THEVAPORIZATION THERMODYNAMICS O F TRIEUR~OPIUM TETROXIDE probability is proportional to the molecular weight of one of the two involved. Coupling reactions involving end groups alone, for example, could produce the observed changes. That is, if approximately 12% of the chains in the solution polymers had reactive end groups which could somehow couple randomly and rapidly with other end groups in the system during irradiation, (P,)o/P,' would be 0.88 and (Pw),,/Fw' would be nearly the same,26 as observed. However, there are many practical objections to this explanation. The end groups should be chemically similar whether derived from styrene transfer alone, as in the bulk polymers, or from styrene and ethylbenzene transfer, as in the solution polymers.32 It seems particularly strange that the introduction of end groups from ethylbenzene should confer additional reactivity. Also, if we assume that end groups are inot clustered but distributed more or less randomly throughout amorphous polymer solids, then it is difficult to reconcile reactions between end groups with the known immobility of chains well below the glass transition temperature. The nature of the transient reaction therefore remains uncertain.
4235
Summary The pregel behavior of polystyrene prepared by uncatalyzed bulk polymerization shows no evidence of reactions other than random cross-linking and chain scission. The behavior of polystyrene prepared in ethylbenzene solution indicates, in addition to crosslinking and chain scission, a rapid reaction which goes to completion in the pregel region. The relative increases in P , and Pwcaused by this reaction are consistent with a coupling process involving chain ends. The random-cross-linking reaction proceeds at the same apparent rate regardless of molecular weight or sample preparation and thus shows no evidence of intramolecular cross-linking which can be related to coil conformation.
Acknowledgments. This work was supported by the Advanced Research Projects Agency of the Department of Defense, through a grant to the Northwestern University Materials Research Center. (32) W. W. Graessley, L. M. Alberino, and H. M. Mittelhauser, Amer. Chem. Soc. Polym. Chem. Preprints, 7, 1018 (1966).
The Vaporization Thermodynamics of Trieuropium Tetroxide by John M. Haschke and Harry A. Eick Department of Chemistry, Michigan State University, East Lansing, Michigan
48883 (Received June 3, 1068)
Trieuropium tetroxide has been prepared by reaction of sesquioxide, oxide chloride, and lithium hydride in a tungsten crucible under vacuum. The vaporization of the tetroxide according to the reaction 3Eu304(~)-+ 4Euz03(s,monoclinic) Eu(g) has been studied over the temperature range 1604-2016'K by target collection effusion techniques, The condensed effusate has been analyzed by X-ray fluorescence. For the reaction at 1810°K, AHT' = 86.2 f 1.4, kcal/gfw and AST' = 28.20 =t0.82 eu. Heat capacity data for EusO((S) have been estimated, and second- and third-law enthalpies for the reaction are presented. For Eu&~(s): AHrozss= -542.4 f 3.6 kcal/gfw, AGroz9s = -514.4 + 3.6 kcal/gfw, and 8 ' 2 ~=~ 48.6 f 2.6 eu.
+
Introduction The thermodynamic properties of the lanthanide oxides have probably been studied more extensively than those of any other binary lanthanide species; however, many uninvestigated areas still exist. I n his extensive review of the thermodynamics of binary lanthanide compounds with particular emphasis on the oxides, Westruml cited the need for additional investigation. Although fragmentary or estimated data are reported for most oxides, no thermochemical values are currently available for trieuropium tetroxide, E U 3 0 4 , although numerous preparatory, crystallographic, and magnetic investigations appear in the literature. The
present investigation was initiated in an effort to characterize the vaporization behavior and thermodynamics of this phase. Because there is a dearth of thermodynamic experimental data, the Eu304 system has afforded an opportunity to employ and evaluate the semiempirical scheme of Grgnvold and Westrum' for estimating lattice and magnetic contributions to lanthanide oxide entropies. I n addition, it has been necessary to approximate the heat capacity of EusO4.
(1) E. F. Westrum, Advances in Chemistry Series, No. 71, American Chemical Society, Washington, D. C., 1967, p 25.
Volume 72. Number 12 November 1068
4236 Experimental Section Preparative Procedures. Trieuropium tetroxide was prepared by an adaption of the method reported by Baernighausen. Europium oxide chloride was prepared by dissolving Euz03 (99.9%, American Potash and Chemical Corp., West Chicago, Ill.) in 6 M HC1, evaporating to obtain the hydrated trichloride, and igniting at 500" in air for several hours.3 EuOC1, which was identified by its X-ray diffraction pattern,4 was combined in a 1:1:2 stoichiometric ratio with Eu203 and LiH (Metal Hydrides Inc., Beverly, Mass.). The blended reactants were placed in an outgassed tungsten crucible in a He-filled glove box and were heated subsequently by induction in a water-cooled Vycor vacuum system whose residual pressure was 10-6 to lo-' torr. The temperature was increased slowly to 900" such that the system pressure never exceeded torr. Heating was continued for approximately 5 hr, by which time the pressure had decreased to torr, an indication that the loss of HB, LiCl, and excess LiH, and consequently the reaction, was complete. The product often contained a few small particles of a white phase which could be separated physically from the reddish black Eu304. The X-ray diffraction pattern of this white phase did not correspond to, that of any known lithium-europium-oxygen or lithium-or europium-oxygen phase. These white particles were combined with additional EuOCl and LiH on the assumption that their molecular weight corresponded approximately to that of EuS03 and were mixed with the Eua04 phase again before the entire sample was reheated to a maximum of about 1200". The final product was a homogeneous reddish black solid. Analytical Procedures. The sample was analyzed by both chemical and crystallographic techniques. Metal analysis was effected by hydrolysis of weighed specimens with 1 M HN03 and subsequent ignition at 900" to the sesquioxide. X-Ray powder diffraction patterns of the polycrystalline phases were obtained with a Hiigg-type Guiner camera using CuKal radiation (A 1.54051 A) and Pt or KC1 internal standards. Vaporization. One trieuropium tetroxide sample was subjected to weight-loss vaporization measurements. A weighed specimen was heated to constant weight at 1400-1500" by induction in a thoroughly outgassed molybdenum effusion cell. The vaporization mode of the tetroxide was also investigated mass spectrometrically with a Bendix time-of-flight mass spectrometer, Model 12-107. The species emanating from a molybdenum effusion cell heated by electron bombardment to 1400-1700" were analyzed with a 25-eV ionizing electron beam. Target collection Knudsen effusion measurements were made with an apparatus similar to that described by Ackermanna6 The residual pressure in the Pyrex and Vycor system at crucible temperature of 13301745" was 10-6 to 10-6 torr. The effusing vapor was The Journal of Physical Chemistry
JOHN M. HASCHKE AND HARRY A. EICK condensed on liquid nitrogen cooled copper targets which previously had been cleaned with dilute HC1, burnished with steel wool, washed, and subjected to background counting with the X-ray fluorescence spectrometer. To ensure reproducible geometry in analysis, the fraction of effusate was determined by a knife-edged insert placed inside the rim of the target during analysis. The effective perpendicular distance from orifice to target was obtained by adding the distance from crucible to target rim, as measured with a cathetometer, to the distance from the rim to the target face. Targets were exposed at both successively increasing and decreasing temperatures. Temperature measurements were made with a National Bureau of Standards calibrated Leeds and Northrup disappearing-filament-type pyrometer by sighting via a prism and windows into a blackbody hole drilled in the bottom of the effusion cell. Temperatures were corrected for prism and window reflectance, which was measured with a standard lamp. After exposure of the targets, replacement of the target holder by an optical window allowed comparison of orifice and blackbody hole temperatures over the experimental range. These measurements indicated a temperature difference of no more than *4". Molybdenum Knudsen cells with knife-edged orifices were used in all measurements. Planimeter measurements of micrographic photographs on which both the orifices and a micrometer slide were exposed indicated areas of 6.7 X loF4,21.1 X and 59.9 X cm2 for the three crucibles employed in the vaporization experiments. The cylindrical cells (internal height = internal diameter = 0.795 cm) were charged with 0.40.5 g of Eu304and 0.05-0.10 g of Eu203. X-Ray fluorescence analysis was employed for determination of the quantity of effusate condensed.6 A linear external standard curve was obtained with a Norelco generator, tungsten tube, and Siemens spectrometer for Eu Lp1 radiation (28 = 56.94" for LiF) over the 1-10 pg of Eu range. The LO1 radiation was found to be more intense than the La1 because of selective enhancement of the p transition by secondary a radiation of the copper target. The standard curve was prepared by weighing quantities (0.05-0.10 pg) of standard europium solution (50-100 pg/ml) onto copper targets, The fraction of effusate collected was defined reproducibly by the 45" beveled insert described previously. X-Ray diffraction patterns of the solid vaporization (2) H. Baernighausen, J . Prakt. Chem., 34, 1 (1966). (3) W.W.Wendlandt, J . Inorg. Nucl. Chem., 9, 136 (1959).
(4) D. Templeton and C. Dauben, J. Amer. Chem. Soc., 7 5 , 6069 (1953). (5) R. J. Ackermann, U. S. Atomic Energy Commission Report ANL-5482, Argonne National Laboratory, Argonne, Ill., 1955. (6) J. M. Haschke, R. L. Seiver, and H. A. Eick, to be submitted for publication.
THEVAPORIZA'FION THERMODYNAMICS OF TRIEUROPIUM TETROXIDE products were obtained with the Guinier camera. A sample removed from the oxide-molybdenum cell interface after completion of measurements at the highest temperatures was also analyzed crystallographically.
Results Both chemical and crystallographic results substantiate the presence of trieuropium tetroxide. Metal analysis of the europium oxide phase used in vaporization experiments together with the standard deviation indicated 87.62 f 0.35 wt % Eu (calculated as 87.67%) or EU304.03*0.02. The invariant X-ray powder diff raction patterns which were indexable on orthorhombic symm$y (ao = 10.089 f 0,009 A, bo = 12.056 f 0.009 As and co = 3.503 rt 0.004 A) agreed to within *0.01 A with lliterature X-Ray diff rabction results indicated that only Eu30.1 and B-Eu2O3 were present during the vaporization measurements. The powder diffraction data for the B-Eu203phase in equilibrium with Eu304 were indexable on monoclinic symmetry. Although lattice parameters reported by various investigators differ considerablyjZpresent values agree quite closely with those of Baernighausenz and Rau.8 No evidence for crucible interaction was detected. Combination of X-ray powder diffraction, weight loss, and mass spectrometric data indicated vaporization according to the equation 3E304(s)-+4Euz03(s,monoclinic)
+ Eu(g)
(1)
Weight-loss measurements yielded 99,2y0 of the theoretical changes far the above reaction. The mass spectrometric investigation also confirmed this reaction, since only masses attributable to europium-151 and europium-153 were observed over most of the temperature range. A t the maximum temperature (1700"), a faint spectrum of EuO(g) (masses 167 and 169) was observed. The relative intensity of Eu(g) :EuO(g) was determined to be 1 2 0 0 at 1700". Five independent vaporization experiments on the composite analyzed sample were made over the temperature range 1604-2016°K. A graph of log PE" vs. 1/T is presented in Figure 1. The linear least-squares equation with associated standard deviation which describes the 42 data points is 2.303R log
=
-8620"
T
* l4Oo + (28.20 f 0.82)
From this equation the following thermodynamic data, together with their standard deviations, are calculated for reaction 1: AH"1glo = 86.2 f 1.4 kcal/gfw and AX"1g10 = 28.20 * 0.82 eu. These second-law values have been corrected to 298°K by use of published heat content and enthalpy data for the monoclinic sesquioxideg and gaseous europium1° and by use of estimated
4237
"i 4 -0
6.01
,
48. 30 A
,
5.0
,
,
5.4
, 5.8
,
,4 62
I / T x 10'
Figure 1. Pressure of gaseous europium in equilibrium with trieuropium tetroxide.
values for the heat capacity of the tetroxide. heat capacity was approximated as
This
Cp(Eu304)= 1.5CP(EuzO3)- 0.5Cp(oxygen) The heat capacity equation for B - E ~ z 0 3and ~ Kopp's approximation for oxygen in solids" were employed to give C,,(E~304) = 43.61 (6.24 X lo-')), H O T H0288 = 43.61T (3.12 X 10-3)T - 13,270, and S O T - S"zgs = 43.61T (6.24 X 10-')T - 250.31. The results of this data reduction, with the listed error indicating the composite of standard deviation and estimated error in the thermodynamic values, follow: AH"298 = 93.5 f 2.5 kcal/gfw and Ahso298 = 39.4 I 1.7 eu. Third-law calculations have been made using free energy functions calculated from published datag,l0and the approximated high-temperature data. No experimental X"Q98 data are available for either the sesquioxide or tetroxide. Thus the S O 2 9 8 value (35 eu) estimated by Westruml was used for the sesquioxide, and the entropy of the tetroxide was obtained, using the following method and data given by Westrum.l To the sum of the lattice contributions of europium and oxide ions (38 eu) was added the magnetic contribution of two Eu(II1) ions (7 eu) and the magnetic value estimated for EuO (4 eu), for a 8 " 2 9 8 value of 49 eu.
+
+
+
(7) H. Baernighausen and G. Brauer, Acta Crystallogr., 15, 1059 (1962). (8) R. C. Rau, "Rare Earth Research 11," K. Vorres, Ed., Gordon and Breach, New York, N. Y., 1964, p 117. (9) L. B. Pankratz, E. G. King, and K. K. Kelley, U. S. Bureau of Mines Report of Investigations, 6033, U. S. Department of the Interior, Washington, D. C., 1962. (10) R. Hultgren, private communication. (11) G. N. Lewis and M. Randall, revised by K. 8. Pitzer and L. Brewer, "Thermodynamics," 2nd ed, McGraw-Hill Book Co., Inc., New York, N. Y., 1961. Volume 72. Number 12 November 1968
JOHN M. HASCHKE AND HARRY A. EICK
4238
Combination of these data yielded a third-law ennoninteracting crucible material for growing single thalpy of AH"298 = 92.28 f 0.56 kcal/gfw, with no crystals of europium monoxide from the melt at temtemperature trend observable in the data. peratures 200-300" above the present temperature Combination of the second-law enthalpy of reaction 1 maximum. l5 and published enthalpies of formation of the sesquiThe lack of essential thermodynamic data for both oxide12 and gaseous europium1° yield for the enthalpy the sesquioxide and tetroxide has necessitated the use of formation of trieuropium tetroxide a value of of several approximations in data reduction, The AHf0~98(EusO4)= -542.4 f 3.6 kcal/gfw. The scheme for measuring entropy values a t 298°K for error limits reflect the estimated uncertainty in data the lanthanide oxides is obviously quite good. It reduction and a 2.6-kcal discrepancy in measured enshould be noted that the stoichiometry of reaction 1 is thalpies of formation for E u ~ ~The ~ entropy . ~ ~ , such ~ ~that a change of 1 eu in the entropy or the free change for reaction 1, the entropy of formation of energy functions of Et1203 or Eu304 gives rise, respecgaseous europium,1° and the estimated entropy of tively, to a 7.2- or 5.4-kcal change in the third-law formation for Euz03l have been used to give ASfozgs enthalpy calculated at 1800"Ii. The necessity for (EuaOd) = -94.1 f 2.6 eu, which when comaccurate, or at least internally consistent, approximabined with the enthalpy value yields = -514.4 tions is obvious for the attainment of second- and thirdf 3.6 kcal/gfw. An entropy of solid Eus04has been law enthalpy agreement. The approximation for obtained from the entropy of europium gas,'O the esti8"298(EUaO4)based on Westrum's scheme (49 eu) agrees mated entropy of EuzO3,l and the second-law entropy with that obtained experimentally (48.6 eu) ; however, change. This calculation yields 8"298(EUeO4) = 48.6 f the present data do not verify the absolute accuracy 2.6 eu, with the error indicating the estimated uncerof the estimates but rather the expected difference betainty in A8'298 plus a 2.0-eu uncertainty in the Eu203 tween the entropies of Eu304and Eu203. Likewise, the value. high-temperature heat capacity approximation and related enthalpy and entropy functions must be reasonDiscussion ably accurate for agreement of second- and third-law In light of high-temperature measurements made of calculations. Since the second-law data are believed to the vapor species in equilibrium with Eu203,l4 the be accurate, the close agreement of second- and thirdpresence of gaseous EuO should be anticipated at the law values is probably more meaningful as a test of the upper end of the temperature range. The sesquioxide thermodynamic approximations than of the experivaporizes congruently by two competing modes to mental data. produce in one case Eu(g) and O(g) and in the other Although the present data should be reevaluated as EuO(g) and O(g). These two equilibria and reaction better data for Euz03 become available, consistency of 1 are all subject to the gas-phase equilibrium described the enthalpy and the entropy values with other therby the reaction mochemical data is evident. The enthalpy of formation of Eu8O4(s) (- 542.4 ltcal/gfw) would be expected EuO(?d _r E u ( d O(g) (2) to be more negative than the sum of the enthalpies of formation of EuO16 and Eu20a9 (-539.1 kcal/gfw). If Panish's pressures14for Eu(g) and EuO(g) at 2000°K Likewise, the enthalpies of formation of the europium are used, an approximate equilibrium constant can be oxides should become increasingly negative with incalculated for reaction 2. If the congruent vaporizacreasing oxygen content, as follows: AHf0z9g(EuO) = tion of Eu203to Eu(g) and O(g) is assumed t o be the - 145.2 kcal/gfw,16 AHr"z98(E~OI.33) = - 180.8 kcal/ dominant mode, the pressure of monatomic oxygen can gfw, and AHr0zes(Eu01.so)= - 196.9 It~al/gfw.~The be Calculated from the observed europium pressure by entropy of formation (-94.1 eu) agrees well with values taking cognizance of the mass differences. A value for reported for other M 3 0 4 phases? Fe304 (-82.5 eu), KZ = 2.6 X atm is thereby obtained. If the MnsO4 (-85.2 eu), and Pbs04 (-94.0 eu). Although oxygen pressure at the extremum dictated by the two the absolute entropy value is less easily evaluated vaporization modes of Eu203is fixed and if the present because of the magnetic nature of europium, 8'298 = experimental pressure of Eu(g) at 2000"13: is used, anticipated upper and lower limits for the EuO(g) (12) E. J. Huber, G. C. Fitzgibbon, and C. E. Holley, J . Phys. Chem., pressure can be set for the present system to be 2 X 6 8 , 2720 (1964). (13) J. M. Stuve, U. S. Bureau of Mines Report of Investigations, 10-6 < PEuO (atm) < 7 x 10-5. The value observed 6640, U. S. Department of the Interior, Washington, D. C., 1965. mass spectrometrically is 5 3 x lo+. Thus the con(14) M.B. Panish, J . Chem. Phys., 34, 1079 (1961). tribution of europium monoxide to the target collection (15) C. F. Guerci and M.W. Shafer, J . Appl. Phys., 37, 1406 (1966). measurements is inconsequential, and, therefore, no (16) J. L. Burnett and B. B. Cunningham, U. S. Atomic Energy correction of the data has been undertaken. Commission Reaort URCL-11126. Lawrence Radiation Laboratory, Berkeley, Calif.: 1964. NOindications for involvement of molybdenum in the (17) F. D. Rossini, D. D. Wagman, w. H. Evans, 8. Levine, and equilibrium could be found by visual or crystallographic I. Jaffe, Circular 500, National Bureau of Standards, U. S. Government Printing Office, Washington, D. c., 1952. analysis. Molybdenum has been found to be a suitable,
+
The Journal of Physical Chemistry
THEREACTION OF TRIFLUOROMETHYL RADICALS WITH HYDROGEN SULFIDE 48.6 eu is consistent with the values for Mn304 (35.5 eu), in light of the Fe304 (35.0 eu), and Pb304 (50.5 mass diff erencee of the metals.
4239
Acknowledgment. The support of the U. S. Atomic Energy Commission (COO-716-037) is gratefully acknowledged.
The Kinetilcs of the Reaction of Trifluoromethyl Radicals with Hydrogen Sulfide1 by Jayavant D. Kale and Richard B. Timmons Department of Chemistry, The Catholic University of America, Washington, D. C. 80017
(Received June 9, 1068)
The gas-phase reactions of trifluoromethyl radicals with hydrogen sulfide have been studied over the temperature range of 95-161". The trifluoromethyl radicals were generated by photolysis of hexafluoroacetone. The rate constant for the reaction CF3 HzS CF3H SH is found to be k = 101l*z*O.l exp(-1200 It 100)/RT cc mol-' sec-'. This value is calculated using the reported value for trifluoromethyl radical recombination of 2.3 x 10lacc mol-' sec-l.
+
Introduction The photocliemistry of hexafluoroacetone (HFA) has been the subject of many papers since the original work of Ayscough and Steacie.2 Recently, much emphasis has appeared on fluorescent measurements in HFA photochemistry. From these studies a rather detailed picture of the photochemistry of this molecule is emerging. 3 , 4 I n addition, the importance of chemical quenching of excited HFA molecules has been raised. The photolysis of HFA has been used extensively as a source of CFa radicals for kinetic studies. One of the recent papers on CF3 reactions is the work of Arthur and who studied the kinetics of abstraction of hydrogen atoms from HzS by CF3 radicals. At the time this paper appeared we were in the process of investigating this reaction over somewhat different reaction conditions and under conditions such that any chemical quenching of HFA by H2S would be minimized. The kinetic parameters determined in our studies differ from those reported in ref 5, and as such, our results are of interest. Experimental Section Materials. The HFA (Allied Chemical Corp). was degassed initially a t -196" and was then allowed to distil to a trap a t -145" to --150" (2-methylbutane slush) where further degassing took place. It was found necessary to carry out degassing at a temperature of --150° in order to remove sizable C2Fs and CF3H impurities in the HFA. The HFA was then distilled from a trap a t -80" to a storage bulb with only the middle fraction being retained for reaction.
--f
+
Hydrogen sulfide, research grade (Matheson), was degassed a t -196" and then distilled from a trap at -80" to the storage bulb. Mass spectrometric analysis failed to reveal any impurities. Apparatus. Reactions were carried out in a 200-cc cylindrical quartz reaction vessel. The total reaction system actually comprised a volume of 5.1 1. and included an all-glass circulation pump.6 The gases circulated through the quartz reaction vessel which was suspended in a tubular furnace. Photolysis was carried out with a Hanovia medium-pressure mercury ltmp. The effective wavelengths were limited to X >3100 A by insertion of a Pyrex filter between the lamp and the reaction cell. With this filter arrangement, no H2 was produced by circulating 20 mm of HzS through the cell for 30 min with the lamp turned on. Reactions were carried out over a temperature range of 95-161". After a run, the products noncondensable a t -196" were collected and measured in a combination Toepler pump-gas buret. The per cent conversion of the H2S v a s never allowed to exceed 5% in any given run. The temperature of the traps containing the (1) Abstracted, in part, from the Ph.D. Thesis of J. D. Kale, The Catholic University of America, Washington, D . C. 20017. (2) P. B. Ayscough and E. W. R. Steacie, Proc. Roy. SOC., A234, 476 (1956). (3) R. K. Boyd, G. B. Carter, and K. 0. Kutschke, Can. J . Chem., 46, 175 (1968). (4) J. S. E. McIntosh and G. B. Porter, Trans. Faraday SOC.,64, 119 (1968). (5) N.L. Arthur and T. N. Bell, Can. J . Chem., 44, 1446 (1966). (6) J. 8. Watson, Can. J . Technol., 34, 373 (1956). Volume 78, Number 18 November 1968
