ENTHALPY OF DISSOCIATION OF NITROGEN TRIFLUORIDE
359
The Enthalpy of Dissociation of Nitrogen Trifluoride
by G. C. Sinke Thermal Research Laboratory, Dow Chemical Co., Midland, Michigan
(Received July $9,1966)
The enthalpies of explosion of mixtures of hydrogen and excess NF3 were measured in a bomb calorimeter. From the measurements is derived the enthalpy of dissociation of the excess NF3 and an enthalpy of formation of -31.44 f 0.30 kcal/mole.
Introduction I n working with NF3 as an oxidizer in calorimetry, it was noted’ that when mixtures of NFa and an oxidizable substance are detonated, excess NF3 is dissociated to the elements. This suggested the feasibility of determining the enthalpy of dissociation of NF3 by measuring the enthalpy of explosion of mixtures of an oxidizable substance and a large excess of NF3. For the present work, mixtures of hydrogen and NF3 were used to define successfully the enthalpy of dissociation of NF3.
Experimental Section Materials. Materials were the same as those employed in previous work. Procedure. An improved platinum-lined combustion bomb of 350-ml volume was developed in cooperation with the Parr Instrument Co. The bomb cylinder and head were dried a t 110” for 1 hr, assembled, connected to a vacuum line, and evacuated while still hot. After cooling to room temperature, the bomb was surrounded by R constant-temperature water bath and pumped until a pressure of at least 1 p was achieved. The bomb was then charged with 799.0 mm of hydrogen as measured by a Wallace and Tiernan precision dial manometer which could be read to 0.1 mm. Nitrogen trifluoride was contained in a small stainless steel cylinder which could be weighed on a 200-g capacity analytical balance. The cylinder was connected to the vacuum line and the bomb charged with about 0.75 g of NF3 (10% excess) or 1.38 g of NF3 (100% excess) in alternate runs. Sample remaining in the connecting lines was recondensed in the cylinder with liquid nitrogen and the exact amount charged determined by reweighing. In the 10% excess runs, an additional 0.5 g of research grade Nz was charged to the bomb in order
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to make the final state of the bomb products as similar as possible in the two cases. The bomb was placed in a conventional combustion calorimeter and the charge was fired by discharging a condenser through a fine platinum fuse wire. The condenser voltage before and after firing was a measure of the ignition energy. The calorimeter was calibrated with National Bureau of Standards benzoic acid 39i. An average of 0.55 g of acid was used and the bomb was charged with 1 ml of water and 30.3 atm of oxygen. In the notation of Hubbard, Scott, and Waddington, the calorimeter equivalent &(calor) was 3436.18 cal/deg with a standard deviation of 0.52 cal/deg for five experiments. After the calorimetry was completed, the gaseous reaction products were passed over a NaF trap to remove H F and into a bulb containing mercury to react out the fluorine. The remaining gas was examined by infrared and mass spectroscopy. Only nitrogen and a small amount of CF, (from reaction of fluorine with a fluorocarbon gasket) was found. In some of the 100% excess NF3 runs a trace of NF3 (less than 0.05% of the original sample) was observed. The bomb was flushed with nitrogen and then opened and the surfaces washed with 6 N HC1 to dissolve small amounts of PtF4 and AuF3 formed by fluorine attack on the fuse wire and electrodes. The solutions were analyzed for platinum and gold by atomic absorption techniques.
Results The results of six 10% excess NF3 experiments are given in Table I and of seven 100% excess NF3 experiments in Table 11. The final temperature of the (1) G. C. Sinke, J . Phys. Chem., 70, 1326 (1966). (2) W. N. Hubbard, D. W. Scott, and G. Waddington, “Experimental Thermochemistry,” Vol. I, F. D. Rossini, Ed., Interscience Publishers, Ino., New York, N. Y . , 1956, Chapter 5, pp 75-128.
vo~unte71, Number 8 January 1967
G. C. SINKE
360
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Table I: Results for 10% Excess NF3 Runs Mass 8
Tot. cal
0.7559 0.7581 0.7485 0.7584 0.7401 0.7486
- 1744.0 -1740.0 - 1745.6 -1737.2 - 1749.2 -1745.4
of NFa,
Corrections in calories AuFa
CFI
PtFi
0.8
1.6 1.1 1.0 1.0 1.3 1.5
1.0 0.7 0.5 0.6 0.6
0.1 0.3 0.4 0.1 0.3 0.1
kn
Adj
0.2 0.7 0.2 0.7 0.2 0.2
-2.6 -3.5 0.7 -3.6 4.3 0.6
Net cal
- 1743.9 -1740.4
- 1742.6 -1738.5 -1742.5 - 1742.4
AV -1741.7
Table 11: Results for 100% Excess NF, Runs Mass of NFa,
Tot. cal
g
- 1477.0
1.3754 1.3880 1.3961 1.3783 1.3765 1.3655 1.3775
- 1471.5 - 1473.9 - 1478.2 - 1475.7 -1483.2 -1474.0
CF4
PtFi
5.4 6.5 5.7 4.6 5.0 5.6 4.6
2.5 2.9 2.6 2.6 1.8 1.8 1.5
Corrections in calories AuFs
0.2 0.2 0.2 0.3 0.2 0.1 0.1
---. Ign
Adj
Net cal
0.4 0.2 0.2 0.2 0.2 0.2 0.2
2.0 -3.5 -7.0 0.8 1.5 6.4 1.1
- 1466.5 - 1465.2 - 1472.2 - 1469.7 - 1467.0 - 1469.1 - 1466.5
AV -1468.0
bomb was 25.00 f 0.01’ in all experiments. The total calories column in the notation of Hubbard, Scott, and Waddington2 is equal to E(calor)(ti - tr At,,,) Gi(cont)(ti - th) 4- E‘(cont)(th - tf 4- At,,,). The correction for fluorine attack on the fluorocarbon was taken as 123.7 kcal/mole of CF4 f ~ r m e d . ~The enthalpy of formation of PtF4 was estimated a t -180 kcal/mole and of AuF3 as -130 kcal/mole. The runs were adjusted to an exact quantity of 0.7500 g of YF3 in Table I and 1.3800 g of NF3 in Table I1 by means of an interative procedure for the energy of dissociat,ion of NF3. The difference between the two sets of experiments, 273.7 ca1, is then the energy of dissociation of 0.6300 g of NF3. Atomic weights of 14.0067 and 18.9984 for nitrogen and fluorine, respectively, are employed to derive AErm = 30.85 kea1 for the reaction
+
XF3(g)
+
l/zNZ(g)
+
+ 3/2Fz(g)
The reverse process when calculated to constant pressure conditions is the enthalpy of formation of NR Aflr 9g8(1CTF3, g) = -31.44 kcal/mole
The complex final state of the experiments makes es-
The Journal of Physical Chemistry
timates of corrections to standard states of dubious value. The experiments were designed to keep such corrections to a minimum. The corrections are believed to be small compared t,o the over-all uncertainty of 0.3 kcal/mole based on twice the standard deviation.
Conclusions The present value can be combined with previous work4 on the enthalpy of reaction of Hz and NF8 to derive an enthalpy of formation of HF (1 in 123 HzO). The result, -77.0 f 0.2 kcal/mole, lies between the latest “selected best value” of the National Bureau of Standards5 and the recent s h d y by Cox and Harrop.6
Acknowledgment. This work was sponsored by the U. S. Air Force under Contract No. AF04(611)-11202. ~~
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(3) E. 5. Domalski and G. T. Armstrong, J . Res. A’atl. Bur. Std., A69, 137 (1965). (4)G.C. Sinke, J. Chem. Eng. Data, 10, 295 (1965). (5) D.D. Wagman, W. H. Evans, I. Halow, V. B. Parker, 5. M. Bailey, and R. H. Schumm, National Bureau of Standards Technical Note 270-1, U. S. Government Printing Office, Washington, D. C., Oct 1, 1965. (6) J. D. Cox and D. Harrop, Trans. Faraday SOC.,61, 1328 (1965).
