608
J. R. LACHER,LIBERTYCASALIAND J. D. PARK
pendence on telluride proportion exists, would indicate that (SZQ) groups, perturbed by telluride, are the centers. The shift of the peak wave length to longer wave lengths with increasing amounts of ZnTe is in accord with previous experience that substitution with ions of larger ionic radius (Tefor S-) results in luminescence emission whose peak wave length is at longer wave lengths. The phenomenological effect of halide fluxes on the luminescence of zinc sulfide phosphors has been discussed by Rothschild.’ With further advances in the work on fluxes for phosphors, Smith8 suggested that the chloride is incorporated into the crystal, forming crystal defects, and Kroger and Hellingmans have reported finding incorporated chloride in sulfide phosphors. The crystal perturbationsdue to incorporated chloride, as evidenced.by magnetic studies, have been described by Larach and Turkevich.’o The nature of curves ABEF (Fig. 2) suggests that the emitting center created by addition of telluride to the ZnS lattice is not essentially equivalent to (7) 8.Rothachild, Trane. Faraday Soc., 41,635 (1940). (8) A. L. Smith, J . Electrochem. Soc., 96, 7 5 (1949). (9) F. A. Krager and J. E. Hellingmsn, ibid., 95,68 (1949). (10) 8.Larach and J. Turkevich, Phys. Rev.,98, 1015 (1955).
Vol. 60
the effective center created by the presence of C1 in the ZnS lattice or the Zn(S:Te) lattice. The nearlyidentical emission of 98ZnS :2ZnTe and ZnS(C1) is probably coincidental. The effect of increasing telluride concentration in the lattice on the ZnS(C1) emission can be accounted for by assuming that the Zn(S :Te) efficiency is increasing (due to increasing number of centers), while the centers related to the presence of C1 are being “poisoned” by the lattice distortion created by the telluride concentration. The exact nature of the Zn(S :Te) center is entirely conjectural. The efficiency plot with telluride suggests that paired defects may constitute the nucleus of the elemental crystal volume giving rise to the emission.1° Parallel situations exist when Ag and Cu are present to give rise to their characteristic emission. The Zn(S :Se) systems has no limited solubility, leading one to argue that low concentrations of Se in ZnS do not cause “poisoning” perturbations. Confirmation is offered by the observation that no marked loss of efficiency is experienced with phosphors containing selenide in small amounts.” ,
(11) “Preparation and Characteristics of Solid LuminesoeDt Mate-
riala,” edited by G.R. Fonda and F. Seitr. John Wiley and Sons,Inc., New York, N. Y., 1948, p. 238.
REACTION HEATS OF ORGANIC HALOGEN COMPOUNDS. V. THE VAPOR PHASE BROMINATION OF TETRAFLUOROETHYLENE AND TRIFLUOROCHLOROETHYLENE’ BY J. R. LACHER,LIBERTYCASALIAND J. D. PARK Contribution of the University of Colorado, Boulder, Colorado Received October 87,1066
A calorimeter has been constructed in which one can study vapor phase reactions at about 103”. It has been used to measure the heat of formation of hydrogen bromide from its gaseous elements and also to measure the heat of addition of bromine vapor to tetrduoro- and trifluorochloroethylene.
In the previous paper,2 a calorimeter was described that uses a .condensing vapor as a source of constant temperature. When diphenyl ether is employed, an operating temperature of 248” is produced. Under these conditions, some simple alkyl chlorides could be hydrogenated. A “low temperature” calorimeter also has been constructed that utilizes condensing toluene vapor as its constant temperature environment. The present papar describes its operation and presents data on the vapor phase bromination of tetrafluoroethylene and trifluorochloroethylene. Materials and Experimental Methods.-Except for minor changes, the “low temperature” calorimeter is identical to the one previously described. Instead of a platinum resistance thermometer, thermistors were employed to sense any temperature change of the condensing toluene. They formed one arm of a conventional Wheatstone bridge. Any unbalance of the bridge was used to modify the pressure on the boiling toluene so as to maintain the temperature constant to 0.001” during an experiment. The calorimeter catalyst chamber was immersed in a (1) This research waa supported by the Atomic Energy Commiasion, Contract No. AT(l1-1)-168. (2) J. R. Laoher, E. Emery, E. Bohmfalk and J. D. Park, THIS JOURNAL,60,492 (1956).
quart dewar flask containing n-butyl ether. The reaction heat was transferred to the ether, and the latter could be cooled by bubbling dry nitrogen through it. I n order to measure the temperature differential between the n-but 1 ether in the dewar and the surrounding temperature bat{, a twenty-four junction copper-constantan thermel was used. One end of the thermel was located in a well made of telescopic iron tubing extending about midway into the n-butyl ether, whereas the other end was suspended in a brass well in the condensing vapor bath. The procedure used in making an experiment was identical to that previously described. The trifluorochloroethylene was prepared by the following dechlorination reaction8 CF2Cl-CFClI
in + Zn -----+CFz=CFC1 + ZnClz n-butanol
The CF-CFCl obtained was then purified on a 100-plate Podbielniak Hypercal column. The starting material was a gift from the du Pont Company. Part of the tetrafluoroethylene used in this work was supplied by the du Pont Company, and the remainder was prepared by the following three reactions': 700” 2CFsHC1 CFz=CFz 2HC1
+
(3) M. L. Sarrah, Ph.D. Thesis, Univeraity of Colorado, 1948. (4) J. D. Park, et al., J . I n d . Eng. Chem., 89, 354 (1947).
-
VARORPHASE BROMINATION OF TETRAFLUOROETHYLENE
May, 1956
+ Brt
CF2=CF2
100
carbon Zn
CFoBr-CF2Br
CFZ=CFZ methanol The crude tetrafluoroethylene obtained by the pyrolysis of difluorochloromethane was brominated to give the dibromide. The dibromide was then distilled on the Podbielniak column. Whenever needed, it was dehalogenated to tetrafluoroethylene and the product condensed into a liquid nitrogen trap. It was then purified by trap to trap distillation and stored in B bomb containing several mi. of "Terpene B" inhibitor to revent polymerization. The inhibitor was removed from t f e gas stream as it was metered to the calorimeter by a method previously described.6 Reagent bromine (Mallinckrodt) was used without further purification. It was carried to the calorimeter in a stream of nitrogen. The nitrogen gas was dried over phosFhorus pentoxide and then bubbled through bromine a t 0". he gas could be sent either to the calorimeter or to an absorption tower to determine the flow rate of bromine. The bromine was absorbed in 3 N potassium iodide solution. The liberated iodine was titrated with standard sodium thiosulfate in the usual way. CFzBr-CFzBr
____f
Experimental Results.-In order to determine whether or not the calorimeter was working properly, it was decidedko measure the heat of formation of hydrogen bromide from its gaseous elements. Electrolytic hydrogen of 99.8% purity from the Mathieson Company was used. The oxygen present was removed by methods previously described.2 A hydrogenation catalyst consisting of palladium on asbestos brought about a complete reaction of the bromine which was the limiting reactant,. Of the eight runs made, four were carried out without experimental difficulties, and the results are given in Table I. The difference between our measured AH and the enthalpy change, AH", for the reaction when the reactants and products are in their standard states as hypothetical ideal gases is small in comparison to our TABLE I VAPORPHASE HEATOF FORMATION OF HYDROGEN BROMIDE AT
Hn flow, molea/min. x 10'
HBr formation, moles/min.
x
10'
103" Energy rate cal./mlin.
-AH, oal./mole, of HBr
6.689 12 ,580 6.350 12,340 6.204 12,430 5.863 12,530 Av. - A H = 12,473 Twice the standard dev. of the mean' = f0.850Jo
10.0 10.4 10.5 10.5
5.318 5.146 4.991 4.679
experimental errors and is neglected. No direct calorimetric determination of the heat of formation of hydrogen bromide from its gaseous elements has been reported in the literature. However, values of -12.33 a t 300°K. and of -12.41 kcal./mole at 400" are given by the Bureau of Standards.? The interpolated value of - 12.39 kcal./mole at 375°K. is in satisfactory agreement with the present average of -12.47 kcal./mole. Using heat capacity data our value for the heat of formation becomes -12,377 f 130 cal./mole a t 298.16"K. (5) J. R. Lacher. J. D. Park, et ol., J . Am. Chem. Soc., 71, 1330 (1949). ( 6 ) F. D.Rossini, Chem. Rsve., 18, 233 (1936). (7) "Seleoted Values of Chemioal Thermodynamic Properties," Seriw 111, National Bureau of Standards.
609
The Bromination of Tetrafluoroethy1ene.-Two catalysts were used in the determination of the vapor phase heat of bromination of tetrafluoroethylene. Antimony bromide on activated charcoal was prepared by mixing finely powdered antimony with charcoal, and the mixture was then placed in a catalyst chamber. Bromine gas entrained in a stream of nitrogen was then passed over it at about 150". During pilot plant studies, it was found that this catalyst would bring about the quantitative addition of bromine to CF*=CFp, CFz=CFCl and CF2=CHz at 103". In the case of CF2-CF=CF-CFz, the conversion was very '
-
low owing, possibly, to an unfavorable equilibrium. A second catalyst consisting of ferric bromide on activated charcoal was prepared in a similar way. In making a run, an excess of tetrafluoroethylene over bromine was used, and the rate of formation of the dibromide was taken equal to the rate bromine was led into the calorimeter. The experimental results are given in Table 11; all runs are TABLE I1 VAPORPHASE HEATOF BROMINATION OF TETRAFLUOROETHYLENE ~ ~ 1 0 3 " Olefin flow, moles/min. x 10'
Brr flow, moles/min. x 104
12.9 12.1 9.7 11.4 13.4 11.7
3.164 2.581 2.803 2.581 2.545 2.454
Energy
.
c arate ~/m?m.
11.572 10.020 10.288 10.729 10.260 9.3664
-AH CFZ~I-CFIB~ cal. mole', of
36,570 38 ,820 36,700 41,570 40,310 38,170
Av. - A H = 38,586 Twice the av. dev. of the mean = &4.2% 9.3 9.3 12.7 8.7 9.7 11.9 3.5 11.5 13.2 12.8
4.446 4.305 4.285 2.855 2.822 4.222 2.843 2.823 5.850 6.130
17.386 16.945 16.761 10.481 10.454 16.644 10.545 10.994 22.213 23.755
39,100 39 ,360 39,120 36,710 37,040 39 ,420 37,090 38,940 37 ,970 38 ,750
Av. - A H = 38,350 Twice the av. dev. of the mean = i1 . 7 %
included except one in which a plugged frit stopped the experiment. The first six runs were made using the antimony bromide catalyst and in every case there was a slight trace of bromine in the exit gases. Also the scattering in the resulting AH values was greater than desired. They give an average of -38,586 cal./mole. The apparatus was then dismantled and a ferric bromide catalyst installed in the reaction chamber. With this catalyst the reaction was quantitative. During the remainder of the runs, a special effort was made to vary the ratio of the olefin to bromine flow. The experimental values seem to be independent of these variations. The values for AH are more consistent in the second series of runs owing,
KENZITAMARU
610 TABLE I11
VAPORPHASE HEATOF BROMINATION OF Olefin flow moles/nhin.
x 10' 14.5 11.9 10.3 10.1 11.5 12.7 9.4 10.1 10.1 9.3
TRIFLUOROCHLOROETHYLENE AT 103" Br: flow moles/min. x 10'
4.665 4.318 4.512 5.149 5.158 5.074 4.416 4.439 4.103 5.120 Twice the av.
10.2 9.6 10.1 9.8 9.8 8.7
4.916 4.928 5.124 4.518 5.213 2.844 Twice the av.
Energy rate oal./m\n.
-AH oel./mole', of CFsBr-CFCIBr
14.439 13.294 13.815 16.704 16.160 16.013 13.788 13.902 13.015 16.128
30,950 30,790 30,620 32,440 31,330 31 ,560 31 ,220 31,320 31,720 31 ,500 Av. -AH = 31,345 dev. of the mean = &l. 1 % 16.056 32,660 16.120 32,710 16.064 31,350 14.657 32,440 16.362 31,390 9.0233 31,730 Av. -AH = 32,047 dev. of the mean = fl. 6%
Vol. 60
perhaps, to a better experimental technique. Their average value is -38,350 cal./mole. This is in essential agreement with the first average and shows that the measured heat of reaction is independent of the catalyst used. The average over all sixteen experiments is -38,478cal./mole. The Bromination of Trifluorochloroethy1ene.The bromination of trifluorochloroethylene was carried out by the same procedures as with tetrafluoroethylene. The results are given in Table 111. The first ten runs were made using antimony tribromide as a catalyst and the last six with ferric bromide. The average of the sixteen runs gives a heat of reaction of -31,608 cal./mole. The runs made with both tetrafluoro- and triffuorochloroethylene show a greater variation in AH values than were obtained in the measurements of the heat of formation of gaseous hydrogen bromide. It is believed that this is due to the relatively large amount of products adsorbed on the carbon catalyst. A slight variation of the flow rates will upset the adsorption equilibrium, making it difficult to obtain a steady state. Very little adsorption will take place with the palladium on asbestos catalyst, and a steady state readily is obtained.
THE THERMAL DECOMPOSITION OF 'TIN HYDRIDE BY KENZITAMARU Frick Chemical Laboratory, Princeton University, Princeton, N . J . Received October BO, 2066
The kinetics of decomposition of tin hydride have been studied by a static method. The decomposition is a first-order reaction in respect to tin hydride, being independent of hydrogen pressure, and !he activation energy of the reaction is 9.1 kcal./mole between 100 and 35". The hydrogen-deuterium exchange reaction dld not proceed at a measurable rate on tin film a t 60", where tin hydride decom osed fairly fast. Similarly, when tin deuteride was decomposed in the presence of hydrogen, no hydrogen deuteride cod! be detected in the reaction product. On the contrary, when mixtures of tin hydride and tin deuteride were decom osed together, hydrogen deuteride was produced. The effect of oxygen on the decomposition was studied. It waa found t i a t a small amount of oxygen can stop the decomposition, forming an oxide film on the tin surface. The rate-determining step of the over-all reaction has been discussed briefly.
the As has been shown in previous decomposition of arsine on arsenic surface is a first-order reaction in respect to arsine and the germane decomposition on germanium surface is zero order, while stibine decomposes on antimony surface as a fractional order, between first and zero order. In all these cases hydrogen does not affect the reaction rate. It was suggested that the rate-determining step of the arsine decomposition is the chemisorption of arsine, while that of germane decomposition is the desorption of chemisorbed hydrogen from the germanium surface. I n the latter case it was shown6 that the germanium surface is virtually covered by the chemisorbed hydrogen atoms during the reaction. These reaction systems of the hydride decompositions are ideally suitable to study becaus.e of the simplicity of the reactions and the fact that clean (1) K. Tamaru, THls JOURNAL,59, 777 (1955). (2) K. Tamaru. ibid., 69, 1084 (1955). (3) 'K. Tamaru, M. Boudart and H. Taylor. - . ibid., 59, 801 (1955). (4) P. J. Fensdam, K . Tamaru, M. Boudart and H. Taylor, ibid., 69, 806 (1955). (5) K. Tamaru, in preparation lor J . P h y s . Chem.
elemental surfaces are continuously maintained through fresh deposition of the elements. The decomposition of another hydride, tin hydride, SnH4,has been studied. Experimental The decomposition of tin hydride was studied by a static method. Since hydrogen is the only gaseous constituent resulting from the decomposition, the rate of the reaction was followed by observing the total pressure of the system, which, on completion of the decomposition, was twice the initial pressure. Preparation of Tin Hydride.-Tin hydride was pre ared by adding chemically pure anhydrous stannic chloride [quid to a solution of lithium aluminum hydride in ethylene lycol dimethyl ether in a dry nitrogen atmosphere. The sofution of lithium aluminum hydride was cooled with liquid nitrogen as the stannic chloride was added. After the chioride addition the liquid nitrogen was removed to allow the solution to warm up gradually to room temperature. The tin hydride thus prepared was condensed in a liquid nitrogen trap and was purified by distilling several times between solid carbon dioxide and liquid nitrogen. For the preparation of tin deuteride, lithium aluminum deuteride was used instead of hydride.6 (6) Anhydrous stannic ohloride was obtained from General Chemical Division, Allied Chemioal and Dye Corporation, New York,and lithium aluminum hydride and deuteride from Metal Hydrides, Inc., Ma@.
