0)5),freezing i t in a trap cooled by liquid nitrogen under reduced pressure, allowing it, t,o melt,, distilling off and disrarding part of the liquid ( t o remove any H2), and then using the middle f r a ~ t i o n . ’ ~The last, unused portion of the liquid rontained a white solid which probably x i s CO?. Airro Ly.6.P. oxygen \vas collected in liquid nit’rogeri and distilled from the liquid. Carbon dioxide of unst,atcd purity from the Liquid Carbonic Co. was condensed in liquid nhrogen, any non-condrnsable gases present were pumped off, and t,he carbon dioxide m%st,hen sublimed for use. The retiction mixtures ww: prepared by placing a known pressurc of Bro in previously evacuated s t o r q e bulbs and thcri adding inert gas until the total pressure mts the desired value. The Bra pressure was measured on a h t , y l phthalate manometer arid the total pressure on a Hg manometer. The mixtures w r o :tllo~vedto mix in t8hebid1 hours before use. Preliminary Calculations.-From a k n o d c d g e of the enthalpy fiinctions for the various gases as a function of temperatmurellit is posFible to calculate t,he presslire rat’io, density ratio arid velocity of the shock ivavi’ as :i fiinrtion of the. final temperature, d measurement of the velocity thvn Serves to determine all of the remaining properties, i n particular the density rat,io and temperat’ure. Thr, calru1:itions were made assuming that room tenipera:/ire WV&P 300” K . The actual value vas usually about 301 and the extremes m r e 5’ on eit,her side of this. A 5” error in the assumed room t,emperature n-odd lead to :I 5’ c r i m in the calrulated high temperaturcl. This was not correcsted for. These calculations can be made on the assumption t h a t the gas is vihr:ttion:tlly frozen at its original tenipei,ature or that, it is vibr:ttionally a t equilibrium a t the high temperature behind the shork ~x-ave. For example, in :t FIIOCICw v e in :r 2% Br:,-SGq Oy mixture in which the observetl shorl; velocity mas 160 r m . :millisec. the temperature at tl ivonld bo r:tlculatecl to be 1750°K. if it n-err (9) W. C:. Fatel,, H. A . Rent and B. Crawford, .Tr.. T . Chem. Phiis ,
( 7 ) In a f r i r of This r r ~ q i i i r r d1 I
thii
i,:trly exl>eritnents t h e mirrors n e r c not used. ighi:r pwssiires. T h e mirror arrangement aion on w h i c h t h e P y r ~ spipe failed t o con-
tain t h e shock vax-e. ( 8 ) I n m a n y of the rxpcriment- il t y p e 53/54D high-gain diffi3rential pliig-in iinit vn. w e d h i i t tlir risc t i m e of thin unit \\-as seyeral microwconds. an11 it did not p r r m i t the l i s p of two o h s r r r a t i o n Bt,ntions. Ttic iliial trncr init n n i u late i m i ~ r ~ i v c m e n t .
3 1 , 204 (1959).
(IO) I n t h e first experiments with t h e CO i t was not purified. This resulted in a precipitate forming slowly when i t m a s niixed with bromine. Shocks in this mixture seemed t o indicate a w r y high efficiency for CO as a third body. This probably n-as due t o the prt’sence of hydrogen a n d t h e rapid reaction between hydrogen und h r o i n i n e . (11) “Selected Valiies of Chemical Thermodynamic I’riii)ertie.-~’’ Serips 111, Katl. Bnr. Rtds.. WashinRton, 1954.
J - I - I - -Loo
'
500
1
_ I
1000
,
'
~L 1
1500
T,"Ieven dissociation evperiments at the. highest temperatureq, 26-20 t o 191Oo1i., nere the. only one1 111 n hich it as felt that the initial clop+ could bc determined with any certainty. Table I h t s these values and also thoqe obtained usiiie a relaxation time four times that for the pure ga.. The coriected values are thought to be the b e t , but the abnormally high temperature dependcncae indicates that they are probably not too reliable. 7'hc equa(IS) RZ Windqor, U D a ~ i d s o n a n dR Tziloi (Intrrnntional) on Conibiistion," London lQ59 1) RO
+
t ~ o nfor tht2se points IS log k~ = 6.222 33'74 (f 40 21, T . \I c'le rLu1 111 2% Oxygen.---Seven expcrimci Br2-9801, 0 2 arid twelve in J3r2-95y& O2 mixturei. In the four experiment\ at the lowest temperatures. 1120 to 1280°1\:, whcre negligible dissociation occurred, vibrational relaxation could he qeen t a h g place at tinieq just barely long enough to be detertahle. Thii agrees nith the results'6 in pure 0 2 The cqerimental extirict~oncoefficients iiidicated that iii all of the cho the relaxal ion 15 a i completein a time short compared to the re:wtioii time. In four experiments near 1800"Ii.. 1 he eatinctioii coefhcientc and the shapes of the cuIves ieemed to indicate that these nere not conipl-tely relaxed at the shock front and that rrlaxation took place in a time comparable to the experimental time Thir conclusion I \ coiiiisteiit nith the experimental values of the relaxation time for pure O2 ah determined by The expermental points in the 2 and 5% mixtures comlined (Jvilh the omiwion of the four points near 1800") g a i e log 1~ = 7.844 1409 (i 504)lT, and an average value of log k~ of 8 835 + 0 023 at 142'3°K. The /iR values for the four points near 1800' agwed with this fairly n-ell I f they vere corrected for \ ibratioiial relaxation. Carbon Dioxide.-Eleven experimentq were run in 2 5 Bi- 08c; C 0 2mixtures. Three of these were at temper:itiue- too low for any appreciable amount of diswci: tion t o take place The eight other- at temperatures from 1380 to 1622OK gave log Xn = '7.885 12ici ii tj54)/T, and an average value of log kp, of 3 686 0.030 a t 1474°K h s \lac the case in th(1 experiments with IZin CO?,?the extinction coeffil~entsindicated clearly that the CO? was vibrationally relaxed. I n the experiments with I? it mas iioiiced that the density of the gas behind the shock front naq not very smooth and it mas cuggested that t h l c might be due to the nearness of the cold front to the shock front in the COZs h o c k , thii iiearneis 1 lemg a 1iece.sary concequence of the high heat capalzity of CO) This w n e unei-enneqs n-as present I I I :ill of the experiment. reported here where the cold froiit naq tn o to three times further away. In the I?shocks the observation. were made 10-12 t u b diameters from the membrane i n the prc"ent e\perimentc the observations 11ere made a h u t 40 tuhe diameters from the membrane. Since the I 11 o v t a 11 eie equally uneven thii une\ enneqs cannit be attributed to the neariieSs of the rold froiit nor t o the recentness of the bursting process. although it itill may be a consequence of the high heat caparity. Bromine-The que5tion of the efficieiicy of bromine atoriy and moleculei aq third bodies is one which the data reported here do not answer wellq *ince none of the experiment:, was performed with a large amount of Br2 relative to the inert gas. Ron ever. n ith both argon and oxygen t n o qets of experiment:, nere made, one set a t 2TGBr2,and the other a t ;jcG. i n each case the constants determined in the 5C& mixtures TIere, on the average, higher than those determined in the 2% mixtures, although the cpreatl in the points was such that the t n o qetq 01 ~r1:ipped T-qing the valueq from the
+
+
+
smoothed curves through the argon data (kR = 2.90 X lo8 liter2 mole-2 see.-' for 2 r 0 and 3.74 x IO8 for 370) one finds k~ = '3.:3 ( 5 0.2) X IOg for argon and 3 0 ( 1 3 ) X IO8 for Ur?. The ratio / Y ~ ~ = I ~13(* B ~ 3~) . The errorh arv c.itiniated from the least squares probable error.: and are certainly optimistic. The results of Gi\-eiis and Willardjb gave a ratio of 130 a t 30OoI
