T H E
J O U R N A L
OF
PHYSICAL CHEMISTRY Registered i n
U.8. Patent Ofice 0Copyright,
1966, by the American Chemical Society
VOLUME 70, NUMBER 1 JANUARY 14, 1966
Fluorine Bomb Calorimetry.
XV.
The Enthalpy of Formation
of Boron Trifluoridel
by Gerald K. Johuson, Harold M. Feder, and Ward N. Hubbard Chemical Engineering Division, Argonne National Laboratory, Argonne, Illinois
(Received August 19, 1966)
The energy of formation of boron trifiuoride was measured by direct combination of the elements in a combustion bomb calorimeter. Two different combustion techniques on the same specimen of zone-refined boron yielded a value for the standard enthalpy of formation, L U Y ~ ~ ~ ~ ~ . Iof~ (-271.65 B F ~ , ~j)=, 0.22 kcal. mole-'. This value is more negative (by 1.77 kcal. mole-') than one previously determined by a similar method in the same laboratory. Reanalysis of the specimen of Moissan boron used in the previous work uncovered additional impurities which account for the difference.
Introduction Paper 1112of this series described a determination of the enthalpy of formation of gaseous boron trifiuoride by direct combination of the elements in a combustion bomb. This value of AHH~"(BFs,~) was combined by Hess' law with subsequent measurements of the heats of fluorination of boron nitride and zirconium diboride to evaluate their enthalpies of formation. Comparison of these values with those determined by other methods revealed an apparent low bias. The consistency of this bias made us suspicious of our previously reported value for AH!" (BF3,g). The logical sources of significant error in this value were (a) a faulty determination of the energy of combustion of the specimen used in the earlier work or (b) a valid determination of the energy of combustion combined with a faulty correction for impurities in the specimen. In addition, there was a possibility that AHfO(BF8,g) was correct as reported; the apparent bias might have arisen from an error connected with the use of a two-
chambered reaction vessel for the combustion of the boron compounds, whereas boron itself had been burned in a single-chambered bomb. The results of an exhaustive investigation of these problems are reported here.
Re-examination of Previous Results Two combustion experiments were conducted on the specimen of Moissan boron previously used. The technique employed differed only in minor ways from that already describeda2 The results obtained for the energy of combustion per gram of sample, uncorrected for impurities [AEcO/M(sample)] were -24,857 and -24,825 cal. g.-l. These results are in good agreement (1) This work waa performed under the auspioea of the U. 8. Atomic Energy Commission. (2)8. 8. Wise, J. L. Margrave, H. M. Feder, and W. N. Hubbard, J. Phya. C h . ,65, 2167 (1961). (3) (a) 8. 8. Wise, J. L. Margrave, H. M. Feder, and W. N. Hubbard, dbid., 70, 7 (1966); (b) G. I(. Johnson, E. Greenberg, J. L. Margrave, H. M. Feder, and W. N. Hubbard, to be published.
1
2
with the value for AEc"/M(sample) of -24,837 f 9 cal. g.-' previously obtained and indicate that the calorimetry of the earlier work was not in error. The specimen was then submitted for complete reanalysis. The results of chemical analysis were (in per cent): C, 0.0925; H, 0.0045; 0,0.0235; N, 0.234. The results of the spectrochemical analysis were (in per cent): Ca, 0.27; Zr, 0.29; Cr, 0.013; Fe, 0.1; Mn, 0.093; Mg, 0.033; Si, 0.13; and AI, Cu, Ni, Ti, 0.01 each. By difference, the specimen contained 98.68% boron. The isotopic ratio B"/B'O (3.97 f 0.08) was found by mass spectrometric analysis of BFa to be within the limits for naturally occurring boron as was assumed by Wise.2 The original analysis of this specimen (on which nitrogen had not been determined) showed (in per cent): C, 0.24; H, O.OOO4; 0, 0.05; Fe, 0.15; Mn, 0.12; Mg, 0.007; Si, 0.05; and, by difference, 99.38% boron. The results of the reanalysis are considered more reliable owing to the use of improved techniques. The correction for impurities in the specimen as reanalyzed is -262 cal. g.-I; the original correction was -98 cal. g.-'. The original value for AHHfo298.16 (BF3,g) was -269.88 -f 0.29 kcal. mole-'. When the correction for impurities is based on the reanalysis, Mf" becomes -271.6 f 0.9 kcal. mole-'. The use of this more negative value for hHf0 removes the apparent bias in the calculated values of the enthalpies of formation of boron nitride and zirconium diboride. However, the uncertainty associated with the large impurity content of the specimen used made a redetermination with a purer specimen desirable; therefore, zone-refined boron was procured. Combustions were carried out in a two-chambered reaction vessel (series I) and in a one-chambered bomb (series 11) so that the possibility of discrepancy in their results could be explored.
Series I Calom'metric System. In combustion series I the two-chambered reaction vessel4 for substances that react spontaneously with fluorine was used in spite of the fact that crystalline boron does not ignite spontaneously in fluorine at room temperature. The calorimeter, laboratory designation ANL-R1,S and the reaction vessel, laboratory designation Ni-2-T-1,6 have already been described. The system was calibrated with benzoic acid (Nsr tional Bureau of Standards Sample 39i) whose certified energy of combustion was 26.434 h 0.003 abs. kjoules g.-l under prescribed conditions. Eight experiments, some preceding and some following the fluorine combustions, yielded an average value for The Journal of Physical Chemistry
G. K. JOHNSON, H. M. FEDER, AND W. N. HUBBARD
&(calor.), the energy equivalent of the calorimetric system, of 3399.63 cal. deg.-l. The standard deviation of the mean was 0.10 cal. deg.-l or 0.003%. Materials. Zone-refined boron (stated spectrographic purity greater than 99.9995%) was obtained from the United Mineral and Chemical Corp., New York, N. Y., in the form of a 10-g. bar. The results of chemical analysis were (in p.p.m.): C, 450; 0, 580; N, 260; H, 7. Carbon was determined by combustion to COZ, oxygen and hydrogen by vacuum fusion, and nitrogen by the Kjeldahl method. No metallic impurities were detected by spectrochemical means. The specimen was 99.87% boron, by difference. The X-ray powder diffraction pattern of the zonerefined specimen corresponded to that of the p-rhombohedral form.' The isotopic ratio B"/B'O was determined to be 3.87 f 0.08. Molybdenum foil (0.1 111111. thick) of 99.97% purity, from the same batch used to determine the enthalpy of , ~ used as a fuse. Molybdenum formation of M o F ~was wire (0.1-mm. diameter) of 99.9% purity was used to ignite the foil. Purified fluorine (99.99%) was prepared by distillation of commercial fluorine in a low-temperature still.5v9 Bhnlc Experiments. In the two-chambered reaction vessel fluorine is stored in a tank surrounding the bomb. The combustion is initiated by opening a valve on the tank to allow the gas to enter the bomb. At this time the interior surfaces of the bomb are exposed to fluorine, and a reaction with moisture adsorbed on the bomb walls, or with the walls themselves, occurs. Because the expansion and reaction occur during the calorimetric period, a correction for their net thermal effect must be applied. This correction was determined by blank experiments in which fluorine was allowed to expand into a bomb which had been pretreated by exposure to boron trifluoride at 2 atm. pressure for 1 hr. and then overnight evacuation. (In preliminary experiments it was found that, following such a pre-
s. Wise, and W. N. Hubbard, Rev. Sci. Instr., 32, 1402 (1961). ( 5 ) E. Greenberg, J. L. Settle, H. M. Feder, and W. N. Hubbard, J . Phys. Chem., 65, 1168 (1961). (6) 5. 8. Wise, J. L. Margrave, H. M. Feder, and W. N. Hubbard, ibid., 67, 815 (1963). (7) J. L. Hoard and A. E. Newkirk, J . Am. Chem. Soc., 82, 70 (1960). (8) J. L. Settle, H. M. Feder, and W. N. Hubbard, J . Phys. Chem., 65, 1337 (1961). (9) L. Stein, E. Rudzitis, and J. L. Settle, "Purification of Fluorine by Distillation," kgonne National Laboratory, ANL-6364, 1961. (Available from O5ce of Technical Services, U. 8. Department of Commerce, Washington, D. C.) (4) R. L. Nuttall,
ENTHALPY OF FORMATION OF BF3
3
Table I: Result3 of Series I Boron Combustions Combustion no.
I
2
1
1. m‘, B reacted, g. 2. m”, Mo reacted, g. 3. Atol deg. 4. &(calor.)(-Atc), cal. 5. AEcontentai cal. 6. AEknition,cal. 7. A E Mfuaer ~ cal. 8. AEgaBlcal. 9. u b l a n k j tal. 10. O N i rescted, cal. 11. &‘cO/M(sample), cal. g.-l
0.24955 0.05155 1.89882 6455.29 -9.02 0.31 199.47 0.05 3.97 1.32 -25,081.91
-
0.29523 0.05470 2.23869 - 7610.72 -10.66 0.31 211.66 0.06 3.97 0.55 -25,081.56
3
5
6
0,24776 0.05280 1.88743 - 6416.56 -8.96 0.30 204.31 0.05 3.97 0.55 -25,090.17
0.23484 0.05264 1,79126 -6089.62 -8.50 0.28 203.69 0.05 3.97 0.30 - 25 ,080.18
0.31235 0.04545 2.35423 -8003.51 -11.22 0.29 175.87 0.06 3.97 0.41 -25.081.26
Mean hEcO/M(sample) = -25,083.0 cal. g.-1 Std. dev. of mean = 1 . 8 cal. g.-1
treatment, the blank correction was smaller and more reproducible.) Blank experiments preceded each combustion, The values obtained were, in chronological order, -2.41, -5.07, -3.47, -4.49, -1.60, and -6.76 cal. The mean value is -3.97 cal. with a standard deviation of 0.77 cal. Combustion Procedure. Preliminary experiments showed that fragments (0.05 to 0.15 g.) of a zone-refined bar of boron burned smoothly, completely, and without shattering in fluorine. For the calorimetric experiments several fragments of boron were weighed directly on the balance pan and transferred to a weighed nickel dish (-3 g.) in the bomb. (The density of boron was taken as 2.35 g. CC.-’ to reduce the weight of boron to true mass.) Because the fragments do not ignite spontanwmsly, they were ignited with the aid of a molybdenum foil-and-wire fuse. (Molybdenum was chosen as the fuse material because it burns to give a gas, MoFe, which does not interfere with subsequent observation and handling of combustion residues.) The fuse was weighed, attached to the electrodes, and positioned close to the sample. The tank, which had previously been evacuated and charged with fluorine to 10.5 atm. pressure, was attached to the bomb. The bomb, which had been rinsed with water and dried between experiments, was then subjected to the pretreatment already described, placed in the calorimeter, and the experiment was started. At the end of the forerating period the tank valve was opened, and the fluorine was allowed to expand into the bomb. Combustion of the sample was started by electrical ignition of the molybdenum fuse. Postcombustion Analyses. After completion of the calorimetric experiment, the reaction vessel was re-
moved from the calorimeter, thoroughly evacuated to remove the gaseous products, and opened. No boron residues were observed in any of the scceptable experiments. The molybdenum residue, consisting of two short stubs of wire attached to the electrodes and sometimes one or two fused beads, was removed, washed, dried, and weighed. The mass of this residue (0.5 to 1.2 mg.) was subtracted from the original mass of molybdenum to obtain the mass burned. The nickel dish, also, was weighed after each combustion to determine the weight gain (0.1 to 0.3 mg. per combustion) which had occurred. The weight gain was attributed to the formation of NiF2 during combustion. Portions of the product gases resulting from some of the combustions were taken for infrared analysis. An 8-cm. long nickel cell, equipped with AgCl windows, was charged to 700 torr for the infrared spectrometer scans. These showed peaks due only to BFa and MoF6, from the sample and fuse, respectively, and to CF4 from the carbon impurity in the sample. The characteristic peaklo of B2F4 at 1151 cm.-’ was not discernible. The spectral sensitivity was such that 0.1% BzF4in BFa would have been detected. Results. The results of five combustions are presented in Table I. combustion no. 4 was discarded because a residue of boron was fused to the nickel dish, and quantitative recovery was uncertain. The results are expressed in terms of the defined calorie equal to (exactly) 4.184 abs. joules. The corrections to standard states were applied in the usual manner.11 (10) J. N. Gayles and J. Self, J . Chem. Phya., 40, 3530 (1964). (11) W.N. Hubbard in “Experimental Thermochemistry,” Vol. 11, H. A. Skinner, Ed., Interscience Publishers Ltd., London, 1962, Chapter 6.
Volume 70, Number 1 January 1966
G. K. JOHNSON, H. M. FEDER, AND W. N. HUBBARD
4
The entries in the table are either self-explanatory or have been previously explained. 11,12 For calculation of item 8 the coeficients p (in the equation of state PV = izRT[l - PPI) and (dE/dP)T were estimated by the method of Hirschfelder, et al.," from the intermolecular force constants for F9," BFa,16 A r , I 3 and M o F ~ . The ~ coefficients as functions of composition at 25" are given by
Table II: Auxiliary Data (25') cpo,
tal, deg.-' g.-l
Ni (0.1061)," B (0.245),6Mo(0.585),' Teflon (0.28)d Fz (5.50)," BFs (10.07),6 MoFa (26.3),' Ar (2.981)' B (2.35), Mo (10.2), Ni (8.907) BhC ( - 12.7),bBzOs ( - 304.10),' BN ( -59.97),h HzO ( -68.32)," CF4 (-221),' H F (-64.8)' B (1.403),b FZ(48.45)," BFI (60.71)* Mo fuse ( -3.8695),' Ni ( -2.76)k
C,", cal. deg.-l mole-'
-I
p =
+ + + + + + 5.242223 + 5.792224 + 2 . 2 3 ~ ~ atm.-l 4)
6.49 X 10-4(17.36~12 5.7122' 23' 1 . 2 4 ~ 4 ~ 20.60x1x2 9 . 8 3 ~ i ~ a 10.7721x4
+
and
+ + +
+ + + +
(dE/dP)T = -1.475(9.05~1~ 3.2222' ~3~ 1.21X4' 11.15x122 6.562123 7.052124 3.63~2~3 3.992254 2 . 2 1 ~ cal. ~ ~ atm.-' ) mole-'
+
+
in which xl, x2, x3, and x4 represent the respective mole fractions of MoF6, BF3, Ar, and Fz. The internal volume of the empty bomb was 0.309 l., and of the tank, 0.24 1. Auxiliary data used to calculate various numerical quantities are given in Table 11.
Series 11 In series I1 the calorimetric system, boron specimen, molybdenum, and fluorine were the same as in series I. The two-chambered reaction vessel was converted to a one-chambered bomb by insertion of a Teflon plug in the line connecting the evacuated tank to the bomb. The combustion procedure in series I1 was similar to that in series I except that the assembled bomb, which contained the sample, nickel dish, and fuse, was evacuated overnight and then charged with 5000 torr of fluorine and 8000 torr of argon. The dilution with argon was found to be an effective means of preventing fluorination of the nickel dish. The bomb was allowed to stand for 2 hr. so that reaction of fluorine with surface contaminants could occur. Auxiliary studies showed that under these conditions a typical boron sample underwent a weight loss of 0.01 mg., presumably by reaction with fluorine to give BFa. In argon-diluted fluorine a small residue of unburned boron remained on the nickel dish after combustion. This residue was removed and weighed. Results. The results of nine boron combustions in series I1 are presented in Table I11 in substantially the same manner as in Table I. Although the standard deviation of the mean was small, less than O.Ol%, a close examination of the values of AEc'/M(sample) suggested the possibility of a trend with mass of sample The Journal of P h y s i d Chemistry
P,
g. cc*-l
Mf', kcal. mole-' So, cal. deg.-l mole-' AEofluorination, cal. mg.-'
' R. Hultgren, R. L. Orr, P. D. Anderson, and K. K. Kelley, "Selected Values of Thermodynamic Properties of Metab and Alloys," John Wiley and Sons, Inc., New York, N. Y., 1963, p. 198. "JANAF Thermochemical Tables," The Dow Chemical Co., Midland, Mich., Dec. 1964. F. D. Rossini, D. D. WagLevine, and I. Jaffe, National Bureau of man, W. H. Evans, Standards Circular 500, U. S. Government Printing Office, Washington, D. C., 1952. W. D. Good, D. W. Scott, and G. Waddington, J. Phys. Chem.,60, 1080 (1956). e W. H. Evans, T. R. Munson, and D. D. Wagman, J . Res. Natl. Bur. Std., 55, 147 (1955). J. Gaunt, Trans. Faraday Soc., 49, 1122 (1953). W. D. Good, M. Mhnsson, and J. P. McCullough, Symposium on Thermodynamiw and Thermochemistry, Lund, Sweden, 1963. See ref. 3a. W. H. Evans, National Bureau of Standards, Washington, D. C., private communication. See ref. 8. E. Rudeitis, Argonne National Laboratory, u n p u b b h e d results.
'
s.
'
reacted. To obtain an impartial analysis, a leastsquares fit of the results to a linear regression of the form Z(items 4-8) = am'
+b
was machine computed. If the apparent trend in the data is unreal, b should not differ from zero by a statistically significant amount. The value obtained for b was -4.48 cal. The standard deviation of b, 1.4 cal., was sufficiently small to indicate that the apparent trend was statistically real. A negative real value of b implies the existence of some side reaction accompanied by the evolution of a constant amount of heat. It was concluded that a constant correction should be applied to each combustion experiment. Additional experiments were conducted in the hope of ascertaining the nature of the side reaction. The (12) E.Greenberg, J. L. Settle, and W. N. Hubbard, J . Phys. Chem., 66, 1345 (1962). (13) J. 0.Hirschfelder, C. F. Curt&, and R. B. Bird, "Molecular Theory of G a m and Liquids," John Wiley and Sons, Inc., New York, N. Y.,1954. (14) D. White, J. H. Hu, and H. L. Johnston, J. Chem. Phys., 21, 1149 (1953). (15) J. C. McCoubrey and N. M. Singh, Trans. Faraday SOC.,53, 877 (1957).
ENTHALPY OF FORMATION OF BFs
5
results lead us to surmise that under the conditions of series 11, BF3 produced in the combustion reacts with the bomb walls. Attempts to eliminate this possibility by altering the combustion technique were curtailed by exhaustion of the stock of boron. Item 10 in Table 111, AEc"/M(sample, adj.), is the energy of combustion per gram of boron sample after adjustment by the empirical correction of 4.48 cal.
Derived Data Values of -25,083.0 f 1.8 and -25,086.3 i 2.2 cal. g.-l were obtained for the mean energy of combustion of the boron sample in series I and 11, respectively. The mean values do not differ by an amount significantly greater than the combined standard deviations; the weighted average, -25,084.5 i 1.4 cal. g.-l, was used in subsequent calculations. Impurity corrections for C, 0, N, and H in the sample were based on their presence as B4C, BzO3,BN, and H20, respectively. The combustion products of these impurities were assumed to be gaseous CF,, 02, N2, and HF. The impurity corrections were: C, -3.5 A 0.6 cal. g.-l; 0, -16.3 =k 4.6 cal. g.-l; N, -7.7 f 2.2 cal. g.-l; and H, -1.5 f 1.7 cal. g.-'. The indicated precisions allow for the errors of the analyses and for uncertainties in the states of combination of the impurities. No correction was made for the possible presence of B2F4 in amounts less than 0.1% in the combustion product. Application of the combined impurity correction of -29.0 i 5.4 cal. g.-l results in a value for the energy of combustion of crystalline boron of -25,113.5 cal. g.-l. For calculation of the derived data presented in Table IV the atomic weight of the boron in this specimen was calculated from the experimentally determined isotopic ratio to be 10.805 f 0.004. The uncertainties given are uncertainty equal to twice the combined standard deviations arising from all known sources. U
F9 U
*
." -I O43
Discussion The standard enthalpy of formation of gaseous boron trif-luoride has been redetermined by direct combination of the elements to be -271.65 f 0.22 kcal. mole-l. This value should supersede the previously reported value2 of -269.88 i 0.29 kcal. mole-'. The latter value, when corrected on the basis of reanalysis, becomes -271.6 f 0.9 kcal. mole-'. Another reported determination of the enthalpy of formation of BF3 by direct combination of the elements (16) F. D. Rossini in "Experimental Thermochemistry," F. D. Rossini, Ed., Interscience Publishers, Inc., New York, N. Y., 1956, Chapter 14.
Volume 70,Number 1
January 1966
G. K. JOHNSON, H. M. FEDER, AND W. N. HUBBARD
6
B(c)
Table IV : Derived Data a t 25" B(Brhomb.1
+ 8/zFz(g)
M c " / M , cal. g.-1 M c " = M f " , kcal. mole-1 AHc" = AHf", kcal. mole-' Asf", cal. deg.-1 mole-1 AGf", kcal. mole-'
+
BFdg)
-25,113.5 f 1 8 . 3 -271.35f0.22 -271.65=t00.22 -13.37 -267.66 f 0 . 2 2
+ 3/2F2(g)
Hz(g)
+ '/202(g)
B(c)
+ '/402(g) + 18.67(HF.3.064H20)+ HBF,. 14.67HF.58.72H~O (1) AH1 = -173.42 j= 0 . 2 0 ~
BFa(g)
+ 15.67(HF.3.747H20)+ HBFl. 14.67HF.58.72H20 (2) A H 2 = -28.29 f 0.0710
The J O U To j ~Physical Chemistry
BF&) A H a = -271.65
-+
0.064H20(1)
+ 0.22
HzO(1)
(3) (4)
AH, = -68.32
f
O.0lza
+ HF.3HzO *HF.3.064H20
(5)
A H 5 = -0.00924
0.683H,O(1) wm made by Gross, et al.," who obtained a value1*of -270.57 kcal. mole-' for crystalline boron. However, the specimen used in their work was not analyzed for C, 0, H, or N, and thus their value is likely to have a positive bias. Gunnl9 has measured the enthalpy of reaction of BFs(g) with hydrofluoric acid. He assumed that the h a 1 state was thermochemically equivalent to that obtained by Good, Minsson, and McCullough20 via oxygen bomb calorimetry. He combined his result with theirs, in a cycle similar to that given below, to obtain a value for AHf" (BFa). The value so obtained, -269.27 kcal. mole-', is in disagreement with our present result. However, the cycle considered by Gunn involved the formation of hydrofluoric acid for which there is increasing evidence21*22 that the currently selected AHf value may be too positive. Before AHfO(BF,) can be reliably deduced from aqueous solution data, the persistent problem of the correct value for AHf" (HF,aq) must be resolved. We have essentially reversed Gunn's cycle to obtain an estimate of the enthalpy of formation of hydrofluoric acid.
-
+ HF.3.064H20 +HF.3.747H20 m6=
'/2H2(g)
+ '/2F2(g) + 3Hz0(1)
-0.088
j=
(6)
0.00224
+HF.3JW
(7) AH7
The enthalpy of formation according to eq. 7 is given by AHf(HF'.3H20) = AH',= '/a(-AHi a/2AH4
+
A H 2
+
- 3 U . 5 f 15.67AHtj)
The value obtained is -76.78 j= 0.10 kcal. mole-'. Despite the indirectness of this means of estimating AHf(HF.3H20), the result does lend support for revision of the currently selected26value, -75.98, to a more negative one.
Acknowledgments. The performance of special analyses by J. A. Goleb, B. D. Holt, and R. V. Schablaske is gratefully acknowledged. (17) P. Gross, C.Hayman, D. L. Levi, and M. C. Stuart, "Measurements of the Heats of Formation of Inorganic Fluorides Especially the Elements of Atomic Number Below 20," Final Technical Report R.146/4/23, Fulmer Research Institute Limited, Stoke Popes, England, Nov. 1960. (18) The reported value, -270.8 kcal. mole-', has been converted to the 1961 atomic weight of B = 10.811 j=0.003. (19) S. R.Gunn, J. Phys. Chem., 69, 1010 (1965). (20)See footnote g, Table 11. (21) See footnote k, Table 11. (22) J. D. Cox and D. Harrop, Trans.Faraday Soc., 61, 1328 (1965). (23) See footnote c, Table 11. (24) Calculated from heat-of-dilution data from V. B. Parker, "Thermal Properties of Aqueous Uni-univalent Electrolytes," NSRDS-NBS 2, April 1965. (26) See footnote i, Table 11.
