ENTHALPY OF REACTION OF SULFURAND NITROGEN TRIFLUORIDE
361
The Enthalpy of Reaction of Sulfur and Nitrogen Trifluoridel
by Lynn C. Walker Thermal Research LabOTatOTU, Dow Chemical Go., Midland, Michigan
(Received July ,029, 1966)
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Rhombic sulfur has been burned in NF3(g) using a nickel combustion bomb and static calorimeter to yield SFe(g) and N2(g). Combining the measured heat of reaction, -228.26 f 0.2 kcal/g-atom of sulfur, with the published value for AHf0298.15(SF6, g)2 we obtain AHro2s8.1s(NF3, g) = -31.75 kcal/mole.
Introduction
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Experimental Section
ture measurements. Qv = G(calor)(ti - m t At,,,) &'(contents)(ti - t h ) Ef(contents)(th - tf Atco,). Materials. Two samples of sulfur were burned in NF3. The first, laboratory designation DOW 1-S, was supplied by the Inorganic Research Group of the analytical laboratories at the Dow Chemical Co. XRay analysis showed the sample to be of the orthorhombic crystal structure and neutron activation analysis indicated 40 ppm of oxygen and a maximum of 0.1% chlorine. Infrared analysis showed no chlorinecontaining organics. The purity was taken as 99.9%. A second sample, USBM-47, was supplied by W. D. Good, U. S. Bureau of Mines, Bartlesville, Okla. This material was of the same batch that was used in ref 2 for the determination of the AHro298.ls(SFe,9). An accompanying analysis showed the sulfur sample to be of the orthorhombic variety and to contain total impurities amounting to 109 ppm. The sample purity was taken as 99.99%. Research grade NF3 was purchased from Air Products Corp. Mass and infrared spectral analysis showed the only impurity to be 0.15% CF,. By difference, the nTF3 was taken to be 99.85% pure.
Calorimetric System. The calorimeter used for this study was a conventional 25" isothermal jacket static bomb type. A nickel combustion bomb of 0.3522-1. volume was equipped with O-ring seal valves for vacuum work. The energy equivalent of the system was measured by combustion of benzoic acid (National Bureau of Standards sample 39i) in oxygen under the prescribed conditions. Eight determinations gave a value of &(calor.) = -3200.7 cal/deg with a standard deviation of the mean equal to *1.7 cal/deg (1 cal = 4.1840 absolute joules). The following expression was employed to calculate combustion heats from tempera-
(1) Presented at the 21st Annual Calorimetry Conference, Boulder, Colo., June 22-24, 1966. (2) P. A. G. O'Hare, J. L. Settle, and W. N. Hubbard, Trans. Faraday SOC.,62, 558 (1966). (3) 0. Ruff and H. Wallauer, Z. Anorg. Allgem. Chem., 196,421 (1931). (4)G. T. Armstrong, S. Marantz, and C. F. Coyle, J . Am. Chem. Soc., 81, 3798 (1959). (5) D. D. Wagman, W. H. Evans, I. Halow, V. B. Parker, S. M. Bailev. and R. H. Schumm. National Bureau of Standards Technical Note-270-1, U. 8. Government Printing Office, Washington, D. C., Oct 1, 1965. (6) J. D. Cox and D. Harrop, Trans. Faraday SOC.,61, 1328 (1965). (7) G . T. Armstrong and R. 5. Jessup, J . Res. Nail. Bur. Std., A64, 49 (1960).
Nitrogen trifluoride has become increasingly useful in the past few years as a calorimetric fluorinating agent mainly owing to its ease of handling. Volatile hydrides and fluorocarbons can be mixed with NF, without prereaction until the mixture is sparked, thereby eliminating the double-compartment type of bomb necessary when elemental fluorine is used. Also, solid inorganics which spontaneously burn in Fz may be treated with NF3 under combustion conditions again eliminating the need for complicated reactor designs. While simplicity of design is possible due to the "inert nature" of NF3, precision calorimetry requires a well-established value for the AHfo2ss.la(NFa, g). Most experiments reported in the literature have involved HF(associated gas) or HF(aq)3*4 as a product. The Wf0298.16(HF, as) is not yet well established,6r6 making its choice as a product in NFa calorimetry a serious source of systematic error. The uncertainty involved when HF(associated gas) is produced is described by Armstrong and Jessup.'
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Volume 71,Number 2
January 1967
LYNNC. WALKER
362
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Table I: S-NF3 Combustion Data
-
Corrections in celoriep
tot. cal
Run
- AEc0/M,
Qv,
8, g
no.
7 8 9 11 12b 14b l7b lgb
0.95642 0.48179 0,47001 0.96877 0.92868 0,91896 0.93002 0.93922
-6714.7 -3249.3 -3160.0 -6813.7 -6536.3 -6452.4 -6536.3 -6639.8
NFs
Mo
Ign
-115.0 -213.3 -231.2 -116.4 -128.6 -126.6 -115.8 -97.1
31.4 31.4 43.2 37.7 41.1 42.6 28.9 38.6
1.6 1.6 1.5 1.4 1.4 1.0 1.0
1.1
cal/g”
0.2 -0.3 -0.3 0.2 0.2 0.2 0.2 0.2
7106.2 7119.1 7120.7 7112.9 7130.8 7111.5 7120.3 7130.4
Av
a
Std dev of mean = 6.6 cal/g.
a Denotes Bureau of Mines sulfur.
Nature of the Reaction. Pelletized samples of sulfur were found to burn smoothly and completely in 5 atm of NF3 when ignited by electrical fusion of an 8-10-cm length of 0.005-in. molybdenum fuse wire. The reaction is shown below. S(c, rh) f 2NFdg)
-+
SF&)
+ Ndg)
(1)
Mass and infrared spectral analysis showed SFe gas as the only fluoride of sulfur. A small amount of NF3 was thermally dissociated to the elements and necessitated a correction. The molybdenum fuse wire burned quantitatively to illoF~(g)with the exception of a small, easily weighed piece. Data for this correction were available.8 Reaction with the crucible apparently did not occur as weight checks showed a constant mass. Ignition energy was measured by discharging a standardized capacitor. The sum of the above corrections amounted to approximately 1-2% of the measured heat in reaction 1. Procedure. After the benzoic acid calibration experiments were carried out, the nickel bomb was passivated by carrying out several of the initial exploratory determinations. Between runs the bomb was kept under vacuum to preserve the nickel fluoride coating and all loading operations were carried out in a nitrogenatmosphere glove box. Sulfur was pelletized on 0.5- and l-g quantities, weighed in air on a microgram balance, and transferred to the drybox with the evacuated bomb and fuse. After placing the pellet in the nickel crucible the molybdenum fuse was placed around the pellet and attached securely to two electrode posts. The bomb was then evacuated to less than 5 p and charged with 5 atm of NF3. The bomb was weighed before and after charging on a 10-kg capacity balance to determine the mass of NFg. The loaded bomb was then placed in the calorimeter and the heat of conibustion measured. The J O U Tof ~Phy8ical ~ Chemistry
7119.0
Immediately after the heat measurement the product gases were analyzed for free fluorine by reaction with mercury. The analysis vessel consisted of a 250-ml Pyrex flask with a tube extension on the bottom to contain 3-5 ml of mercury. The flask was evacuated, then filled to 1 atm with product gases from the bomb. The gases were allowed to condition the bulb and vacuum system for a minimum of 4 hr before a gas sample was taken for quantitative analysis. This technique was checked out both with mixtures of fluorine and nitrogen and with fluorine of 99.5% purity. Recoveries of fluorine on these gases averaged =!=0.20j, of theory. “Blank” experiments showed that NF3 and SF6 did not react with the system. From the gas sampling one could calculate the grams of F per gram of gas and from this the moles of NF3 decomposed. The residual gases SFs, NF3, and Nz were then checked by mass and infrared spectroscopy.
Results and Discussion I n Table I (S-NF3 combustion data) we have listed eight determinations. The first four are on the Dow sulfur sample, the last four are on the Bureau of Mines sulfur. Columns 4-7 are thermochemical corrections to QY,the total observed heat. These have been discussed earlier. AE gas is a correction to standard state involving critical constants9 for the gases NF3, SF6, Nz,and Fz. The correction was calculated in the usual manner.’O The standard state enthalpy of reaction is calculated using 32.064 for the atomic weight of sulfur. (8) J. L. Settle, H. M. Feder, and W. N. Hubbard, J . Phys. Chem., 65, 1337 (1961). (9) M.Stacey, J. C. Tatlow, and A. G. Sharpe, Advan. Fluorine Chem., 4, 189 (1965). (10) W. N. Hubbard, “Experimental Thermochemistry,” Vol. 11, H. A. Skinner, Ed., Interscience Publishers Ltd., London, 1962, Chapter 6.
ENTHALPY OF REACTION OF SULFUR AND NITROGEN TRIFLUORIDE
AEoo298.15= m c " 2 9 8 . 1 6 = -228.26 f 0.2 kcal/gatom of sulfur. The uncertainty interval, 2u, is twice the over-all standard deviation of the mean as determined from calibration experiments and the SNF3 combustions. Taking AHf02~8.16(SF~, g) = -291.77 f 0.24 kcal/mole2, we calculate AHf'298.16 (NF3, g) = -31.75 f 0.2 kcal/mole. The work by Sinke on the enthalpy of dissociation of NF3 appearing in the preceding paper is in agreement within experimental error. Ludwig and Copper1' carried out the combustion of boron using NF3 as an oxidizer. Their measured AH," yields -32 kcal/mole for Hrozgs.16(NFa, g) using the latest BFa heat of formation = -271.65 kcal/ mole. l 2 Derived Bond Energy. The average (N-F) bond energy of 67.1 kea1 is calculated from the above result and "JANAF" values for heats of atomization. Armstrong and co-workers have measuredla the heats of reaction of NF3 and NzF4 with NH3 to yield NH4F(c). Their data is combined with the present work for AHro2s8.1s(NF3, g) to derive AH~'ZS~.I~(NZF~, g) = - 5 kcal/mole. This value is combined with the heat of dissociationla to two NFz radicals to give AHfO298.16
363
(NF2, g) = 8.5 f 2 kcal/mole. From this we calculate the energy required for the dissociation of the first F from NF3. NF3(g) +NF2(g)
+ F(g)
(2)
D(NF2-F) = 59.1 2 kcal/mole. This is considerably less than the average and is in agreement with the electron paramagnetic resonance work by Kennedy and Colburn14in which they report D(NF2-F) = 57.1 f 2.5 kcal/mole. Thus both spectral and thermal evidence shows that NFS differs widely from NH3in that it first forms NF2 radicals which are apparently stabilized by some bond hybridization. Acknowledgment. This work was supported by the
U. S. Air Force under Contract No. AF04(611)-11202. (11) J. R. Ludwig and W. J. Cooper, J . Chem. Eng. Data, 8 , 76 (1963). (12) G. K. Johnson, H. M. Feder, and W. N. Hubbard, J. Phys. Chem., 70, 1 (1966). (13) "JANAF Thermochemical Tables," The Dow Chemical Co., Midland, Mich. (14) A. Kennedy and C. B. Colburn, J . Chem. Phys., 3 5 , 1892 (1961).
Volume 71, Number 8 January 1967
