2308
H. A. PORTE, E. GREENBERG, AND W. N. HUBBARD
Fluorine Bomb Calorimetry. XII.
The Enthalpy of Formation
of Ruthenium Pentafluoride"'
by Howard A. Porte, Elliott Greenberg,3and Ward N. Hubbard Chemical Engineering Division, Argonne .Vational Laboratory, Argonne, Illinois
(Received February 23, 1966)
The energy of formation of ruthenium pentafluoride was measured by direct combination of the elements in a combustion bomb calorimeter. The standard enthalpy of formation, AHfo298.15, was calculated to be -213.41 f 0.35 kcal. mole-'.
Introduction Higher fluorides of transition metals have been the object of considerable recent experimental and theoretical i ~ i t e r e s t . ~Properties of these materials, with few exceptions, are known only qualitatively. This investigation is part of a continuing program5 to obtain precise thermochemical data by fluorine bomb calorimetry, and deals with the enthalpy of formation of ruthenium pentafluoride. RuF5was first prepared by Ruff and Vidic6 by passing fluorine over finely divided ruthenium. This reaction probably constitutes the easiest method for making the compound. In the pure form a t room temperature, RuF6 is a pale green solid with a vapor pressure of about 0.1 nimS7 RuF6 reacts with moisture immediately to form HF, Ru04, and lower oxides. Because of the reactivity of RuF5 arid the great difficulties experienced in handling it, the compound has not been studied extensively. Even the melting point and boiling point are in d o ~ b t . ~ , ~
Materials. Specially fabricated high-purity ruthenium, in the form of 3-mm. diameter rod and 0.1-mm. foil, was generously donated by Mr. R. Vines of The International Nickel Co., to whom thanks are due. The impurities in the samples are summarized in Table I11 and will be discussed in a later section. Purified fluorine (99.94%) was prepared by distillation of commercial fluorine in a low-temperature still.g-ll Because of the unavailability of ruthenium wire, high-purity molybdenum wire,120.1 mm. in diameter, was used as fuse material. Combustion Technique. The sample arrangement, ignition, and combustion technique were essentially
(1) This work was performed under the auspices of the U.S. Atomic Energy Commission. (2) Presented in part at the 18th Calorimetry Conference in Bartlesville, Okla.. Oct. 1963. (3) T o whom inquiries concerning this paper should be addressed. (4) B. Weinstock, Chem. Eng. News, 42, No. 38, 86 (1964). (5) E . Greenberg. C. A. Natke, W. N. Hubbard, J . Phys. Chem., 6 9 , 2089 (1965) : see also previous papers in this series. (6) 0. Ruff and E . Vidic, 2. anorg. allgem. Chem., 143, 163 (1925). Experimental (7) J. H. Holloway and R. D. Pencock, J . Chem. Soc., 527 (1963). Calorinaetrzc System. The calorimeter, laboratory (8) H. A. Bernhardt, R. L. Farrar, Jr., R . A. Gustison, and S. S. Kirslis. "The Preparation of Ruthenium Pentafluoride and the Dedesignation ANL-R1, arid combustion bomb, laboratermination of I t s Melting Point and Vapor Pressure," U. S.Atomic tory designation Si-5, have already been d e s ~ r i b e d . ~ ~ Energy '~ Commission Report AECD-2390 (1948). (Available from Office of Technical Services, U. S.Department of Commerce, WashThe system was calibrated with benzoic acid (Il'ational ington 25, D. C.) Bureau of Standards Samples 39h and 39i) whose (9) E. Greenberg, J. L. Settle, H. M. Feder, and W. N . Hubbard, J . Phys. Chem., 65, 1168 (1961). certified energy of combustion was 26.434 f 0.003 abs. (10) E. Greenberg. J. L. Settle, and W. N. Hubbard, ibid.. 66, 1345 kjoules g. -l under prescribed conditions. Kine ex(1962). periments, some preceding and some following the (11) L. Stein, E. Rudzitis, and J. L. Settle, "Purification of Fluorine ruthenium combustions, yielded an average value for by Distillation," Argonne National Laboratory, ANL-6364 (1961). (Available from Office of Technical Services, U. S. Department of &(calor.), the energy equivalent of the calorimetric Commerce, Washington 25, D. C.) system, of 3545.11 cal. deg.-', with a standard devia(12) J. L. Settle, H. M .Feder, and W. N . Hubbard, J . Phye. Chem., tion of the mean of 0.4 cal. deg.-'. 65, 1337 (1961).
The Journal of Physical Chemistry
ENTHALPY OF FORMATION OF RUTHENIUM PENTAFLUORIDE
the same as describedg for the combustion of zirconium in fluorine. The sample consisted of a 0 . 5 - ~ m .piece ~ of foil inserted in a slot in the end of a 3.5-cm. length of rod. The bomb was evacuated overnight with an oil diffusion pump in order to eliminate traces of moisture and was then charged with 3900 mm. of pure fluorine. A sample exposed to fluorine under these conditions showed no significant change in weight. The calorimetric measurements were made in the usual manner. Post-Combustion Examinations. iifter one of the calorimetric experiments, the gas in the bomb was condensed into a liquid nitrogen cold trap, fluorine and other permanent gases were pumped off, and the condensable gases were evaporated into an infrared absorption cell equipped with silver chloride windows. 80 absorption was observed in the infrared region of the spectrum. This was taken to indicate that ruthenium hexafluoride was not present since it shows a strong absorption peak13at 735 cni.-'. The sensitivity of this absorption peak mas such as to fix the possible RuF6 concentration a t less than 0.1% of the total ruthenium combustion product. Immediately after the bomb was opened (in an inertatmosphere glove box) , the only visible combustion product was a uniform green solid which was determined to be ruthenium pentafluoride by comparison of its X-ray powder diffraction pattern with that reported by Holloway, Peacock, and Small.14 No lines due to RuF3, RuO2, Ru, or ruthenium oxyfluorides were detected. However, X-ray diffraction analysis might have failed to detect small amounts of these possible Contaminants or lower fluorides formed during combustion. More sensitive experiments were carried out in which weighed samples were heated in vacuo in order to evaporate away the relatively volatile RuF5 product. Such experiments were made difficult by the fact that in handling RuF6 it reacts readily to yield nonvolatile hydrolysis products. In the most successful experiments a nonvolatile residue of 0.4y0 was obtained. Thus, a t least 99.6% of the combustion product was RuF6. We believe that the 0.4% nonvolatile residue was a hydrolysis product formed in handling the RuF5 rather than a lower fluoride of ruthenium formed during combustion. The combined evidence of the post-combustion examinations has been interpreted to mean that the reaction studied was Ru(c)
+ 5/~F~(g)
+RuFdc)
(1)
The amounts of unburned ruthenium and molybdenum were determined as follows. The interior of the bomb was washed with water, and the unburned stub of ruthenium rod was scrubbed with a brush to
2309
remove the adhering insoluble hydrolysis products, dried, and weighed. The difficulty in effecting a quantitative separation and determining the mass of unburned ruthenium is reflected in tthe precision of the results. The remnants of the molybdenum fuse wire were isolated and weighed separately.
Results The results of six combust'ion experiments, expressed in terms of the defined calorie equal to (exactly) 4.184 absolute joules, are summarized in Table I. The corrections to standard states were applied in accordance with the procedure illustrated for t,he combustion of molybdenum in fluorine.'5 The entries in the table are identical wit,h those previously used and explained.'O For the calculation of items 5 and 7, the auxiliary data shown in Table I1 were used. The contents of the bomb included 78.17 g. of nickel and 0.17 g. of Teflon. The internal volume of the empty bomb was 0.318 1. The value of -3868.4 cal. g.-I for the energy of conibustion of inolybdenum12 was used for the calculation of item 8. For tlhe calculation of it,em 9, the data shown in Table I11 were used. Except for oxygen, which was assumed to be present as Ru02, all of the impurities in the sample were assumed to be present' uncombined. The corrections for hydrogen and nitrogen are negligible and the assumptions made regarding their presence in t'he sample are therefore not critical. The net correction made for all impurities was 2.19 + 0.09 cal. g.-l of ruthenium sample burned. The remaining corrections to standard states were all negligible. AEc"/M is just the sum of items 4 through 9 divided by the mass of sample burned. The energy of formation (AEf" = AEcO) and enthalpy of formation ( AHfO) of ruthenium pent,afluoride at 25", in accordance with reaction 1, were calculated to be -211.93 + 0.35 and -213.41 f 0.35 kcal. mole-', respectively. The atomic weight,16 of rut'henium was taken as 101.07. The uncertainties given are uncertainty intervals'' equal t,o twice t,he combined standard deviations arising from known sources. The uncertainty in the results is due primarily t'o t'he un(13) H. H. Claassen, H. Selig, J. G. Malm, C. L. Chernick, and B. Weinstock. J . A m . Chem. Soc., 8 3 , 2390 (1961). (14) J. H. Holloway, R. D. Peacock, and K . W. H. Small, J . Chem. Soc., 644 (1964). (15) W. N. Hubbard in "Experimental Thermochemistry," Vol. 11. H. A. Skinner, Ed., Interscience Publishers. Ltd.. London. 1962, Chapter 6. (16) A. E. Cameron and E. Wichers, J . A m . Chem. SOC.,84, 4175 (1962). (17) F. D. Rossini in "Experimental Thermochemistry," F. D. Rossini, Ed., Interscience Publishers, Inc., Kew T o r k , N. Y.. 1956, Chapter 14.
Volume 69,Number 7 J u l y 1966
2310
H. A. PORTE, E. GREENBERG, AND W. N. HUBBARD
Table I: Results of Combustion Experiments
1. 2. 3. 4. 5.
6. 7. 8. 9. 10.
Combustion no. m', g. Atc, deg. &(calor.)(- A t , ) , cal. AEeontt.nts,cal. AElgnltlon,cal. AEga8,cal. AEM,fuse, cal. AElmpurltles,cal. AEc"/M, cal. g.-l
1 2.21286 1,31456 - 4660.26 -11.36 0.58 -0.42 14.58 4.85 - 2102.27
2 2.36557 1.39981 - 4962.48 -12.26 0.45 -0.59 15.78 5.18 -2094.18
3 2.30688 1.36703 -4846.27 -11.99 0.58 -0.59 15.32 5.05 - 2097.16
4 2.63436 1.56054 - 5532.29 -13.76 0.49 -0.60 17.60 5.77 - 2096.44
7 2.24629 1.32878 - 4710.67 -11.66 0.49 -0.59 16,29 4.92 - 2092.88
8 2.27021 1.34577 - 4770.90 -11.80 0.45 -0.58 14.82 4.97 - 2098.06
Mean AEe"/M = -2096.8 cal. g.-l Std. dev. of mean = 1.3cal. g.-l
*
Table 11: Auxiliary Data for Calculation of Items 5 and 7 in Table I Substance
Ni Teflon Ru RuF~ Fdg)
cp, cal. deg. -1 g. - 1
P,
g. cc.-1
8.907b 2,24" 12.ld 3.92'
0.1061" 0.28' 0.0569" 0.16"
Table 111: Data for Calculation of Item 9 in Table I
' A
a R. Hultgren, R. L. Orr, P. D. Anderson, and K. K. Kelley, ['Selected Values of Thermodynamic Properties of Metals and Alloys," John Wiley and Sons, Inc., New York, N. Y., 1963, pp. 198,243. * H. E. Swanson and E. Tatge, "Standard X-Ray Diffraction Powder Patterns," Vol. I, National Bureau of Standards Circular 539,U. s. Government Printing Office, Washington, D. C., 1953,p. 13. W.D. Good, D. W. Scott, and G. Waddington, J. Phys. Chem., 60,1080(1956). Experimental determination. A. Glassner, "The Thermochemical Properties of the Oxides, Fluorides, and Chlorides to 2500"K.," Argonne National Laboratory, ANL-5750 (1957). (Available from U. S. Government Printing Office, Washington, D. C.). See ref. 14. ' C , = 5.50cal. deg.-lmole-'; W. H. Evans, T. R. Munson, and D. D. ~ ~ Wagman, J . Res. Natl. Bur. Std., 55,147(1955). c ( =~ 0.000803 atm.-l and (dEI/dP)298 = -1.781 cal. atm.-l mole-'; see J. 0. Hirschfelder, C. F. Curtiss, and R. B. Bird, "Molecular Theory of Gases and Liquids," John Wiley and Sons, Inc., New York, N. Y., 1954,and D. White, J. H. Hu, and H. L. Johnston, J . Chem. Phys., 21, 1149 (1953).
'
Impurity
C" 0" Ha N" Fe
os
Rh Ni Pd MO
Impurity concn.. p.p.m.
Assumed combustion product of impurity
135 400 0.5 0.7 750 100 50 20 10 2
AHfozqs of combustion product, kcsl. mole-'
-221b OC -64.@ 0 -235* -215e - 175' - 159.5" - 112* - 372.4'
Impurity correction, cal. g.-l of Ru
2.19 -1.49 0.03 0.00 1.57 -0.09 -0.02 0.01 -0.01 0.00
' Chemical analysis by Chemical Research Services, Inc., Addison, Ill.; all others are spectrochemical data reported by The International Nickel Co. These values have been tentatively adopted by the National Bureau of Standards, Washington, D. C., for the revised edition of NBS Circular 500. ' A H f o z g s (RuOz) = -52.5 kcal. mole-'; see "Selected Values of Chemical Thermodynamic Properties," National Bureau of Standards Circiilar 500, U. S. Government Printing Office, Washington, D. C., 1952. L. Brewer, L. A. Bromley, P. W. Gilles, and N. L. Lofgren, "The Chemistry and Metallurgy of Miscellaneous Materials: Thermodynamics," L.L. Quill, Ed., McGraw-Hill Book Co., Inc., New York, N. Y., 1950, pp. 76-192. e Estimated. See footnote e of Table 11. See ref. 12.
certainties in the impurity corrections and in determining the mass of unburned ruthenium. The only previous value for the enthalpy of formation of RuF6 is an estimate'* of - 300 kcal. mole-'.
ments, R. V. Schablaske for X-ray diffraction analyses, and H. hI. Feder for his continued interest and suggestions during this research.
Acknozuledgment. We wish to thank C. A. Natke and R. Terry for making the calorimetric measure-
(18) See footnote e of Table 11.
The Jwrnal of P h y a k l Chemistry
