July, 1962
HEATSOF FORMATION OF TITANIUM AND HAFNIUM TETRAFLUORIDES
macroscopic value of the dielectric constant is used throughout, On the other hand, it is interesting to note that substitution of a very low value for the dielectric constant D in the inner brackets of (8), while retaining the macroscopic value in the outer factor, allows the use of a more believable value of hydroxyl ion dipole moment to make the calculated distance of closest approach equal the sum of ionic radii. This procedure perhaps could be justified by viewing the outer D as containing a factor correcting for changes in the solvent that otherwise are ignored in the calculation. The inner term, however, should make use of the proper local value of the dielectric constant, which may be very much less than the macroscopic value because of dielectric saturation effects. A more realistic appraisal may be that dielectric saturation must be explicitly taken into account for all interactions when the ions can readily approach each other very closely, as by a proton transfer step. Although S ~ h e l l m a n ’ s calcula~~ (29) J. A. Sohellman, J . Chem. Phys., 26, 1226 (1967).
1345
tions suggest that the dielectric saturation effect is slight even at very small distances, it seems unlikely that the macroscopic dielectric constant would apply when the ions are adjacent with no intervening water molecules. With dielectric saturation, all contributions to the interaction energy are enhanced, and it is not necessary to suppose that any other than the simplest electrostatic interactions occur. Yet, if covalent bonding is excluded from consideration, comparison with Cs+ or Rb+ (which are about the same size as T1+) would seem to point to the higher polarizability of the thallous ion as a principal reason for its tendency toward association in solution, a property not shared by Rb+ and Cs+. It is possible that the artifice of assigning different values to D in the inner and outer terms of (8) may have some merit, for in this way contributions of the polarizability terms are increased in significance. Acknowledgments.-The author wishes to acknowledge the valuable assistance of Mr. T. S. Bulischeck with the experimental work.
FLUORINE BOMB CALORIMETRY. IV. THE HEATS OF FORR/IATION OF TITANIUM AND HAFNIUM TETRAFLUORIDES’.2 BY ELLIOTT GREENBERQ, JACKL. SETTLE, AND WARDN. HUBBARD Chemical Engineering Division, Argonne National Laboratory, Argonne, Illinois Received Febrzbarg I d , 186.8
The heats of formation of titanium and hafnium tetrafluorides were measured by direct combination of the elements in a combustion bomb calorimeter. The standard heats of formation, A H f o ~ s . ~ aof, titanium and hafnium tetrafluorides were found to be -394.19 rt 0.35 and -461.40 f 0.85 kcal. mole-’, respectively.
Introduction The determination of the heats of formation of the tetrafluorides of titanium and hafnium is part of a continuing p r ~ g r a m ~to- ~obtain precise thermochemical data by fluorine bomb calorimetry. This investigation completes the measurements of the heats of formation of the Letrafluorides of the group IVA elements, the heat of formation of zirconium tetrafluoride having been reported previo~sly.~ Experimental Calorimetric Sysl em.-The calorimeter, laboratory designation AKL-R1, and combustion bomb, laboratory desig nation Ni-T, already have been described8 except for a modification of the gasket which sealed the cap to the body of the bomb. An annular U-shaped groove was cut in the gold gasket and filled with a Teflon ring in order to facilitate sealing (Df the bomb. In effect, the gold gasket was itself gasketed with Teflon. Eight calibration experiments with National Bureau of Standards standard samples of benzoic acid (39g and 39h), some preceding and some following the fluorine combustions,
-
(1) Thia work was performed under the auspices of the Energy Commission.
yielded an average value for G(calor.), the energy equivalent of the calorimetric system, of 3565.41 cal. deg.-l, with a standard deviation of the mean of 0.4 cal. deg.-’, or 0.01%. Materials.-Titanium and hafnium were obtained in the form of iodide-deposited crystal bars. These were arcmelted, cast, and rolled to 0.1 X 0.1 in. rods and 0.005 in. foil. The outer portion of the rods was removed by taking surface cuts with a milling machine. Several rods, each of which was analyzed, were required for the calorimetric combustions. Table I summarizes the impurities found in the samples. E o other metallic impurities were detected. The chemical state of the impurities was unknown.
TABLE I IMPURITIES IN TWE SAMPLES Mean values, parts per million Element
Cr
Al Fe Si Zr
U. 8. Atomic
(2) Presented in part at the International Calorimetry Confer-
ence in Ottawa, Canada, August, 1961. (3) E. Greenberg, J. L. Settle, H. M. Feder, and W. N. Hubbard, J . Phy8. Chem., 611, 1168 (1961). (4) J. L. gettle, H. M. Feder, and W. N. Hubbard, ibid.. 611, 1337 (1961). (5) 9. S. Wise, J. L. Margrave. H. M. Feder. and W. N. Hubbard. ibid.. 611,2167 (1961).
C 0 H N
Ti
Hf
Spectrochemical analysis 1200 200 300
.. ..
Chemical analysis 140 360 25 30
...~
...I 20 25 1.43 x 104
60 160 15
50
Because of the unavailability of hafnium wire, high-purity zirconium wire, 0.010 in. in diameter, was used as fuse
E. GREENBEICG;, J. L,~ETTLI,A N D W. N. HUBBARD
1346
Vol, 66
TABLE TI RESULTSOF TITAXIUM COMBUSTIOS EXPERIXENISa 1. mass, g. 0 85599 0.85713 0.86908 0.86279 0 85550 0.85799 0.84818 2. At,, deg. 1.96360 1 96510 1 99219 1 97826 1.96405 1 96927 1 ,94545 3. 8(ca10r.)(-AtG), cal. -7001.04 -7006.39 -7021.25 -7102.97 -7053.31 -7002.64 -6936.33 4. AEcontents, ~ a 1 . b ' ~ -15.13 -15.16 -15.34 -15.25 -15.16 -15.18 -14.96 5. AEignition, CaI. 1.15 1.04 1.30 1.10 0.97 1.02 1.23 6. L\Egas.,cal.d -1.26 -1.26 -1.28 -1.27 -1.26 -1 25 -1.26 So correct,ion necessary 7. AEf,,,, cal. 8. AEimpurities, cal. -9 10 -9.11 -9.24 -9.17 -5 02 -5.04 -4.98 9. AEC",W, CELL g. -1 -8207.32 -8202.82 -8201.24 -8203.50 -8209 36 -8207.22 - 8201.43 Av. AEc"/.M = -8204.7 cal. g.-* Std. dev. of mean = f l . 2 cal. g.-l a The symbols employed are explained in ref. 8. AEoontents = Gi(cont.) (t, - 25) GE(cont.)(25 - tf At,,,.) in which ti waB approximately 22.78'. The bomb contents included 65.42 g. of nickel and 0.09 g. of Teflon. AEgas. = [ AE1 (gas.)]fi(sas.) [AEf(gas.)]Opf(gaB8.). I
+
+
+
'FABLE 111 RESULTSOF HAFNIUMCOMBUSTION EXPERIMENTS~ 1. 2. 3. 4. 5. 6. 7. 8. Y.
mass, 6 . At,, deg. E(calor.)(-AQto), cal. i\Eoontonts. oaLbSc AEignition. oal. AE,,,., ca1.d AEzr fuse,oal. U i m p u r i t i e s , cal. AEco/iM, cal. g . -1
1.85729 2.05689 1.82159 1.37236 1.51552 1.34466 -4893.04 -5403.47 -4794.28 -10.58 -11.72 -10.37 1.35 1.38 1.28 -0.78 -0.86 -0.76 45.35 47.30 55.74 59.20 60.36 66.85 -2577.38 -2877.91 -2578.86'
2.05485 1.51349 -5396.23 -11.70 1.35 -0.85 35.91 71.28 -2578.38'
1.87393 1.38410 -4934.90 -10.68 1.25 -0.78 46.85 66.00 -2679. 21e
1.98066 2.99433 2.43775 3.11682 1.46073 2.20270 1.79775 2.29126 -5208.12 -7853.56 -6409.73 -8169.30 -11.28 -17.18 -13.93 -17.83 1.21 1.58 L56 1.47 -1.20 -1.00 -1.24 -0.83 43.25 48.40 54.19 38.91 68.71 105.13 85.59 109.43 -2578.46 -2677.81 -2577.51 -2579.09 Average AEc'/;M = -2578.4 cal. g.-1 Std. dev. of mean = 1 0 . 3 cal. g.-'
+
+
0 The symbols employed are explained in ref. 8. b L\Econtents = Gi(cont.) ( t i 1 2 5 ) &'(conk) (25 - tf Atcor,). Because of va,rying sample size t i varied from 22.6 to 23.4". e The bomb contents included 66.01 g. of nickel and 0.09 g. of A E gas. = [ AEi(gas.)]OPi(gas.) [AE'f(gas.)]opf(E&s.).e These combustions were carried out with high-purity Teflon. commercial fluorine.
+
material for the hafnium combustions. For the titanium experiments, the fuse consisted of a 0.01 in. wide strip of 0.005 in. t.itanium foil. Purified fluorine (99.947;) was prepared by distillation of commercial fluorine in a low temperature sti11.316 High-purity argon (99.9%) was obtained commercially. Combustion Technique .-Trial exposures of weighed samples t o fluorine indicated the absence of any reaction prior to ignition. The sample arrangement and combustion technique xere t,he same as describedYfor the combustion of zirconium in fluorine. The gas mixture ronsisted of 2000 mm. fluorine for the titanium experiments, 2500 mm. fluorine for the hafnium experiment.s, and, in each case, sufficient argon to raise the total pressure t'o 12.0 atm. Argon diluent was substituted for the helium used in the zircouium experiments because of handling convenience in the high pressure manifold. The assembled bomb always was pretreated with several atmospheres pressure of fluorine for a few minut'es before final evacuation and charging. The calorimetric measurements were made in the usual manner. Analysis of Combustion Products.-Af ter t'hc completion of each calorimetric measurement the bomb gas was discharged and the unburned metal was recovered and weighed. The unburned hafnium was soaked in water t80loosen adhering tet,rafluoride, scrubbed with nylon tweezers, and scraped clean with a razor blade. The remnants of the zirconium h e xire were isolated a.nd weighed separately. The washing procedure for titanium was modified t o avoid possible attack by concentrated solutions of tihnium tetrafluoride on titanium metal. The product of the hafnium combustions was determined to be monoclinic hafnium tetrafluoride by comparison of the X-ray powder pattern with the pattern previously obtained and verified to be that of @-(monoclinic)zirconium tetrafluoride.3 The two powder patterns were identical except, for slight shifts in line locations due to cell size differences. (6) L. Stein, E. Rudeitis. and J. L, Settle, "Purification of Fluorine by Distillation," Argonne National Laboiatory, ANL-6364 (1961). (Available from Office of Technical Services, U. S. Dept. of Commerce, Washington 25, D. C . )
The product of the titanium combustions, which was a white solid, gave an X-ray powder diffraction pattern that showed only a few diffuse bands. After the sample was annealed a t about 300' for approximately half an hour, a diffraction pattern was obtained which was in general agreement with that reported for titanium tetrafluoride.' Electron microscopic examination of the combustion product showed spherical particles of the order of 1 U , in diameter, which are large enough particles to have given a satisfactory X-ray diffraction pattern if the particles were crystalline. Proof that the product was titanium tetrafluoride uncontaminated by lower fluorides was furnished by chemical analysis for fluorine, which showed 61.4 =t 0.3% of the poduct as fluorine compared to 61.3% fluorine in TiF,. After several of the calorimetric experiments, a portion of the gas in the bomb was reacted with mercury to remove fluorine, and then was analyzed mass spectrometrically. The bomb atmosphere after combustion was thus shown to contain 0.09% nitrogen and 0.27, oxygen. The remainder of the gas was cooled with liquid nitrogen, fluorine and other non-condensables were pumped off, and the residue was evaporated into an infrared cell equipped with silver chloride windows. All the observed absorption bands could be attributed to fluorides OF oxyfluorides of carbon and to silicon tetrafluoride. The gaseous impurities found can all be attributed to contaminants in the metal saniple and in the fluorine charge.
Results Experimental Results.-Tables I1 and I11 summarize the results of the titanium and hafnium combustion experiments, expressed in terms of the defined calorie equal to (exactly) 4.184 absolute joules. The corrections to standard states were applied in accordance with the procedure illustrated for molybdenum hexafluoride.8 (7) IC. 8. Vorres and F. B. Dutton, J . Am. Chem. Soc., 77, 2019 (1965).
July, 1962
H E A T S OF FORMATION O F
TITANIUM AND HAFNIUM TETRAFLUORIDES
1347
The entries in the tables are: (1) The mass in argon in the mixture. For estimation of the intervucuo of the sample reacted, which was determined nal volume of the bomb in the initial and final
states, the densities used were: 8.907, 2.24, 4.51, 2.780, 13.36, and 7.13 g. ml.-l for Ni,I9 Teflon,lo Ti,zo TiF4,21Hf,Z2 and HfF4,23respectively. The internal volume of the empty bomb was 0.358 1. The previously report’ed3value for the heat of formation of zirconium tetrafluoride was used for calculation of item 7. For calculation of item 8 these a.ssumptions were made regarding the states of combination of the impurities : carbon, oxygen, and nitrogen were assumed to be present as Tic, HfC, TiO, HfOz, TiN, and HfN, respectively; hydrogen was assumed to be present in solid solution; silicon was assumed to be present in the hafnium sample as Hf4Si. The remaining impurities were assumed t’o be present in the sample as elements, and to form t’heirmost st’ablefluorides during combustion. The required heats of formation were taken from the following sources: T i c and TiN,Z4 T i 0 25 Ti(c) + 2F2(g)-+ TiF4(s,amorph.) (1) Ti-H system,26 HfC,27HfOz and HfN,2* Hf-H sisHE(C ) + 2Fz(g) +HfFd(C) (2) Hf4Si,30CF4,31HF,32ZrF4,3SiF4,33FeF3,B4and in which equations the reactants and products are reference 16 for CrFa a,nd AlF,. The approximate in their standard states at 25’. corrections to the measured heat for the titanium For calculation of item 4 the following values sample were: oxygen, -0.077,; chromium, were used; heat capacities at constant pressure -0.05%; iron, -0.02%; hydrogen, +O.Ol% ; were 0.1061, 0.28, 0.1248, 0.2204,0.0342, and 0.100 carbon, +O.Ol% ; aluminum, +0.007% ; nitrogen, cal. Cteg.-’ g.-1 for Xi,9 Teflon,” Ti,ll TiF47 l 2 H f , 1 3 -0.005%; and, for the hafnium sample: zirand HfF4,I respectively; heat capacities at con- conium, +1.34%; oxygen, -0.07%; hydrogen, stant volume were 5.50 and 2.981 cal. deg.-l mole-1 +0.03%; carbon, +0.02%; nitrogen, -0.02%; for F P and Ar,16respectively. and silicon, +O.Ol%; other impurity corrections The coefficients (bE/bP)T and p (in the equa- were negligible. The net, correction made for all tion PV = nRT (I - p P ) ) , which were required for impurities (item 8) was (-0.11 f 0.06)% for t’he calculation of item 6, were estimated by the method titanium sample and (+1.32 f 0.18)% for the of Hirschfclder, et u1.,17 from the force constants (19) H. E. Bwanso~iand E. Tatge, “Standard X-ray Diffraction for F218 and ,4r.I7 The coefficients as functions of Powder Patterns,” Vol. I. National Bureau of Standards Circular 5d9, composition at 25’ were U. S . Govt. Printing Office, Washington, D. C., 1953.
by subtracting the mass of unburned 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 = t f - ti Ateor. ; (3) the energy equivalent of the calorimetric system, multiplied by At,; (4) the energy equivalents of the initial and final contents of the bomb cach multiplied by its appropriate portion of -Atc to correct t o the hypothetical isothermal process at 25’; (5) the measured electrical energy input for ignition of the fuse; (6) the net correction for the hypothetical compression and decompression of the bomb gas; (7) the energy supplied by combustion of the fuse wire; (8) the net correction for impurities in the sample; and (9) the energy change per gram of metal for the reactions
p =
2.86 X
( ~ E / ~ P=) -3.147 T X
(281
- 55.22
(565.9
+ s2)atni.-l
(3)
atm.-l mole-‘
(4)
- 98.322 + z2)cal.
iii which equations 2 represents the mole fraction of (8) W. N. Kubbatd, “Experimental Thermochemistry,” Vol. 11, H. A. Skinner, Ed., Interscience Publishers, Ltd., London, 1962, Ch. 6, pp. 95-127. (9) R. H. Busey and W. F. Giauque, J . Am. Chem. Soc., 18, 3157 (1952)“ (10) W. D. Good, D. W. Scott, and G. Waddington, J. P h p . Chem., 60, 1080 (1956). (11) C. W. Kothen and €1. L. Johnston, J A m . Chem. Soc., 16, 3101 (1953) (12) R. D. Euler and E. F. Westrum, Jr., J . PAus. Chem., 66, 132 (1961). (13) D. R. Stull and G. C. Sinke, “Thermodynamic Properties of the Elements,” American Chemical Society, Washington 6, D. C., 1956. (14) K. K. Kelley, “Contributions t o the Data on Theoretical Metallurgy, XIII. High-Temperature Heat-Content, Heat-Capaciry, and Entropy Data for the Elements and Inorganio Compounds,” Bureau of Mines Bulletin 584, U. S.Govt. Printing Office, Washington, D. C., 1960. (15) W. H. Evans, T. R. Munson, and D. D. Wagman, J. Res. Natl. Bur. Std., 65, 147 (1955). (16) “Selected Value5 of Chemical Thermodynamic Properties,” National Bureau of Standards Circular 500, U. S.Govt. Printing Office, Washington, D. C., 1952. (17) J. 0. Wirschfelder, C. F.Curtiss, and R. B. Bird, “Molecular Theory of Cases and Liquids,” John Wiley and Sons, New York, iV.Y , 1954. (18) D. White, J. H. Hu, and 13. L. Johnston, J . Chem. Phys., 21, 1149 (1953).
(20) G. Skinner, H. L. Johnston, and C. Beckett, “Titanium and Its Compounds,” Herriok L. Johnston Enterprises, Columbus, Ohio, 1954, p. 3. (21) F. D. Rossiui, P. A. Cowie, F. 0. Ellison, and C. C. Broane, “Properties of Titanium Compounds and Related Substances,” Office of Naval Research Report ACR-17, Department of the Navy, Washington, D. C., 1956. (22) L. B. Prus, “Reactor Handbook,” Vol. I, C. R. ‘I’ipton, Jr., Ed., Second Edition, Interscience Publishers, Inc., New York, N. Y., 1960, Ch. 36, p. 784. (23) Calculated from the data of W. H. Zachariasen, Acta C~yst.,2, 388 (1949). (24) G. L. Humphrey, J. .Am. Chem. Soc., I S , 2261 (1961). (25) A. D. Mah, K. K. Kelley, N. L. Gellert, E . G. King, and C. J. O’Brien, “Thermodynsmic Properties of Titanium-Oxygen Solutions and Compounds,” Bureau of Mines Report of Investigations 5316 (1957). (26) A. D. McQuillan, Proc. R o y . SOC.(London), 8204, 309 (1950). (27) I
