896
E. D. KACFMAR' AND J. F. REED
T'ol. 67
THE VAPOR PHASE DIFFUSIOS FLAME REACTIOS OF SODIUM WITH FLUORIKATED CHLOROMETHASES B Y E. D. KAUF.;LrL4S~ S D J. F.REED Depai tinent of Chemtstiy, Loyola T7naverszty, Chzcago, Illznoas Receized October 24, 196g The rates of the gas phase reactions of CCl,, CFC13, CF*Cl?,and CF3C1with sodium vapor have been measured by the diffusion flame method Products were collected and analyzed. The specific rate constants for the primary processes were found to be 14.9 X 10" cc./mole-sec., 6.2 to 9.1 x 1011 cc./inole-sec., 3.2 to 4.6 X 10" cc./mole-sec , and 0 78 X 10" cc./mole-see., respectively, at 583°K. The energies of activation were calculated using a unit steric factor; the values obtained mere 8.4, 8.7 to 9.2, 9.0 to 9.5, and 10.2 kcal./mole, respectively. Based on the known value of D(CCld-Cl), values were deduced for D(CFzC1-Cl) as 72 f 4 kcal./ mole, D(CFe-Cl) as 62 +C 14 kcal./mole, and D(CFC12-CI) as 71 =t3 kcal./mole. The heats of formation of CFZC1 and CFCL were deduced as -69 =!z 4 kcal./mole and -24 f 6 kcal./mole, respectively. Chemiluminescence is observed in these reactions and based on limited measurements some discussion is presented.
Introduction The "diffusion flame" method, first reported by v. Hartel and Polanyi,2 has been applied to t'he iiivestigation of many fast reaction^.^ In particular, the met'hod has been used extensively in the study of t'he reactions of sodium vapor rvith organic halides. A number of workers have reported on the reactions of sodium mit'h p~lyhalides.~-'Chemiluminescence was reported by Heller and Polanyij in the study of metal polyhalides, Haresnape, Stevels! and Warhurst6 in the st'udy of polybromides, and Bawii and c o - ~ o r k e r sin~ the study of polychlorides. The latter workers furthermore reported t'hat halogen atoms are successively abstracted from the parent halide by reaction with sodium. The present' work has been undertaken in an effort,to gain furt'her understanding of the reactions of fluorinated polyhalides with sodium and of the diffusion flame. The kinetics of t8he gas phase reactions of chlorotrifluoromethaiie, dichlorodifluoromethane, trichlorofluoromethane, and carbon tetrachloride with sodium vapor have been studied. These compounds were selected because t'heir reactions with sodium would yield fluorinated and chlorinated radicals whose characteristics are not yet fully understood. Some of the compounds studied in this work have been studied earlier in much less detail. The reaction of carbon tetrachloride has been studied repeatedly. I n their original work, v. Hartel and Polanyi2 reported that t'he reaction proceeds essentially without activation energy. Heller and Polanyij also investigated the behavior of this system, and found a somewhat lower specific rate coiistant and also expressly stated that this reaction is not accompanied by chemiluminescence. Haresnape, Stevels, and Warhursts reported a specific rate constant virtually identical with that reported by Heller and Polanyi. Bawn and eo-workers' reported complete abstraction of chlorine from carbon t'etrachloride accompanied by intense luminescence. The reaction of chlorotrifluoromethane with sodium has (1) Abstracted from the P1i.D. Thesis submitted to the faculty of Loyola Cniversity by E. D. Kaufman, 1962. ( 2 ) H. v. Hartel and VI. Polanyi, Z. physilc. Chem., B11, 97 (1930). (3) E. W. R. Steacie, "Atomic and Free Radical Reactions," Reinhold Publ. Gorp., Kew York, 11. Y., 1954. ( 4 ) H. v. Hartel, K. JIeer, and M. Polanyi, Z.p i ~ y s i k .Chem., B19, 139 (1 932). ( 5 ) W. Heiler and 31,Polanyi, Tmns. Paraday S O C . , 32, 633 (I9Sfi). (a) J. S , Hareanape, J . M. Stevels. and E. Warhurst, i b i d . , 36, iG5 ( I 940). (7) C. E. H. Bawn and E. F. Hunter, ib.id., 34, 608 (1938); C. E. H. Bawn and W. L. Dunninx, ibid., 36, 185 (1939).
been studied by Hodgiiis and Haines* and by Reed and R a b i n o ~ i t c h . ~These workers reported the same products for the reaction, but they gave different mechanistic explanations for their occurrence. Neither report' chemiluminescence. The theory and range of applicability of the "diffusion flame" met'hod have been examined by several authors.*-12 The mork of Heller12 consisted of an extensive experimental study in which the original assumptioiis of v. Hartel and Polanyi were subjected to careful scrutiny, and the range of their applicability was laid down. One of the original assumptioiis sets the part'ial pressure of the halide reaction partner at an average value. Cvetanovic and LeRoylO and Smithll obtained solutioiis for the differential equations describing a diffusion model when the halide pressure may not be approximated by a constant value throughout the flame. Reed and R a b i n o ~ i t c hin , ~ a different extension, corrected for the integrated int'ensity of the sodium fluoresceiice at the boundary of the flame zone used as a measure of the reaction rate. These authors also took specific cognizance of secondary radical reactions of halogenated radicals in the flame.
Experimental Materials.-Nitrogen, used as a carrier gas, was purified by passage through two sodium-potassium acetophenone t'raps, a cold t'rap, and a silica gel trap.I3 Sodium was purified by distillation in oacuo. Cut,ting of small pieces and filling of the carburetor were carried out under dry nitrogen. Chlorofluoromethane, dichlorofluoromethane, trichlorofluoromethane, obtained from the Matheson Company, Inc. as spectroscopically pure reagent's, and carbon tetrachloride, obtainedfrom the Eastman Chemical Company as a spectroscopically pure reagent, all were further purified by three further bulb to bulb distillations discarding the initial and final fractions. Analytical reagents were obtained commercially and used without further purification. Silver nitrate, sodium fluoride, and sodium chromate were Baker and Adams Reagent Grade chemicals. Titanium trichloride solution was obtained in nominal 20% strength. Hydrogen peroxide, 30%, was obtained as a Certified Reagent Grade chemical from the Fisher Scientific Co. Sodium chloride was obtained with a National Bureau of Standards certification of 98.12% purit'y . Apparatus.-The apparat,us was modeled after the flow system of Reed and R a b i n o ~ i t c h . ~The horizontal reactor had two (8) J . W. Hodgins and R. L. Haines, Can. J . Chem., 30, 473 (1952). (9) J. F. Reed and B. S. Rabinovitch, J. P h y s . Cl,em., 69, 261 (1955); 61, 598 (1967). 110) R. .J. Cvetanovic and D. J. LeRoy. Can. J . Chmn., 29, 397 (1951); R. J. Cretanovic, ibid., 34, r14 (1956). (11) P.Smith, J . Chem. P h y s . , 22, 1GO5 (1954). (12) W. Heller, Trans. Faraday Soc., 33, 1550 11937). (13) L. Fiener, "Experiments in Organic Chemistry," :3rd Ed., L). Heath Co., New York, h-.Y., 1957, p. 299.
c.
April, 1963
T'APOR P H A S E
REACTIOK O F sODIU;1~WITH FLUORINATED CHLORQMETHSNES
plane windows. One of' the windows was placed t o permit headon viewing of the nozizle. To facilitate observation, a prism was mounted in front of this window so that the nozzle could be seen from the side of the reactor. Temperatures were measured with calibrated thermocouples. Runs were made a t a carburetor temperature of 228" (501°K.) and a reaction zone temperature of 246" (519°K.) and also a t a carburetor temperature of 284" (557°K.) and 290" (563'K.) and a reaction zone temperature of 313" (583°K.). The temperature profile in the furnace was adjusted t o maintain the temperature along the carburetor within h0.5" and within + l oin the reaction zone. Procedure.-The system pressure was adjusted t o the desired value after the reactor had attained thermal equilibrium. Circulation of the carrier gas was then started and the halide flow mas initiated and adjusted to give a flame radius between 1.0 and 1.8 cm. as soon as the reaction chamber --as filled with sodium. After a few minutes the flame reached its steady-state size. Runs were made over a range of v / D values of 0.9 t o 16 cm.-'. The flame size was measured on a reticule mounted in front of the viewing prism. The reticule was marked a t one millimeter intervals on cross-hairs. The limiting visible pressure of sodium was established by slowly heating the reactor and observing the lowest temperature a t which sodium could be detected in the nozzle tube. The limiting visible flux of &odium was determined in a closed cell which was substituted for the nozzle tube in the reactor. This cell was equipped with two plane windows one centimeter apart which provided a convenient depth of field for the observations. A small sodium reservoir was attached to the bottom of the cell. The furnace was slowly heated and the temperature a t which sodium could first be seen in the cell was recorded. A number of determinations ere made over a total system nitrogen pressure range of 0.5 to 10.0 mm. A number of observations were made at each pressure by allowing the furnace to heat and cool alternately. The sodium vapor pressures at the temperatures recorded were obtained from the data of Sittig.14 To determine the nature and relative amounts of products, runs of approximately 40 hours duration a t 583°K. were made. Gaseous products were condensed and collected in liquid nitrogen cooled traps. Solid products were collected on a Pyrex glass shovel placed under the reaction zone. The solid products collected on the nozzle tube itself were carefully removed and preserved separately. Product Analysis.-The gaseous products were transferred to Pyrex glass bulbs for tilorage prior to analysis. The analysis of gaseous products was carried out on a Consolidated Model 21103c mass spectrograph. The presence of the species listed in Table I was deduced. The ratio of concentrations of CzF4to CzFe for the CF3C1 reaction was found to be 2.8 from peak heights.
TABLE I ANALYSIS OF GASEOUS PRODUCTS Halide
Product species
CFSCI CFZClz CFCl, cc1,
CsF4,CzF6,trace C4F6 C2F4,C2FdC1, trace of C3 compounds CzFzClz, CzFzC14 None
Solution of the solid products of all reactions left a flaky black deposit. This deposit did not undergo any changes on heatng to 300" in a Fisher-Johns melting point apparatus. From these results it was deduced ihat the black deposit was carbon. The presence of C&le in the solid products of the CCl, reaction was eliminated since the melting point of this substance is 187O.15 Qualitative tests on the solutions obtained when the solid products were dissolved indicated the presence of both chloride and fluoride ions except in the case of the C C 4 reaction. The total amount oi sample collec%edon the nozzle was of the order of 0.5 g., while the total amount of sample collected on the plate was usually much less, being of the order of 20-30 mg. The products collected on the shovel were washed into a suitable container, evaporated t o dryness, and diluted to 10 cc. in a volumetric flask. The product collected from the nozzle tube was dried in an oven a t 80" for three hours and weighed. (1.1) 31. Sittig. "9oriium, tts Manufacture, Properties and Usca." Reinliold Publ. Corp., K e w York, S. Y . , 1956, p. 476. (15) R. 1,. Shriner and R.. C. Fuson, "The Systematic Identification of Oraanic Compounds," John Wiles and Sons, New Yorli, N. Y., 1948, p. 288.
897
Approximately 10 mg. of this product was reserved for carbon analysis, the remainder was reserved for chlorine and fluorine analysis. Carbon, in the nozzle product, was determined by the XicroTech Laboratories of Skokie, Illinois, by measuring the amount of COz evolved in a combustion train a t 800". Comparison of the weight loss on combustion and the amount of COZtrapped calculated as carbon showed that the combustible product was carbon. Chlorine was determined by the Mohr method, calibrated with standard sodium chloride solutions. A one-cc. sample of filtered salt solution was titrated with standard AgXO, solution ill NazCrO, as an indicator. A yellow using 0.1 cc. of 4 X light was used t o facilitate end-point detection. Fluorine was determined by measurement of the bleaching of the Ti0,- ion in solution by F-.'e For the analysis, 5 cc. of the stock reagent, 0.2 cc. of fluoride solution and 0.5 cc. of H202 were mixed and the optical density of the resulting solution was determined with a Beckman DC spectrophotometer at 440 mp. The method was calibrated using standard fluoride solutions. The results of the quantitative analysis of the solid product9 are presented in Table I1 in the form of product ratios.
TABLE I1 RESULTSOF AKALYSIS OF SOLIDPRODUCTS Halide
Source of sample
F/C
CFzCl
Nozzle Plate Kozzle Plate Xozzle Plat'e Nozzle
1.452
CF2Clz CFCl3 CCl,
c1/c
F/C1
0.415
3.49 0.75 1,390
...
...
1,098 ... 0,869
0,788
... 1.315
...
...
...
2.90
0.10 0 , G53 0.00 ...
Measurement of the Luminescence Wave Length .-Lurninescence was observed in all reactions carried out a t 583°K. Carbon tetrachloride was the only compound that showed luminescence a t 519"K. I n general, it appeared that a high concentration of sodium and a relatively low concentration of halide are required to produce luminescence. I n any one series of reactions it was observed that tho visual light intensity is somewhat greater for reactions in which the halide pressure is lower. S o luminescence is observed in the absence of halide f l o ~ . The wave length of the lupinescence was first measured with interference filters with a 25 A. band width. These measurements established that the emission is virtually continuous from 450 to 580 mp with an apparent visual maximum a t 480 mp. The luminescence was then observed with a Gaertner manual monochromator. Of the compounds studied the luminescence of only CC14 could be detected by this instrument, due t o its low optical efficiency and the relative weakness of the observed emission, and indicated a strong line at 472 i 5 mp and a weaker line a t 563 j=5 mp. Measurements were then made as fO11061'S. -4 condensing lens system consisting of a 270-mm. focal length lens and a 35 mm. focal length lens was placed in front of the viewing prisms. The image thus obtained was focused on the 2 mm. slit of a Jarrell-Ash Model 8200 500 cm. grating scanning spectrometer. This instrument was equipped with a 1P21 R C A multiplier phototube powered by a John Fluke Model 405 D C power supply. The output of the phototube was amplified by a Minneapolis-Honeywell Acu-Ata I11 Model DISA-1003 wide band amplifier and recorded on a Brown rscorder. The emission spectrum was scanned at a rate of 500 ,4.per minute over the range 650-300 mp. Eor calibration the emission of the sodium resonance lamp was used as an internal standard. Spectra of the luminescence were obtained both with and without irradiation of the reaction zone with sodium resonance light. These measurements gave a line a t 467 mp for the CC1, reaction and a line at 466 mp for the CFC1, reaction. The luminescence was also observed in a static reactor. Suitable pressures of halide gases u ere introduced into an evacuated 3-liter bulb in which a small sodium reservoir was heated with a bunsen burner. The resulting intense luminescence permitted clear visual color distinction. In the series CCl,, CFCla, CF~CIZ,CF&l all emissions appear blue, becoming a deeper blue in the series as written. For comparison the reaction of CHZCI? w-as also observed in the static experiments. Here it (In) D. Nonnier, R. Vaucher, and P. Wenger Helv. Chim. Acto, 31, 9?9 (1948).
E. D. KAUFMAS AND J. F.REED
898
2
t (a&
01 0
I
1
2
I
. 4
I
I
I
I
6 8 L X 1014, mole/om.2.
,
, 10
,
,
12
Fig. 1.-Behavior of the limiting visible flux as a function of the total pressure. The numbers in parentheses are the number of repetitions of the data point. was noted that the appearance of the originally orange-yellow luminescence faded to a gray-blue glow as the reaction proceeded. Interference filters showed continuous emission in the range from 450 to GOO nip in all cases.
Results Limiting Visible Flux Determination.-The limiting visible flux was determined as a function of total system pressure and is plotted in Fig. 1. Although the actual data show poor precision, it is clearly shown that the numerical value of the limiting visible flux depends on the total pressure. For the calculation of the rate constants in this study, a value of L = 6.13 X 10-14 mole/cm.2 mas adopted as representative of the value of the flux in the pressure region of interest. Xumerically this value agrees satisfactorily with the values obtained by 0thers.~,1~ Calculation of the Specific Rate Constants.-The rate constant, W , was calculated from the relation
Vol. 67
sodium streaming into the reaction zone equal to the total amount of reaction, r is the radial distance from the nozzle, S is the line of sight, and C2 is given by the expression k @ ) p ' / D . Effect of Non-uniform Halide Pressure.-It has been pointed out by Heller'2 that the specific rate constant should be calculated using an effective halide pressure somewhat lower than that calculated from the flow of halide into the system and the total pressure. Mathematical treatments of a diffusion model for the calculation of the halide pressure in the flame have been proposed by Cvetanovic and LeRoy'O and by Smith. 1 1 The Cvetanovic and LeRoy treatment results in a differential equation of which numerical evaluations, l o based on a zero halide pressure at the nozzle, permit the calculation of a correction factor for the rate constant which does not exceed 20% for the cases studied here. The calculations required are somewhat laborious and probably lead to overcorrection since the pressure of halide a t the nozzle is surely not depleted to zero.12 The Smith treatment leads to solutions for the pressure of sodium and of halide using the halide pressure a t the nozzle as a parameter. Unfortunately, it is not possible to obtain a simple correction to the rate constant from this treatment. Solution for the corrected rate constant must be accomplished by successive approximations. The magnitude of the correction here is also of the order of 20y0for the cases studied, although it is usually smaller than that obtained from the Cvetanovic and LeRoy treatment. Neither of the above treatments appear to haye a significant effect on the precision of the calculated rate constants. In view of the relatively small magnitude of the correction, and the large amount of labor required, it is desirable to apply a simpler treatment which mill give similar results. This might be done as follows. Let n be the total number of halogen atoms abstracted from the halide molecule by sodium. Then the quantity of the parent halide molecule consumed will be given by l/nth of the total amount of sodium reaction, i.e., b$n. If F R X is the flovc. of halide into the system, then the average flow of halide in the flame is given by
where p* is the equilibrium vapor pressure of sodium a t the temperature of the carburetor, p , is the least &X = [FRX (FRX- bQ/n)]/2= arQ2uofi'/RT pressure of sodium the fluorescence of which can be detected visually by irradiation with the sodium+ line, substituting nro2vQp'/RT for F R X we have R is the flame radius, ro is the nozzle radius, p' is the 6' = pm' - p*/2n partial pressure of the halide in the reaction, zone, and D is the diffusion coefficient of sodium into the mixture of where p ' is the halide pressure calculated from the halide and nitrogen, taken to be the same as the difflow of halide into the system and the total pressure. fusion coefficient of the sodium into nitrogen alone. As a test, this treatment was applied to the case of A more accurate value of the specific rate constant the reactions of CC1, a t 583°K. where the effect of the may be obtained by considering the limit of visibility halide depletion due to reaction is expected to be most of sodium to be the integrated intensity over the line of The correction was applied to a rate constant sight rather than a fixed value of the sodium p r e s s ~ r e . ~ ~marked. '~ calculated using the total amount of reaction as boundThe value of the rate constant, thus obtained ary condition in order to determine the value of the depends on the experimental measurement of the sodium pressure at the nozzle. For comparison the limiting visible flux by the relation constants obtained from the Cvetanovic and LeRoy and Smith treatments are also presented. The latter constants were calculated using the same lower boundary condition for the sodium pressure. where L is the limiting visible flux, A is a constant of The results of these calculations for an uncorrectcd cc./molespecific rate constant of 49.0 =I= 13.9 X integration obtained by setting the total amount of see. are as follows. The constant corrected by the (17) E. fif. Yemeth, Loyola University, Chicapo, Ill., M.S. Thesis, 1QGO. Cvetanovic and LeRoy method is 61.8 17.1 X 10f'l (18) E. 31. Kemeth and J. E'. Reed, t o be submitted.
+
VAPORPHASE REACTIOK OF SODIUM KITH FLUORIXATED CHLOROXETHANES
April, 1963
cc./mole-sec. ; correction by the Smith method yields cc./mole-sec.; and correction by 54.4 =t 18.1 X the simple technique gives 57.8 i: 16.8 X 10+l1 cc./ mole-sec. It is clearly shown that the magnitude of the simple correction gives results comparable with those of the more complete treatments, while saving considerable time and labor. For this reason, this treatment was used in the calculation of all rate constants reported in this work. Specific Rate Constants.-The constants used in the calculation of the specific rate constants were obtained as follows. The diffusion coefficient of sodium into nitrogen was calculated from the values of v. Kartel and PolanyiZ and Chetanovic and LeRoylO using a 3/2 power dependence for the temperature. The halide diffusion coefficients were calculated from the kinetic theorylg using collision cross sections calculated from viscosity data. The coefficients of viscosity of CF3C1, CF2C12,CFC13, and CC1, mere taken to be 139.6 ;K 10-6,20124.7 X 10-6,21108.4 X 10-6,21 and 195 X The coefficient of l 2 poise a t 25“, respectively. viscosity of nitrogen was taken to be 176.5 X a t 23”. Average values for the calculated specific rate constants are presented in Table I11 for each of the compounds studied. The numbers in parentheses are average deviations from the mean. Detailed data and values for the various experimental parameters for each of the runs may be found elsewhere.23 TABLEI11 CALCULATED SPECIFIC RATECOXSTANTS k(O),
Halide
CFdCl CFaClz CFCl, CC14
Temp., OK.
KO,of
519 583 519 583 518 583 519 583
21 17 7 25 7 14 7 17
runs
x
CC
kea),
lo-“
mole-see.
5 3 6 25 50 47 206 143
x
CC
lo-’’
mole-seo
40(1 75) 42(1 17) 45(1.11) 4 ( 5 95) 5 (16 7 ) 6 (13 7 ) (45 0) (51 8)
1 62(0.28) 1 14(0 13) 2 77(0.31)
10 6 19 8 20 2 57 6 59.7
(1.94) (5.10) (3 78) (6 21) ‘13 0)
+ Ka --+CF2 + T\’a+FCFZ+ Ka C F + Na+F-
CF3
---f
CF
+ Na
--f
C
+ KafF-
899 (b) (c) (d)
2CF3 +CzFs
(e)
2CF2 +CzF4
(f)
The products found in this study in the reaction of CF3C1 were KaC1, KaF, C, CzF4,and C86, in accord with this mechanism. CJs is ignored since it is present only in trace amount. The mechanisms for the reactions of sodium with CF2C12and CFCl3 must be substantially the same, since the gaseous products found correspond to recombination products of halogenated methyl and methylene type species, and the solid products were KaC1, NaF, and carbon. I n the CC14 reaction it appears that the halogen abstraction proceeds so rapidly that no gaseous products are formed, and that the mechanism for this reaction consists essentially of the four halogen abstractions only. Calculation of the Individual Reaction Rates.The over-all rate constants calculated thus far are based on the consideration that the total reaction may be represented by a single reaction term in the original differential equation, implying that there is only one reaction per halide molecule. The mechanism presented in the previous section clearly shows that sodium undergoes several reactions to bring about the observed products. I n order to extract the specific rate constants for the individual reaction steps, the following procedure was used. For simplicity, again consider the reaction of CF3C1. The total amouiit per second of the abstraction reactions, xi, may be represented by integrals of the type xi = I c , p , p ~dV, ~ where Ici is the rate constant for the ith step, p i is the pressure of the halide species reacting in the ith step, and dV is the element of volume in which the reaction occurs. The total amounts per second of the recombination reactions may be represented by integrals of the type xJ = k,pj2dV,where the symsv bols have the same meaning as above. Since sodium is completely consumed in the over-all reaction by the abstraction processes, one may write for the total flow rate of sodium into the reaction zone, bo
Sv
Discussion The Mechanism of the Reactions.-The primary process for the reactions of sodium vapor with organic halides has been reported to be RX S a + Na+XR. For the compounds under consideration here, the radical, R, produced is still halogenated and as xa xb & xd = b0 (1) such is still subject to further attack by sodium. The where the subscripts refer to the individual steps in the presence of carbon in the products indicates that abmechanism. Since all the CF3generated in the primary straction of halogen atoms proceeds until there are step may be accounted for by the CzF6found and the none left on the carbon atom. Similar complete abstractions have been reported by Bawn and D u n n i i ~ g , ~ CF3 consumed in the second abstraction, then ~ Reed and R a b i n o ~ i t c h . ~ Haresnape, et ~ l . ,and 2xe xb = II’a (2) The mechanism proposed by the latter authors is as follows, for the case o.f CF3CI Similarly, for CF2
+
+
+ + + +
CFaCl
+ Na -+ CFa + Na+Cl-
(a)
(19) E. H. Kennard, “Kiiietic Theory of Gases,” McGraw-Hill Book Go., New York, N. Y., 1938 pp. 145, 194. (20) J. 17. Reed and B. ti. Rabinovitch, Chem. Eng. Data Aer., 2, 75 (1967). ( 2 1 ) A. 1’. Bennins and W. €1. Markwood Refrig. E ~ Q 37, . , 243 (1939). (22) “International Critical Tables,” Vol. 11, hIoGraw-Hi11 Book Go., New York, N. Y., 1928. (23) E. D. Kaufman, Lo:iola University. Chicago, Illinois, Ph.D. Thesis, 1962.
2x1
+ xo =
xb
(13)
The only significant reaction of C F is the production of C and XaF by reaction with sodium, therefore
x, = IL’d (4) Since the F/C1 deposit ratio on the nozzle is not the same as that found on the collection plate, the ratio
E. I).KAUFMAK ASD J. F. REED
900
of the concentrations of the different species in the flame must change with distance from the nozzle. Since virtually no carbon was found on the plate, it seems reasonable to say that reaction (d) does not contribute a significant amount of NaF to the plate salt mixture. We now assume that the ratio of the amounts of reactions (a) and (b) is constant throughout the reaction zone. Furthermore, a fraction, 4, of the amount of these reactions occurs in the outer flame region and contributes to the plate deposit, while a fraction, 1 - 4, occurs near the nozzle and contributes to the nozzle deposit. Similarly, a fraction, 6, of reaction (c) contributes to the nozzle deposit and a fraction, 1 8, contributes to the plate deposit. The amount of a solid product found in a deposit may then be obtained by summing the total amounts of the reactions giving rise to the species each multiplied by the appropriate fraction. We now define CY as the fluoride to chloride ratio in the outer flame region, p as the chloride to carbon ratio on the nozzle, and y as the fluoride to carbon ratio 011 the nozzle, and take for these the values found on the collection plate and the nozzlc, respectively. Thus we have a!=
Cb2b
+ (1 - @>G
(5)
9%
a value of pcv3 may be obtained. Substitution of this value into Xb = k b $CFa pNadV will then yield a value Jv of kb. I n order to evaluate lz, by a similar process a value of lif is needed. Lnfortunately no value for this constant is available a t this time. We therefore express our result in terms of the recombination constant, lzf. The result gives lec = 1.09 X lo5 kf112cc./mole-sec. The value found by Reed and Rabinovitchg was 5 X lo6 kf”’. The value of k d cannot be established, since there is no reliable means available for evaluating fiCF.21,22
A further check on the self-consistency of this treatment may be made. In the solution of the original differential equation the reaction term was taken t o be Icpp’, and was considered to be of the form C2p, where C2 = kp‘/D = a constant. It is therefore essential that the total observed rate be related to the rates of the individual steps in such a fashion that the term C 2 p adequately represents the reaction. The reaction term in the differential equation is properly writ ten 1caPCFaCIpNa
y =
(1 - 4)xb
+ fixe +
Jd
(7)
xd
Vo1. 67
+ l”bpCF3PNa +
ICopCF#Xa
+ 1cdpCFpKa
The assumption of constant arerage pressures for the halide species and the factoring of p N a , the variable quantity, gives
Calling the ratio of C2F4t o C2F6found in the products 6 6 = x‘f/xe (8) it is then possible to solve the set of simultaneous equations l through 8 for x,, @, and 0 in terms of bo. The solutions are presented in Table IV. TABLEIV TOTALAMOUNTS OF INDIVIDUAL REACTIONS Xa
Xb
0 566b
0 418bo
+
Xf
2.2
0 074bo
0 0206bo
0 995
=
~.&CF~CI
p~~ dl; and x b
= Icb$cFa
K
p ~dli, , etc.
the term k b f i ~ r ~ / f i may C~~~ be1 shown to be (xb/z,)ic, and the over-all rate constant
0 00GGbo 8
0 146
I n order to calculate the specific rate constants of the individual steps, we proceed as follows. I t is assumed that the partial pressure of the halide may be taken as an average pressure throughout the flame. Taking the value of the pressure used in the calculation of IC@), one may now calculate the value of the specific rate constant for the primary process from the relation k,
xa
x.1
3%
0 O06Gb0
Realizing that
X,
~ c F ~ cZ.Y. I J ~d 1 ~ ’
If one assumes that the partial pressure of the other halide species may also be represented by some average value throughout the flame, the remaining integrals may be solved in the following fashion. Using the value IC, = 2.34 X IOT3 cc./rnole-~ec.*4~*~ in the relation (24) P. B. Ayscough, J . Chem. I ’ h ~ s , 24, 939 (1956). (25) G. 0. Pritchald and J. R. Dacej, Can. J . Chem , 38, 182 (1960).
from which it is seen that li(3) = Ic(bo/z,) = ka/0.5G6. Substitution of the appropriate data into these relations shows that the value of IC, thus calculated agrees with the observed value within 5%. The specific rate constants for the elementary reactions in the other cases, CF2C12,CFCL, and CCl,, cannot be obtained in this way since insufficient data are available. However for the reaction of sodium with CC1, only carbon and NaC1 were found in the products; no gaseous products were obtained. For complete reaction the primary process must necessarily consume 0.25b0. From this, a value for the rate constant of the primary process may be calculated. The primary processes of the reactions of CF2C1, and CFC1, must consume a t least 0.256,. It appears that the reactions of the radicals with sodium become more important in the series CF,, CF2C1, CFC12, CCL as one proceeds from CF3 to CC13. It is reasonable therefore to expect that the primary processes of the reactions of CF2C12 and CFC1, ill consume a fraction of the total amount of sodium streaming into the reaction zone which lies between 0.2560, the rate for CC14, and
April, 1963
VAPOR P H A S E
REACTIOK O F SODIUM KITH
O.s66bo, the rate for CFsC1. It is then possible to calculate limits for the rate constants for these reactions. The values obtained in this fashion are presented in Table V. SPECIFIC Rum Species CFsCl CFI CF2 CF2Ch CFCla CCla a
TABLEV Ea f RX 583OK.
COXSTAXTS“ FOR
Ai
0.78 926 1.1 X 10-%+’* 3.2-4.6 6.2-9.1 14.9
Other values (ref.) 1 . 9 8 (9) 672 (9) 45 X lO-%il’* (9)
...
..-t
Sax f R
AT
Schwarta lower limit
...
20.5 0 . 5 X 10-W’z
...
...
...
70-300 (4,5)
...
Units are 1011 cc./mole-see.
h check on the reliability of the rate constants thus obtained may be made through the application of the Schwartz inequality. Treat’ment by the method of Reed and Rabinovitchg gives the results listed iii Table
v.
Calculation of Bond Energies and Heats of Forrnation.-It has been shown by Ogg and PolanyiZ6that for a series of reactions RX Na 4 KaX R where R is varied, the activation energy changes in the series may be related t o the changes in RX bond energy by the relation ( A E ) =: a f ( A D )where a’ is a simple constant of proportionality. This relation will be expected t o hold as long as there are no significant changes in the magnitude of the polarizabilities of the radicals, R, produced in the reactions, as long as the equilibrium internuclear separation C-X is constant, and as long as there is no appreciable change in the resonance stabilization of the transition state.27 In order to apply this treatment t o the present problem, it is necessary t o assess the validity of the relation ( A E ) = a’(AD) for the series studied. Since specific values for t:he polarizability of the radicals are not available at this time, it is necessary t o proceed with an inductive argument. It would appear on the basis of a molecular volume treatment that the polarizabilities of the radicals CC13, CFC12, and CF2C1will not differ greatly. However, the polarizability of CF3 may be significantly different. Kumerically, it is estimated that doubl.iiig cm., results in an the value of a from 2 to 4 X increase of the activation energy of approximately 1.5 kcal./mole. T l ~ u sin comparing the reaction of CF3C1with that of CC1, one would expect a decrease in activation energy as a result of the change in the radical polarizability. The carbon-chlorine equilibrium separation in the stable R-C1 molecules is constant in the series CC1341, C F C k C l , CF2C1-C1.28~29 a t 1.77 A. The equilibrium separatioon in the CF’,-Cl molecule is somewhat lower2@ a t 1.75 A. On the basis of a simple geometric argument involving the appropriate cut t’hrough the energy surface for the reaction, it may be shown that the activation energy will increase by approximately 0.3 kcal ./m%le when th.e equilibrium separation increases by 0.02 A. This change will tend to offset partially the decrease in activahion energy resulting from the polarizability effect.
+
+
(26) R. A. Ogg and Mi. Polanyi, Trans. Farndag Soc., 31, 1375 (lW5). (27) M. G. Evans and E. Warhurst, ibid.. SS, 593 (1939). (28) R. L. Livingston and D. H. Lyon, J. Chem. Phys., 24, 1286 (1966); Q . L. Karle, ibid., 17, 1052 (1949). (29) L. 9. Bartell and L. 0. Brookway, i b i d . , 23, 1860 (19%).
F L U O R I X s T E D CHLOROMETHAKES
90 1
A decrease in resonance stabilization of the transition state in the series CCl,, CFCL, CF2C12,CF3C1 would result in a further increase in activation energy. On the basis of the number of resonance structures that may be writtenjaOCC14 and CFC1, will not show any variation due to this effect, while CF2C12and CF3C1 would be expected t o show somewhat higher activation energies. The magnitude of the change in activation energy will probably not exceed 1.0 kcal./mole. The forgoing arguments indicate that the application of the relation AE = a f A D t o the data for the reaction of CF&l with sodium is not warranted in view of the variations in activation energy resulting from changes in the polarizability and the radical stabilization. The application of the relation to the data in the series CC14, CFCl,, CF2C12,in which the constancy of the C-X bond length and the estimated constancy of the radical polarizabilities are satisfactory, is reasonable. I n view of the fact that the values obtained for the specific rate constants at 519 and 583OK. are substantially the same within experimental error, a plot of In IC vs. 1/T cannot be expected to give satisfactory results for the activation energy. The energies of activation corresponding to the specific rate constants found in this study may be obtained from a simple Arrhenius treatment using the usual value 5 X 1014 for the pre-exponential factor. The values of the rate constants reported must however be corrected by a statistical factor reflecting the number of identical reacting atoms in the molecule. The value of the constant, a’, is usually taken to be between 0.20 and 0.25.31 These values are adopted as limits in the calculations of the bond strengths. The calculated bond strengths are reported in Table VI, where the value D(CC1,-C1) = 67.9 kcal./mole has been taken as the reference value.,2 TABLEVI BONDDISSOCIATIOK ENERGIES Species E , kcal./mole D ,koal./inole CFI-Cl 10 2 ... CF2Cl-Cl 9.0-9.5 72 f 4 CFClz-Cl 8.7-9.2 71 f 3 ... (75.6 2)33 CClo-C1 8.4 (67.9 f 2)92 CFz-F 3.2 ... CF&l ... 62 =I=14
Taking the values for the heats of f0rmation~~-3~ of the halide molecules, the heats of formation of the radicals produced in the primary steps of the CF2Clz and CFC1, reactions may be calculated. These values were found to be AHf(CF2C1)= -69 f 4 kcal./mole and AHr(CFC12) = -24 f 6 kcal./mole. Considerations of the Observed Luminescence.-The appearance of the maximum intensity of emission a t (30) E. Warhurst, Quait. Res. (London), 5, 44 (1951). (31) N. N. Semenov, “Some Problems of Chemical Kinetics and Reactivity,” Pergamon Press, London, 195Sj p. 27; J. R. Dacey and R. Young, J. Chem. Phys., 23, 1302 (1955); R. N. Haszeldine and B. R. Steele, Chem. and I n d . (London). 684 (1951). (32) J. B. Farmer, L. H. S. Henderson, F. H. Lossing, and D. G. H. Marsden, J. Chem. Phys., 2 4 , 348 (1956). (33) R. K. Curran, ibzd., 34, 2007 (1961). (34) P.W. Kirkbride and F. G. Davidson, N u t w e , 174, 79 (1954). ( 3 5 ) D. R . Stull, “Janaf Thermochemical Data,” Doiv Chem. Co., Midland, Mich., 1961. (36) F. D. Rossini, et at., “Selected Values of Chemical Thermodynamic Constants,” Circular 500, National Bureau of Standards, Washington, D. C., 1962.
0. J. BLEPPA AND 8. Y.MESCHEL
902
466 mp for the CC1, reaction indicates a process that is exothermic to the extent of at least 61 kcal./mole. The apparent shift toward the ultraviolet observed in the static experiments indicates that the corresponding processes in the reactions of CFC13, CF2C12,and CF&l may be more energetic. Ogg and Polanyi26 explained the luminescence observed in the reactions of metal tetrahalides with sodium by a process involving radical disproportionation. Since no evidence for disproportionation was found in these reactions, the mechanism must also be discounted. Evans and TT;alsh3’ have suggested that the luminescence of the CH2C12 reaction arises from the collisional deactivation by sodium of excited CzH4formed by reconibination of two CH2 radicals. Such a mechanism would not explain the luminescnnce observed in the series under consideration here, since the CC1, reaction does not lead to gaseous products. Bawn and Dunning7 have suggested that the observed luminescence in the CH2Clz reaction arises from the rearrangement of the CH2 radicals produced after the second halogen abstraction. The radicals are presumably produced in an excited state and are collisionally deactivated by sodium. Parmetier and G a y d ~ observed n~~ a strong C-H line a t 431 mp in an oxygen flame of CH3Cl. il. further greenish blue coloration of the flame was attributed to CCl fragments. D ~ r i efound ~ ~ some weak lines in CC1, flames in the region from 450 to 500 r n p . Andrews and observed a band spectrum with lines to 480 mp in electric discharge experiments with CF4, which they attributed to the presence of CF. Unfortunately, these workers did not examine the spectrum in the visible region in any detail. It was qualitatively observed that the reaction of CCl, gave a very intense luminescence zone, the reac(37) M. G. Evans, T r a n s . Frsiaday Soc., 95, 193 (1939); A. D. Walsh, Discusszons Faraday Soc., 2, 144 (1947). (38) G. Parmetier and A. G. Gaydon, Compt. rend.. 225, 1139 (1947). (39) R. A. Durie, Proc. Phys. SOC.(London). A211, 110 (1962). (40) E. B. Andrews and R. F. Barrow, Nature, 166, 890 (1950).
Vol. 67
tion of CFC13 gave a zone somewhat less intense, that of CFzClzgave still less and the reaction of CFdZ1 gave a relatively very weak luminescence intensity. The variation of the reaction rates for these species is in the same order, i e . , CC1, gives the fastest over-all reaction rate and CF3C1gives the slowest rate. It would be expected that the flame zone in CC1, reaction would contain more CX2and CX radicals than those of the other reactions, and that the concentration of these radicals in the steady state would perhaps decrease with decreasing over-all reaction rate. The observed luminescence intensity variation is consistent with the conception that the phenomenon is due to one or both of the radicals CX2 or CX. I n view of the emissions attributed to CX species by previous workers, and in view of the fact that the observed radiation appears to be confined to the region in the vicinity of the nozzle where CX radicals are present, it seems reasonable to attribute the observed lumiiiescence to the CX species produced in the third halogen abstraction. Since carbon is one of the reaction products, it is possible that the observed luminescence arises from carbon-carbon rearrangements such as were reported by Gaydon41in the study of CH, flames. The observed blue color of the luminescence is a t least in qualitative agreement with Gaydon’s conclusions. Furthermore, it is entirely possible that the observed effect is due to several processes occurring simultaneously. Unfortunately the data are not sufficiently accurate to dram a definitive conclusion. Acknowledgments.-The authors gratefully acknomledge the cooperation of S. Meyerson of the American Oil Company, who provided the mass spectra, and G. Swenson of Loyola University, whose assistance was invaluable in making the measurements of the luminessence. E. D. K. also wishes to thank the National Science Foundation for a fellowship during the summer of 1961. (41) A. G. Gaydon, Quart. Rev. (London), 4, 1 (1950).
A THERMOCHEMICAL STUDY OF SOLUTIOSS OF CALCIUM ASD CAD3IIUM NITRATES I N LIQUID LITHIUM, SILVER, AND THALLIUM NITRATES BY 0. J. KLEPPAASD S . V. MESCHEL Department of Chemistry and Institute for the Study of Metals, University of Chicago, Chicago 37, Illinois Received October $6, 1968 Some new calorimetric data are reported for the solutions of calcium and cadmium nitrates in liquid lithium, silver and thallium nitrates. The limiting enthalpies of solution of the undercooled, liquid divalent nitrates in the silver and thallium salts are about 2 kcal. more positive than one would have predicted from available information on solutions of calcium nitrate in the alkali nitrates. This is attributed to the significant van der Waals contributions t o cohesion in t5e silver and thallium salts. By extrapolation of the new enthalpy data for the liquid solutions, the heat of fusion of cadmium nitrate was found to be 4.35 kcal./mole a t 300”. The entropy of fusion of this salt is comparable in magnitude t o that of calcium nitrate, but much smaller than that of the corresponding strontiuni and barium salts. An interpretation in terms of the rotational freedom of the NOa- ions in the melt is suggested.
Introduction Recently one of the authors reported Some 1iew therm mochemical data for the liquid solutions of the alkalilie earth nitrates in the alkali nitrates.1 In the present work our study of charge-unsymmetrical fused salt (1) 0. J. Xleppa, J . Phys. Chem., 66, 1668 (1962).
mixtures is extended to the solutions of calcium nitrate in silver and thallium nitrates, and to cadmium nitrate in lithium, silver, and thallium nitrates. MTehave already stressed that the thermal instability of the divalent nitrates imposes very definite restrictions on the temperature of measurement, and consequently also
