ENTHALPIES OF FORMATION OF NIOBIUM AND TANTALUM PENTAFLUORIDES
various solvent systems are similar to those determined by other investigators for electrolytes in mixed and nonaqueous solvents. The general behavior is that deviations from the Debye-Hiickel theory occur at lower concentrations as the dielectric constant of the solvent is decreased.
Fluorine Bomb Calorimetry. X.
2089
Acknowledgment. The authors wish to thank the U. S. Atomic Energy Commission, which supported this work financially under contract AT-(40-1)-2069. Thanks are also due Union Carbide Chemicals Company for use of its IBM 7074 computer in calculating the values of E O .
The Enthalpies of Formation of
Niobium and Tantalum Pentduorides1,2
by Elliott Greenberg, Carol A. Natke, and Ward N. Hubbard Chemical Engineering Division, Argonne National Laboratory, Argonne, Illinois (Receiued January 18, 1966)
The energies of formation of niobium and tantalum pentafluorides were measured by direct combination of the elements in a bomb calorimeter. From these measurements the standard enthalpies of formation, AHf’288.16, of niobium and tantalum pentafluorides were calculated to be -433.50 f 0.15 and -454.97 f 0.19 kcal. mole-’, respectively.
Introduction The determination of the heats of formation of the pentafluorides of niobium and tantalum is part of a continuing program3to obtain precise thermochemical data by fluorine bomb calorimetry. Experimental Calorimetric System. The calorimeter, laboratory designation ANL-R1 , and combustion bomb, laboratory designation Ni-T, have already been described. Twelve calibration experiments were carried out with benzoic acid (National Bureau of Standards Sample 39i), some preceding and some followingthe fluorine combustions. The certified energy of combustion for this sample was 26.434 i0.003 abs. kjoule g.-I. &(calor.), the energy equivalent of the calorimetric system, was 3566.30 cal. deg.-l for the niobium experiments and 3566.24 for the tantalum experiments. In each case the standard deviation of the mean was 1 0 . 2 cal. deg.-l, or 0.005%. Materials. Samples of niobium and tantalum were 8l
obtained from the Wah Chang Corp. and the National Research Corp., respectively, in the form of 0.317-em. diameter rod, 0.0127-em. foil, and 0.0254-cm. diameter wire. The outer portion of the rods was removed by taking surface cuts with a small lathe, using a tungsten carbide tool bit and keeping the sample flooded with Chlorothene (inhibited l,l,l-trichloroethane). The machined samples were then stored under vacuum to minimize any possible surface oxidation. Samples for the combustion experiments and analyses were cut at appropriate positions along the length of a single rod. The impurities found in these samples are summarized in Table I. No other metallic impurities were de~
(1) This work waa performed under the auspices of the U. S. Atomic Energy Commission. (2) Presented in part at the 18th Calorimetry Conference in Bartlesd e , Okla., Oct. 1963. (3) E. Greenberg, J. L. Settle, H. M. Feder, and W. N. Hubbard, J . Phgs. Chem., 65, 1168 (1961). See also succeeding papers in this series. (4) E. Greenberg, J. L. Settle, and W. N. Hubbard, ibid., 6 6 , 1345 (1962).
Volume 69, Number 6
June 1966
2090
ELLIOTT GREENBERG, CAROL A. NATKE,AND WARDN. HUBBARD
tected. The chemical states of the impurities are unknown, but the assumed states of combination are indicated in the table. In each case, the foil and wire samples were of comparable quality to the rod samples and, because these items constituted only a small fraction of the total sample, it was not considered necessary to analyze t’hem for their specific impurity contents.
base of the sample. The assembled bomb was always pretreated with fluorine at operating pressure for a few minutes before final evacuation and charging. The calorimetric measurements were made in the usual manner. Surface Fluorination Experiments. Trial exposures of weighed samples to fluorine indicated a definite spontaneous surface reaction. Since this reaction occurs before intentional ignition of the sample, the associated Table I : Impurities in the Samples thermal effect represents heat evolved which is not detected during the calorimetric experiment, thereby Assumed form ,-P.p.m. necessitating a correction. Therefore, in carrying out of impurity Impurity Nb Ti3 the calorimetric experiments a special effort was made MCb 38 22 C” to reproduce as closely as possible the elapsed time be83 68 05 Mz0: tween first exposure of the sample to fluorine and the 8 0.2 Solid s o h H” initiation of the combustion reaction in the calorimeter. MNb N“ 50 16 Ti Ti 94 . .. After the calorimetric combustions were completed, a W W 60 8 series of blank experiments was run with each metal in .. 51 Fe Fe which typical weighed samples were loaded and treated Zr Zr 50 ... in a manner identical with the procedure employed for . . 16 Nb Nb the calorimetric runs. After exposure of the sample to .. 11 Si Si A1 .. 11 A1 fluorine for the appropriate time interval, the bomb was .. 9 Ca Ca evacuated, flushed with argon, and the sample reNa .. 9 Na covered and reweighed. The reweighing was per.. 7 Mg Mg formed in an inert-atmosphere glove box in order to 7 ... Mo Mo avoid reaction between atmospheric moisture and the 5 ... Ta Ta cu 5 ... cu metal fluoride on the sample surface. From the gain in 3 ... Hf Hf weight of the sample (about 0.4 0.1 mg., assumed to be due to the formation of the corresponding pentaAverage of chemical analyses by manufacturer and Chemical Research Services, Inc., Addison, Ill.; all others are spectrofluoride) an appropriate thermal correction was calchemical data provided by manufacturer and J. Goleb, Argonne culated for the prefluorination of the sample. National Laboratory. M represents either niobium or tantaAnalysis of Combustion Products. After the complelum. tion of each calorimetric measurement, the bomb was discharged and the unburned metal was quickly recovered, flooded with water, dried under vacuum, and Purified fluorine (99.97%) was prepared by distillaweighed. tion of commercial fluorine in a low temperature sti11.3~5 The white, solid, combustion products recovered from Combustion Technique. The sample arrangement and the bomb exhibited the characteristic physical propercombustion technique were similar to those described3 ties of NbF5 and TaF5, respectively, and yielded X-ray for the combustion of zirconium in fluorine. Prediffraction patterns which were quite spotty, possibly liminary combustion experiments indicated a tendency owing to the coarseness of the crystallites. After for preferential attack of the sample at the point of conthese products were ground in an inert-atmosphere tact between it and the nickel support. This attack, glove box, satisfactory X-ray patterns were obtained, which often cut through the sample near its base before provided the inert atmosphere was sufficiently pure to the end of the combustion, was greater a t higher fluorine be unreactive. The patterns agreed with those obpressures. Satisfactory combustions were obtained tained by Edwardss for NbF5 and TaFS. with fluorine pressures of 1500 mm. in the niobium exIn some of the combustion experiments the small periments and 2000 mm. in the tantalum experiments. amount of gas remaining in the bomb after combustion No inert diluent gas was necessary because of the high melting point of these metals. Under these conditions it was satisfactory to machine the lower end of the nio(5) L. Stein, E. Rudsitis, and J. L. Settle, “Purification of Fluorine by Distillation,” Argonne National Laboratory, ANL-6364 (1961). bium sample rods to a diameter of 0.089 cm. but it was (Available from Office of Technical Services, U. S. Department of necessary to increase this dimension to 0.14 em. for tanCommerce, Washington 25, D. C.) talum in order to prevent burning through near the (6) A. J. Edwards, J . C h m . SOC.,3714 (1964).
*
The Journal of Physical Chemistry
2091
ENTHALPIES OF FORMATION OF NIOBIUMAND TANTALUM PENTAFLUORIDES
Table 11: Results of Niobium Combustions Combustion no. 3
J
1
Mass, g.
1.05769 1.37599 -4907.19 -9.88 0.77 -0.10 -1.73 -0.35 -4650.21
1.05270 1.36953 -4884.15 -9.81 0.61 -0.10 -1.73 -0.35 4650.45
At,, deg.
&(calor.)(-At,), cal. AEoontents,
2
tal."'*
cal. AEgsB1 cal.c AEignitionj
cal. cal. AEc"/M, ca1. g.-I
AEpreiluarinationj AEimpuritisw
-
+
6
1.04358 1.04506 1.04976 1.35739 1.35935 1.36601 -4871.60 -4840.86 -4847.86 -9.75 -9.81 -9.82 0.88 0.92 1.14 -0.10 -0.10 -0.10 -1.73 -1.73 -1.73 -0.34 -0.35 -0.34 -4649.04 -4649.46 -4651.23 Mean AEc'/M = -4650.1 cal. g.-I Std. dev. of mean = 1 0 . 3 cal. g.-1
1.04740 1.36254 -4859.23 -9.81 0.84 -0.10 -1.73 -0.35 -4649.97
+
-
5
4
" hEccntents= &'(cont.)(ti - 25) &f(cont.)(25 tf Atoom)in which ti varied from 22.9 t o 23.3'. about 65.2 g. of nickel and 0.08 g. of Teflon. AEgaa= AEi(gas)]oP'(gaS) AE'(gM)I"p,(gas).
+
b T h ebomb contents included
Table III : Results of Tantalum Combustions" c
8
Mass, g. Atc, deg. &( calor.)( -At,), cal. AEoontents, Cal.C'd AEignition, cal. AE,,,, cal." AEprefluorinationi cal. AEimpurioies, cal. AEc"/M cal. g.-l
11
9
2.72870 1.91390 -6825.43 -13.91 1.19 -0.18 -1.72 0.25 -2506.61
2.74600 1.92466 - 6863.80 -13.97 0.67 -0.18 -1.72 0.25 -2505.01
Combuation no? 12
2.72486 1.91106 6815.30 -13.91 0.68 -0.18 -1.72 0.25 -2506.62
-
2.72652 1.91167 -6817.47 -13.90 0.60 -0.18 -1.72 0.25 -2505.91
13
14
15
2.72309 2.67219 2.73363 1.90853 1.87522 1.91716 -6806.28 -6687.48 -6837.05 -13.88 -13.59 -13.89 0.90 0.92 0.68 -0.18 -0.18 -0.18 -1.72 -1.72 -1.72 0.25 0.24 0.25 2504.84 -2507.98 2506.52 Mean AEc'/M = -2506.2 ml. g.-1 Std. dev. of mean = f 0 . 4 cal. g.-1
-
-
a An earlier series of combustion experiments gave somewhat less precise results bemuse of minor experimental dif6culties, and, for this reason, the data are not included in the linal tabulation. However, the mean AEc'/M value for the first series does not differ No data were obtained for run 10 because the calorimeter stirrer failed during the reaction significantly from that reported herein. AEcontents = & ' ( c o n t . ) ( t i 25) 4-Gf(cont.)(25 tr 4- Atoom) in which ti was approximately 22.77". period. The bomb contenta included about 65.0 g. of nickel and 0.09 g. of Teflon. = AEi(gas)]oPi(gaS) AEf(gas)]o~f(gas).
-
'
-
was transferred to an infrared cell for analysis. The analyses confirmed that the carbon and silicon impurities in the samples were burned to their respective tetrafluorides.
Results Experimental Results. The results of the niobium and tantalum combustion experiments, expressed in terms of the defined calorie equal to (exactly) 4.184 absolute joules, are summarized in Tables I1 and 111. The correctiom to standard states were applied in accordance with the procedure illustrated for the combustion of molybdenum in fluorine.' The entries in the tables are (1) the mass in vacuo of the sample burned, which was determined by subtracting the mass of un-
+
burned metal recovered after combustion from the mass of sample originally introduced into the bomb; (2) the observed increase in the calorimeter temperature, corrected for heat exchanged between the calorimeter and its surroundings, Ato = tt - ti - Atarr; (3) the energy equivalent of the calorimetric system minus the contents of the bomb, multiplied by -At,; (4)the energy equivalents of the initial and final contents of the bomb, each multiplied by its appropriate portion of -Ato to correct the results to the energy of the hypothetical isothermal process a t 25'; (5) the ~-
(7) W. N. Hubbard, "Experimental Thermochemistry," Vol. 11, H. A. Skinner, Ed., Interscience Publishers Ltd., London, 1962,
Chapter 6.
Volume 69,Number 6 June 1966
ELLIOTTGREENBERC, CAROLA. NATKE,AND WARDN. HUBBARD
2092
measured electrical energy input for ignition of the fuse; (6) the net correction for reducing the pressure of the bomb gas to standard-state conditions; (7) the correction for the spontaneous surface reaction of the sample with fluorine before intentional ignition of the sample; (8) the net correction for impurities in the sample; (9) the energy change per gram of metal for the reaction
M(c)
+ ‘/&’&)
-
MFs(c)
(1)
where M represents either niobium or tantalum. For calculation of item 4 the following values were used : heat capacities a t constant pressure--0.1061, 0.28, 0.0633, 0.0335, 0.1715, and 0.118 cal. deg.-l g.-l for Ni,8 TeflonJgNb,lo Ta,l0 NbFs,ll and TaFS,l2 respectively; heat capacity at constant volume-5.50 cal. deg.-’ mole-’ for fl~0rine.l~ The coefficients (bE/bP), and p (in the equation PV = nRT(1 - p P ) ) , which were required for calculation of item 6, were estimated by the method of Hirschfelder, et aZ.,l4from the force constants for fluorine.lS The coefficients at 25’ were 0.000803 atm.-l and -1.781 cal. atm.-’ mole-1 for p and (bE/bP),, respectively. For estimation of the internal volume of the bomb in the initial and final states, the densities used were 8.907, 2.24, 8.53, 16.626, 3.54, and 5.19 g. cc.-l for Ni,16 T e f l ~ n ,Nb,” ~ Tall6 NbF5,6 and TaFs; respectively. The internal volume of the empty bomb was 0.358 1. For calculation of item 8 the assumptions indicated in Table I were made regarding the states of combination of the impurities. After the combustion, carbon, oxygen, hydrogen, and nitrogen were assumed to be present as CF4,02,HF, and N2, respectively. The remaining impurities were assumed to form their most stable fluorides during combustion. For the small silicon impurity in the tantalum ample there is no significant change in the correction if the silicon is assumed to be combined. The required enthalpies of formation were taken from the indicated sources: NbC,18119 TaC,18v20Nb20s and T ~ O S Nb-H , ~ ~ sysNbN and TaN,2a C F4, 24 HF,26TiF49 * WF61 26 SiF4,nAIF3,28FeFa,29MgF2,30CaF2,31and NaF.al The approximate corrections to the measured heat for the niobium sample were: oxygen, -0.018%; carbon, + 0 . 0 0 9 ~ ; nitrogen, -0.009%; hydrogen, +0.008%; titanium, +0.007%; tungsten, -0.003%. Corrections for the tantalum sample were : oxygen, -0.023% ; carbon, +O.O1l~o; silicon and aluminum, +0.005% each; nitrogen, -0.004%; iron, +0.004%; calcium and magnesium, +0.002% each; niobium and sodium, +O.OOl% each; other impurity corrections were negligible. The net correction made for all impurities (item 8) was (-0.007 i 0.014)% for the niobium The Journal of Physical Chemistry
sample and (0.004 f 0.019)% for the tantalum sample. In each case the uncertainty attached to the net impurity correction is relatively large as compared to the actual correction owing to fortuitous partial cancellation of the individual corrections. These uncertainties include a generous allowance for analytical uncertainties and for the possibility that the impurities existed in states of combination other than those assumed. All other corrections to standard states were neg(8) R. H. Busey and W. F. Giauque, J . Am. Chem. SOC.,74, 3157 (1952). (9) W.D. Good, D. W. Scott, and G. Waddington, J . Phys. Chem., 60, 1080 (1956). (10) 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. 189, 272. (11) A. P. Brady, 0. E. Myers, and J. K. Clauss, J . Phye. Chm., 64, 588 (1960). (12) Estimated. (13) W.H. Evans, T. R. Munson, and D. D. Wagman, J . Res. Natl. Bur. Std., 55, 147 (1955). (14) J. 0.Hirschfelder, C. F. Curtias, and R. B. Bird, “Molecular Theory of Gsaes and Liquids,” John Wiley and Sons, Inc., New York, N. Y., 1954. (15) D. White,J. H. Hu, and H. L. Johnston, J . Chem. Phys., 21, 1149 (1953). (16) H. E. Swanson and E. Tatge, “Standard X-ray Diffraction Powder Patterns,” Vol. I, National Bureau of Standards Circular 539,U.S. Government Printing 05ce, Washington, D. C., 1953,pp. 13,31. (17) M. R. Nadler and C. P. Kempter, A d . Chem., 31, 1922 (1959). (18) A. N. Komilov, V. Ya. Leonidov, and 5. M. Skuratov, Vestn. Mosk. Univ. Ser 11: Khim., 17,No. 6,48 (1962). (19) E. J. Huber, Jr., E. L. Head, C. E. Holley, Jr., E. K. Storms, and N. H. Krikonan, J . Phys. Chem., 65, 1846 (1961). (20) E. J. Huber, Jr., E. L. Head, C. E. Holley, Jr., and A. L. Bowman, ibid., 67, 793 (1963). (21) A. N. Komilov, V. Ya. Leonidov, and S. M. Skuratov, DOH. Akad. Nauk SSSR, 144, 355 (1962). (22) E. Veleckis, Ph.D. Thesis, Lllinois Institute of Technology, 1960. (23) A. D. Mah and N. L. Gellert, J . Am. C h m . SOC.,78, 3261 (1956). (24)D. W. Scott, W. D. Good, and G. Waddington, ibid., 77, 245 (1955). (25) The value of -64.8 kcal. mole-’ for the standard enthalpy of formation of HF(g) has been tentatively adopted by W. H. Evsns, National Bureau of Standards, Washington, D. C., for the revised edition of N.B.S. Circular 500 (private communication, 1963). (26) 0. E. Myers and A. P. Brady, J . Phys. Chem., 64, 591 (1960). (27) S. S. Wise, J. L. Margrave, H. M. Feder, and W. N. Hubbard, ibid., 67, 815 (1963). (28) “Janaf Thermochemical Tables,’’ The Dow Chemical Co., Midland, Mich., Sept. 1963. (29) L. Brewer, L.A. Bromley, P. L. Gilles, and N. L. Lofgren, “The Chemistry and Metallurgy of Miscellaneous Materials: Thermcdynamics,” L. L. Quill, Ed., McGraw-Hill Book Co., Inc., New York, N. Y., 1950,pp. 76-192. (30) E. Rudaitis, H. M. Feder, and W. N. Hubbard, J . Phgs. Chem., 68, 2978 (1964). (31).“Selected Values of Chemical Thermodynamic Properties,” National Bureau of Standards Circular 500,U. S. Government Printing Office, Washington, D. C., 1952.
ENTHALPIES OF FORMATION OF NIOBIUM AND TANTALUM PENTAFLUORIDES
ligible. AEc"/M is just the sum of items 3 through 8 divided by the mass of sample reacted. Derived Data. Table IV presents derived standard thermal data for the formation of niobium and tantalum pentafluorides at 25' as shown by reaction 1. The atomic of niobium and tantalum were taken as 92.906 and 180.948 g. (g.-atom)-l, respectively. The entropies, rS", at 25O, of Nb(c),I0 Ta(c),l0 NbFs(c),l' TaF6(c),33and F2(g)13were taken aa 8.70, 9.92, 38.3, 40.6, and 48.45 cal. deg.-l mole-l, respectively. The uncertainties given are uncertainty intervalP equal to twice the combined standard deviations arising from known sources. Table IV : Derived Data a t 25" NbFdo)
l'aFs ( 0 )
Energy of formation, AEf" = AEc", kcal. mole-'
-432.02 f 0 . 1 5 - 4 5 3 . 4 9 f 0.19
Enthalpy of formation, AHf ", kcal. mole-'
-433.50 f 0 . 1 5 -454.97 zkO.19
Entropy of formation, Ah'f", cal. deg.-1mole-'
-91.5
mined to be -433.50
f 0.15 and -454.97 f 0.19kcal. mole-l, respectively, by direct combination of the elements in a combustion bomb calorimeter. The only previous thermochemical study reported is that by Myers and BradyZ6for niobium pentafluoride. They carried out three different heat of solution measurements, with each of the thermochemical cycles involving estimation of unknown auxiliary data. Their values were -439 f 8, -423 f 10, and -435 f 8, with the average reported aa -432 kcal. mole-l. Considering the uncertainties involved in their work, their value is in surprisingly good agreement with the value reported herein. G1assners5and A ~ O S O V ~ ~ estimated -342 and -370 f 30 kcal. mole-', respectively, for NbF6, and -360 and -380 f 20 kcal. mole-l, respectively, for TaFs, while Brewer, et U Z . , ~ ~ estimated -300 kcal. mole-1 for TaF6. These estimates, in common with a number of others for metal fluorides, were very wide of the mark.
Acknowledgment. We wish to thank R. V. Schablaske for the X-ray diffraction analyses required in this work.
-90.4
Gibbs energy of formation, AGf" = AHf" kcal. mole-1
2093
- TASP, -406.22 f 0 . 1 5 -428.02 f 0.19
Conclusion The standard enthalpies of formation of crystalline niobium and tantalum pentafluorides have been deter-
(32) A. E. Cameron and E. Wichers, J . Am. Chem. SOC.,84, 4175 (1962). (33) V. M.Amosov, Im. Vuashikh Uchebn. Zavedenii, Tsvetn. Met., 6 , No. 2, 103 (1963). (34) F.D.Rossini,"Experimental Thermochemistry,"F. D. Rossini, Ed., Interscience Publishers, Inc., New York, N. Y.,1956, Chapter 14. (35) A. Glassner, "The Thermochemical Properties of the Oxides, Fluorides, and Chlorides to 2500°K.," Argonne National Laboratory, ANL-5760 (1957). (Available from the U. 5. Government Printing Office, Washington 25, D.C.)
Volume 69,N u m b 6 June 1966
