ENTHALPY OF FORMATION OF YTTRIUMTRIFLUORIDE
values obtained for the constant C in the B.E.T. equation are larger for the lower density UOz samples. This may indicate that heat of adsorption for the first layer of xenon is higher when densities are low, which is in agreement with Smith's results' for krypton adsorption on UOZ.
Conclusions It is possible to measure small surface areas of about 1 cm.2with a precision of h O . 1 cm.2 using a mixture of natural xenon and xenon-133. The xenon vapor pressure a t liquid nitrogen temperature was found to be 2.15 torr after a thermal transpiration correction. X The surface area measured for glass samples is equal
Fluorine Bomb Calorimetry, XI.
2305
to the geometrical surface area. For sintered UOe, the B.E.T. surface area is much larger than the geometrical surface area, and the ratio between these increases as the density decreases. The described technique makes it possible for one to measure the surface areas of high activity samples (irradiated U0z for instance).
Acknowledgment. The authors thank 1Iiss Y. Carteret (CEN Saclay) very much for her kind collaboration. They are grateful for the critical comments of their colleagues of their laboratory (CEN Grenoble). (7) T. Smith, NAA-SR-53, October 1960, p. 19.
The Enthalpy of Formation
of Yttrium Trifluoridel
by Edgars Rudzitis, Harold M. Feder, and Ward N. Hubbard Chemical Engineering Division, Argonne National Laboratory, Argonne, Illinois
(Received January 19, 1965)
The energy of formation of yttrium trifluoride was measured by direct combination of the elements in a bomb calorimeter. The standard enthalpy and Gibbs free energy of formation a t 298.15"K. were determined to be -410.7 f 0.8 and -393.6 f 1.0 kcal. mole-', respectively .
This work, which is part of a series of fluorine bomb calorimetric studies, was prompted by the unavailability of thermochemical data on YF3, an important intermediate in the production of metallic yt,trium. The combustion method employed was developed earlier.2 An yttrium metal sample was suspended in a fluorine-filled combustion bomb and ignited electrically. The interior surfaces of the bomb werc protected by a tamped liner of YF3. Extensive experimentation showed that adequate combustion of yttrium in fluorine could be achieved only if the temperature of the burning metal was well in excess of its melting point (1509") and high enough to volatilize the reaction product YFa (b.p. -2300").
Experimental Materials. (a) Y . An yttrium ingot (Lunex Co., nuclear grade) was rolled to a sheet approximately 1 mm. thick. Chemical and spectrochemical analysis showed the following significant impurities (in p.p.m.) : Ta, 2500; 0, 1390; C, 188; H, 42. If the impurities are assumed to be present as the chemical species, Ta, Y203, YCZ, and YH2, the calculated content of elemental yttrium in the sample is 99.37 mole yG. (1) Work performed at Argonne National Laboratory, operated by the University of Chicago under the auspices of the U. S. Atomic Energy Commission, Contract No. W-31-109-eng-38. (2) E. Rudzitis, H. M. Feder, and W. N. Hubbard, J . Phys. Chem., 68, 2978 (1964).
Volume 69, A'umber 7
J u l y 1965
EDGARS RUDZITIS, HAROLD M. FEDER, AND WARDN. HUBBARD
2306
( b j Fz. The fluorine used was purified by distillation. Its impurity content was approximately 0.01%. (c) YF3. The material was prepared by the reaction of Y z O ~and HF in a fluidized bed3 and contained approximately 0.1% oxygen. It was further purified by autoclaving with a mixture of anhydrous hydrogen fluoride and chlorine trifluoride at 250". This treatment reduced the oxygen content to below the limit of detection. Procedurea. The apparatus and experimental procedures used in this work were analogous to those described for combustions of magnesium in fluorine.2 The calorimetric samples were preconditioned by exposure to fluorine. The bomb was initially charged with 8 atm. of F2. After combustion, the unburned metal residue was determined by the hydrogen evolution method The well-crystallized reaction product of the combustion, YF,, was identified by its X-ray diffraction pattern.
Results All combustion experiments attempted for the series were acceptable; the results are summarized in Table I. Energy quantities are expressed in terms of the defined calorie equal to (exactly) 4.184 absolute joules. The entries in the table are: (1) the mass of yttrium sample burned (percentage of introduced sample) , (2) the observed increase in the calorimeter temperature corrected for the heat exchanged between the calorimeter and its surroundings, (3) the energy equivalent of the (Galorimeter multiplied by the negative of the corrected temperature increase, (4) the energy equivalent of the initial and final contents of the bomb, each multiplied by its appropriate portion of the corrected temperature increase (the sum of itenis 3 and 4 is the total evolved heat corrected to the isothermal bomb process at 2 5 " ) , (5) the net correction of the bomb gas to the standard state, (6) the electrical energy input for the ignition, (7) the net impurity correction, and (8) the energy change per gram of yt3/2F2(gj + YF3(c) with trium for the reaction Y(c) the reactants arid products in their respective standard states a t 25". For the calculation of item 1, the mass of sample introduced was corrected for unburned yttrium and for the increase in weight owing to preconditioning. The latter corrections were based on the assumption that YF, was the product formed. The weight increases 0'69, 0'47J 0'43, 0'57J0'46, 0'52, and O ' j 2 mg' for the seven experiments as listed. Item 3 was calvulated using a value of &(calor,)of 3433.45 cal. deg.-'. This was the result of a series of six benzoic acid Calihation experiments. The standard deviation of the
+
The Journal of Physical Chemistry
mean was 0.79 cal. deg.-'. (The same value was used for the magnesium studies2 because magnesium and yttrium combustions were carried out alternately in a continuous series.) For calculation of item 4 the following values were used: C,: Y,5 6.34; S i J 5 6.23; NiF2,6 15.31 cal. mole-' deg.-'; C,: FZJ7 5.50 cal. mole-' deg.-'. The heat capacity of YF3 was estimated to be 22.2 cal. mole-' deg.-' by the application of Kopp's law to the related structures8 of YF3 and CeF3. The following auxiliary values were used: C,: CeF3,9 22.34; Ce,5 6.44 cal. mole-' deg.-'. The contents of the bomb were 0.2 g. of ?UTiF2,196.8 g. of Si and the varying amounts of YF3 of 244.6, 228.2, 231.9, 222.4, 206.5, 193.0, and 182.5 g. for the seven experiments listed. The volume of the empty bomb was 0.354 1. The corrections to the standard states were applied in the usual manner.'o Item 7 was obtained by calculating the thermal effects of the reaction of impurities with fluorine and subtracting the thermal effect which would result if an equal mass of yttrium reacted with fluorine. The following AHf02g8values were used (in kcal. mole-'): ITF3,l' -410.7; TaF6,12 -455.0; Y203,13-455.5; YC2,14-52; CFqJ15-220.4; HF,15 -64.8. The uncertainty in the impurity corrections was estimated to be f 2 cal./'g. of sample. Item 8 was calculated by summation of items 3 through 7 and division by item 1. The following standard thermal data in kcal. mole-' were derived for the formation of YF3(c) according to eq. 1: AEfo = - 409.8 f 0.8; AHrO = -410.7 f 0.8; A G f O = -393.6 f 1.0. The uncertainties in AEfO and AHf" equal twice the combined standard deviations arising from (3) I. E. Knudsen and N. M . Levitz. ANL-6011, May 1959. (4) E. Rudzitis. H. M.Feder, and W. N. Huhbard, J . P h y s . Chem., 67, 2388 (1963). ( 5 ) R. Hultgren, R. L. Orr, P. D. Anderson, K. K . Kelley, "Selected Values of Thermodynamic Properties of Metals and Alloys," John Wiley and Sons, Inc.. New York, N . Y., 1963. (6) E. Catalan0 and J. W. Stout, J . Chem. Phys., 23, 1284 (1955). (7) W. H. Evans, T. R. Munson, and D. D. Wagman. J . Res. 1Vatl. Bur. Std., 5 5 , 147 (1955). (8) A. Zalkin and D. H. Templeton, J . Am. Chem. SOC.,75, 2453 (1953). (9) E. F. Westrum, Jr.. and A. F. Beale, Jr.. J . P h y s . Chem., 6 5 , 353 (1961). (10) W.N. Huhhard in "Experimental Thermochemistry," Vol 11, H . A. Skinner, Ed., Interscience Publishers Ltd., London, 1961, Chapter 6. (11) work. (12) E. Greenberg, C. A. Natke, and W. K. Huhbard. J . P h y s Chem., 6 9 , 2089 (1965). (13) E. J. Huher, Jr., E. L. Head, and C. E. Holley, Jr.. i b i d . , 61, 497 (1957). (14) Estimated. (15) G. T. Armstrong. Chemical Propulsion Information Agency Publication No. 44 ( U ) , Val. I , Johns Hopkins University, Silver spring, Md., 1964, p , 59.
ENTHALPY OF FORMATION OF YTTRIUM TRIFLUORIDE
2307
Table I : Results of Yttrium Combustion Experiments" Expt. no. 1
1. Mass, g.
(76) 2. Atc,b deg. 3. &(calor.)(- Ato), 4. 5. 6. 7. 8.
cal . AE(cont.),' cal. AE(gas), cal. AE(ignition 1, cal. AE(impurities), cal. AE,"/M, cal. g.+
1.33322 (99.82) 1.75540 -6027.1 -102.5 -0.7
3
2
1.04151 (99.53) 1.36663
0.83878 (97.63) 1.09937
1.08003 (99.80) 1.42287
- 4692.3
- 3774.6
- 4885,4
-76.4 -0.5
-62.0 -0.4
-75.2 -0.5 0.1
-30.4
-23.8 -19.1 -4601.9 -4597.1 Mean A E , " / M = -4609.7 Std. dev. of mean, k3.2 cal. g.-l or 0.07%.
- 4620.9
T h e symbols are explained in ref. 10.
Ato
=
tf
- ti
- At,,,.
the scatter of AECo/hfvalues, the impurity correction, and the calibration data. The uncertainty in AGf" includes the estimated additional uncertainty in the ASf" value. The atomic weight of yttrium was taken at 88.905 g. (g.-atom)-'. The entropies S02g8 were taken to be: Y,I6 10.63; F2,I748.5; YF3, 26.2 cal. deg.-' mole-'. The latter value was an estimate based on So29s (CeF3) = 27.54 ~al.deg.-'.~ and Latimer's rule.l8 The present results are the only known thermochemical data on the formation of YF3, except for an estimate by Brewer, et ~ d . ,of' ~AHf"(YF3) = -397 k 7 kcal. mole-'.
Acknowledgments.
4
The authors wish to acknowledge
-24.6 -4616.2
7
6
0.91875 (99.78) 1.21214
7
6
0.81880 (99.74) 1.07710
-4161.8 -51.3 -0.4
-3698.2 -54.4 -0.4
-21.0 -4608.9
- 4606.2
-4919.1 -70.2 -0.5
-
-18.7
AE(cont.) = [Ef(cont.) - Ei(cont.)](25 - ti)
1.08618 (99.83) 1.43267
-24.8 -4616.6
+ Ef(cont.)(-Atc),
the help of F. J. Karasek for fabricating yttrium metal into sheets and L. Ross for valuable discussions. Thanks are due to R. Terry and E. Van Deventer for checking the manuscript and calculations. (16) L. D.Jennings, R. E. Miller, and F. H. Spedding, J . Chem. Phys., 33, 1849 (1960). (17) K . K . Kelley and E. G. King, U. S.Bureau of Mines Bulletin 592, U. S.Government Printing Office, Washington, D. C., 1961. (18) G. N.Lewis, M. Randall, K. S. Pitzer, and L. Brewer, "Thermodynamics," 2nd Ed., McGraw-Hill Book Co., Inc., New York, N.Y., 1961, p. 518. (19) L. Brewer, L. A. Bromley, P. W. Gilles, and N. L. Lofgren, "Chemistrv and Metallurnv of Miscellaneous Material. Thermodynamics," L. L. Quill, g d . , hfcGraw-Hill Book Co., Inc., New York, N. Y.,1950.
Volums 69, Number 7 July 1966
