4358
P. A. G. O’HAREAND WARDN. HUBBARD
Fluorine Bomb Calorimetry.
XIII. The Enthalpy of Formation
of Arsenic Pentafluoride’
by P. A. G. O’Hare and Ward N. Hubbard Chemical Engineering Division, Argonne National Laboratory, Argonne, Illinois
(Receized August 2, 1966)
The energy of formation of arsenic pentafluoride was measured by direct combination of the elements in a bomb calorimeter. From these measurements the standard enthalpy of formation, ARfo298.16(AsFs,g), was calculated to be -295.59 0.19 kcal. mole-I. The average bond strength in AsF5 (92.4 kcal. mole-I) is about 20 kcal. mole-l less than it is in AsF3.
*
This investigation is part of a continuing program to determine the enthalpies of formation of various compoundsby fluorine bomb calorimetry. An accurate value for the enthalpy of formation of arsenic pentafluoride is a necessary prerequisite to the study of arsenic compounds by this method. This paper reports a value for AHf”(AsF5, g) calculated from measurement of the energy of combustion of crystalline black arsenic in fluorine, a spontaneous and complete reaction.
Experimental Section Calorimetric System. The calorimeter, laboratory designation ANL-R2, has been described in detail2 The reaction vessel, laboratory designation NI6-T2, was the bomb-and-tank device previously dsecribed,a except that a direct screw drive of the value stem was substituted for the toggle arrangement. The energy equivalent of the calorimetric system was measured by combustion of ben~oicacid (National Bureau of Standards $ample 39i) in oxygen. A series of eight calibration experiments yielded a value for &(Calor.> of 3363.94 cal. deg.-l with a standard deviation of the mean of 0.21 cal. deg.-l (1cal. =4.1840 abs. joules). Materials. High-purity, zone-refined, crystalline black arsenic (Grade 1, Batch R77) was purchased from Johnson, Matthey and Co., London. No metallic impurities were detected by spectrochemical analysis (detection limit 5 p.p.m.). Chemical analyses by Ledoux and Co., Teaneck, N. J., showed 39 p.p.m. of carbon and 10 p.p.m. of nitrogen. Neutron activation analysis, performed by Drs. E. H. Strain and W. The Journal of Physical Chemistry
ROSS, Oak
Ridge National Laboratory, showed an oxygen content less than 10 p.p.m. Fluorine of 99.99% purity was prepared by distillation of a commercial sample in a low-temperature still.4 Procedure. Arsenic samples could be weighed in air as a 24-hr. exposure showed a weight change of less g. Before the calorimetric series, the than 1 X bomb was preconditioned by several combustions of arsenic in fluorine. All subsequent operations in which the bomb was opened were performed in a helium atmosphere glove box (HzO-0.1 p.p.m.). The sample on a 33-g. nickel dish was introduced into the bomb which was then connected to the tank charged with fluorine at 190 p.s.i.a. pressure. After overnight evacuation, the reaction vessel was transferred into the calorimeter and measurementswere made in the usual mannera5 After selected experiments, the product gases were condensed in a liquid nitrogen-cooled trap. Excess fluorine was removed by repeatedly freezing, pumping, and melting the condensate. The liquid nitrogen (1) (a) This work was performed under the auspices of the U. S. Atomic Enerev -” Commission: (b) for the arevious Daoer in this series. see H. A. Porte, E. Greenberg; $nd W. N. Hubba;d,- J . Phys. Chem., 69,2308 (1965). (2) E. Greenberg, J. L. Settle, H. M. Feder, and W. N. Hubbard, ibzd., 65, 1168 (1961). (3) R. L. Nuttall, 8.S.Wise, and W. N. Hubbard, Rev. Sei. Instr., 32, 1402 (1961). (4) L. Stein, E. Rudzitis, and J. L. Settle, “Purification of Fluorine by Distillation,” Argonne National Laboratory, ANL-6364, 1961. (Available from Office of Technical Services, U. S. Department of Commerce, Washington, D. C.) ( 5 ) W. N. Hubbard, C. Katz, and G. Waddington, J . Phys. Chem., 58,142 (1954).
4359
ENTHALPY OF FORMATION OF AsF5
Table I:
Results of Combustion Experiments 1.21521 1.42474 -4792. 6Sd -6.14 -0.16 1.48 12.44 -3937.64
1. m', g. 2. 3. 4. 5. 6.
At,, deg. E( Calor.)( - Atc), cal. AEeontanta,
cal.a'o
A E ~ ~caI.' ~ ,
cal. 7 . A E N ~ F cal. ~, 8. A Ec / M ( sample ), cal. g.-' AEblank,
1.37874 1.61563 5434.88 -6.99 -0.18 1.48 10.22 - 3938.63
-
1.34229 1.56795 5274.49 -7.47 -0.21 1.48 2.04 3932.57
-
-
0.97003 1.13928 3832.47 -4.80 -0.. 13 1.48 22.07 -3931.68
-
-
0.87221 1.01903 3427.96 -4.30
-0.11 1.48 0.13 - 3933.41
0.70216 0.81992 -2758.16 -3.45 -0.09 1.48
0.00 -3931.04
1.53364 1.79960 -6053.75 -8.23 -0.24 1.48 29.52 - 3932.62
Average AEc"/M(sample) = -3933.94 cal. g.-l Std. dev. of mean I . 1 cal. g.-l 0.5 cal. g.-l Impurity correction = -3933.4 cal. g.-l AEc"/M(As, c, black)
*
-
AJ?&,, = AA"(ga~)]o~~'~"' f AEf(gas)l0pf(g8s). ' The internal a AEcontants = Ei(Cont.)(ti -- 25) f ES(Cont.)(25 tr f At,,). For this experiment &(Calor.)was 3363.90 cal. deg,-I. volume of the empty bomb was 0.307 1. and of the tank, 0.232 1.
was then replaced by a -70" slush bath and a similar procedure was used to remove AsF6. At -70" the vapor pressure of AsFs is minute, whereas that of AsF5 is 290 torr.6 Thus, any Asp3 that might have been produced as a by-product of the combustion reaction should have been retained in the trap. A subsequent infrared analysis of its contents failed to reveal any evidence for the presence of AsF3. In synthetic mixtures as little as 0.02% AsF3 in AsFs could be detected by this method. The nickel dish increased in weight by varying amounts in each experiment. No arsenic was detected spectrochemically in scrapings from the surface of the dish after the calorimetric series was completed. The weight gain was ascribed to the formation of NiF2.
Results Combustion Experiments. The data for seven acceptable calorimetric experiments are summarized in Table I. The symbols in Table I are those used in ref. 7. Standard state corrections were made in the usual manner.7 For the conversion of the weight of arsenic to true mass its density was taken as 5.778 g. cc.-l? For the calculation of item 4 the following heat capacity data were used: c, =: 0.106 and 0.080 cal. deg.-I g.-1 for Ni9and Aslo; C, -- 5.50 and 21.27 cal. deg.-l mole-L for F P and AsF5,12 respectively. For the calculation of item 5, values of p (atm.-l) and (&?3/dP)~(cal. atm.-l mole-I) were derived from the force constants of F213and the estimated force constants for AsP5.I4 These quantities, as functions of composition at 25", are given by p =
O.OOOSOl(l
+ 3.33~[1+ 0.83~1)
and
( ~ E / ~ P=) T1.780(1
+ 1 . 9 8 ~ [ 1+ 0.49~1)
(1) (2)
where x is the mole fraction of AsF6 in the final combustion gas mixture. I n spite of the preconditioning of the bomb and its prolonged evacuation, an exothermic effect ("blank") was observed when fluorine was expanded into the empty bomb. Determinations of the magnitude of this blank were alternated with combustion experiments. Item 6, AEblank, 1.48 i 0.51 cal., is the uniform correction applied. Item 7 is the correction applied for the formation of NiF2, based on a value of -2.76 cal. mg.-I for the energy of combustion of nickel in fluorine.16 No correction was applied for the possible formation of less than 0.02% AsF3. The sum of items 3 through 7 was divided by rn' to obtain AEco/M, the energy of combustion of the sample in fluorine. No obvious trends are apparent in the results although the mass of sample reacted was varied by a factor of two. The carbon, nitrogen, and ~
~
~-
( 6 ) 0. Ruff, 2.anorg. allgem. Chem., 206, 59 (1932).
(7) W. N. Hubbard in "Experimental Thermochemistry," Vol. 11, H. A. Skinner, Ed., Interscience Publishers Ltd., London, 1962, Chapter 6. (8) H. E. Swanson, R. K. Fuyat, and G. M. Ugrinic, "Standard X-ray Diffraction Patterns," Vol. 111, National Bureau of Standards Circular 639, U. S. Government Printing Office, Washington, D. C., 1954. (9) R. H. Busey and W. F. Giauque, J . Am. Chem. Soc., 74, 3157 (1952). (10) K. K. Kelley, U. 8. Bureau of Mines Bulletin 476, U.8. Government Printing Office, Washington, D. C., 1963. (11) W. H. Evans, T. R. Munson, and D. D. Wagman, J. Res. Nail. Bur. Std., 5 5 , 147 (1955). (12) L. K. Akers, Ph.D. Thesis, Vanderbilt University, 1955. (13) D. White, J. H. Hu, and H. L. Johnston, J. Chem. Phys., 21, 1149 (1953). (14) J. 0. Hirschfelder, C. F. Curtiss, and R. B. Bird, "Molecular Theory of Gases and Liquids," John TViley and Sons, Inc., New York, N. Y., 1964. (15) E. Rudzitis, this laboratory, unpublished measurements.
Volume 69, Number 1.8 December 1966
P. A. G. O’HAREAND WARDN. HUBBARD
4360
oxygen in the sample were assumed to be present as uncombined carbon and nitrogen, and as Asz03, respectively ; the products of combustion were assumed to be CF4, Nz, AsF5, and 02. Auxiliary enthalpies of formation (in kcal. mole-1) were taken as -221 for C F P and -155 for Asz03.17 The net impurity correction, 0.46 f 0.1 cal. g.-1, was applied to give the standard energy of combustion of arsenic in fluorine, AEc”/M, according to As(c, black) 3- 5/2F~(g)-+ AsFdg)
(3)
Derived Data. Thermodynamic data for arsenic pentafluoride are presented in Table 11. The atomic weight of arsenic18 was taken as 74.9216. Standard entropies, E”, a t 26” of crystalline arsenic,1° fluorine,ll and arsenic pentafluoride12 were taken as 8.40, 48.45, and 78.00 cal. deg.-l mole-1, respectively. The uncertainties given are uncertainty intervalslg equal to twice the combined standard deviation contributed by all known sources.
Table 11: Derived Data at 25’ As(c, black)
+ 6/&’dg)
+
AsFdg)
Energy of formation AEf” = AEc”, kcal. mo1e-l
-294.70fn.19
Enthalpy of formation AHf”, kcal. mole-1
-295.59iO.19
Entropy of formation Asf”, cal. deg.-l mole-’
-
Gibbs energy of formation A G ~ ”kcal. , mole-’
-280.23
51.53 0.19
The only previously reported value for AHf”(AsF5) was an estimate (- 266 kcal. mole-l) by Glassner.20
Bond Dissociation Energies in Arsenic Fluorides The average As-F bond dissociation energy in AsFs, L)(As-F)A,F,, is given by
T h e Journal of Physical Chemistry
D(As-F)A~F~ =
”5
[AHf”(AS, 4Sa/1) f 5AHf”(F, 2Pa/2)- AHf”(AsF6)I (4)
The enthalpy of formation of gaseous atomic arsenic may be calculated from
AHf” (As, 4i33/1)
= ‘ / z [D(Asz)
+ AHf” (Asz, g) I
(6)
Inserting values of D(Asz) = 90.8 kcal. mole-I 21 and AHf”(As2) = 53.1 kcal. mole-l,zz we obtain A H 7 2 0 8 (As, 4S,/2) = 72.0 kcal. mole-l. The latter value was combined with AHf”(F, zP3/,) = 18.9 kcal. mole-l,ll and LW~~ZLB(ASF~,) = -295.6 kcal. mole-I to give L) (As-F)A,F&= 92.4 kcal. mole-l a t 25”. An equation analogous to eq. 4 can be set up for D(AS-F)A,P,. ~~ Taking AHf”(AsF3, g) = -218.3 kcal. m ~ l e - l ,we obtain D(As-F)A,F, = 115.7 kcal. mole-’. Thus the average bond strength in AsF3 is about 20 kcal. mole-l greater than that in AsF5. A similar situation exists for the corresponding fluorides of phosphorusz4 and antimony.25 Acknowledgments. The authors are grateful to Drs. H. M. Feder and Henry &!tacklefor helpful discussions. They are also indebted to Drs. E. H. Strain and W. Ross for the neutron activation analyses, and to Dr. R. P. Larsen for help in analytical matters. Thanks are due to Mr. G. K, Johnson for preparation and analysis of high-purity fluorine, and to Mr. J. L. Settle for checking the calculations. (16) W. H. Evans, National Bureau of Standards, private communication. (17) A. de Passill&,Ann. chim. (Paris), [11]5, 83 (1936). (18) A. E. Cameron and E. Wichers, J. Am. Chem. Soc., 84, 4175 (1962). (19) E”. D. Rossini, Chem. Rev., 18, 233 (1936). (20) A. Glassner, “The Thermochemical Properties of the Oxides, Fluorides, and Chlorides to 2600°K.,” Argonne National Laboratory, ANL-5750, 1957. (21) H. J. Spangenberg, 2. Chem., 3, 315 (1963). (22) J. Drowart and P. Goldfinger, J . chim. phys., 55, 721 (1958). (23) F. D. Rossini, D. D. Wagman, W. H. Evans, S. Levine, and I. Jaffe, “Selected Values of Chemical Thermodynamic Properties,” National Bureau of Standards Circular 500, U. S. Government Printing Office, Washington, D. C., 1952. (24) P. A. G. O’Hare and W. N. Hubbard, to be published. (25) N. Bartlett, “Chemistry in Canada,” Chemical Institute of Canada, Ottawa, Can., Aug. 1963, p. 33.
