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The enthalpies of formation of uranium mononitride and α- and β-uranium sesquinitride by fluorine bomb calorimetry
M-1239 J. Chem.
thermodynamics
1981, 13, 273-282
The enthalpies of formation of uranium mononitride and oc- and p-uranium sesquinitride by fluorine bomb calorimetrya GERALD
K. JOHNSON
Chemical Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, U.S.A. and E. H. P. CORDFUNKE Netherlands Energy Research Foundation ECN, Petten, The Netherlands (Received 1I August 1980) The energies of combustion in fluorine of uranium mononitride and a- and g-uranium sesquinitride were measured in a bomb calorimeter. The derived standard enthalpies of formation, AH,“(UN,, c, 298.15 K), are - (290.3+ 2.2), - (391.5+ 2.3), - (382.2+2.3), and -(362.9+2.3) kJ mol-’ for UN,,,,,, a-UN,.h74r a-UN,,,,,, and b-UN,.466. respectively.
1. Introduction
Previous fluorine bomb-calorimetric measurements of the enthalpies of formation of uranium mononitride and u-uranium sesquinitride have been reported”’ from this laboratory. These results have subsequently been revised by Oetting and Leitnaker”’ because of a suspected bias in the analytical procedures used for the determination of uranium. These revised results are in general agreement with results from other techniques and have been regarded as the most reliable values available.‘3’ However, a recent revision (4) of AHF(UF,, 298.15 K) by approximately 11 kJ mol- ‘, would presumably affect by a similar amount the enthalpies of formation of the uranium nitrides based on fluorine bomb calorimetry. Such a change, however, thus causes serious disagreement between the fluorine bomb results and those from other methods. In order to resolve these difficulties, a re-determination of the enthalpies of formation of the uranium nitrides by fluorine bomb calorimetry was undertaken. The current availability of well-characterized samples of UN and a-U,N, as well as l3U,N, and procedures which yielded nearly complete combustion, gave us confidence that reliable and definitive results for the enthalpies of formation were obtainable. 0 This work
was performed
under
the auspices
of the U.S. Department
of Energy.
274
G. K. JOHNSON AND E. H. P. CORDFUNKE
2. Experimental CALORIMETRIC
SYSTEM
The calorimetric system consisted of a two-chambered nickel reaction vessel similar to that described by Nuttall et ~1.‘~’ and a rotating bomb calorimeter, laboratory designation ANL-R-3, similar to that described by Hubbard et al.‘@ Calorimetric temperatures were measured with a quartz-crystal thermometer (Hewlett-Packard Model 2801-A). The calorimetric system was calibrated by combustion in oxygen of National Bureau of Standards benzoic acid (Sample 39i), whose certified specific energy of combustion under prescribed conditions is - (26.434 f 0.003) kJ g- ‘. Ten calibration experiments, five performed before and five after the uranium nitride combustions, gave an average value and standard deviation of the mean for .s(calor), the energy equivalent of the calorimetric system, of (14071.17 kO.54) J K-i. MATERIALS
The uranium nitride samples were prepared at the Netherlands Energy Research Foundation ECN, Petten, by the reaction of uranium metal and nitrogen gas. The a-U,N, was prepared as follows: National Bureau of Standards uranium metal (Sample 960), with a purity of (99.975 kO.017) mass per cent, was surface cleaned by mechanical polishing and converted to UH, by reaction with hydrogen at 523 K. The material was then dehydrided at 723 K. Successive hydriding and dehydriding procedures produced a very finely divided uranium powder. The powder was then converted to a-U,N, by reaction with argon-diluted nitrogen (approximately 9 moles per cent of N2) at 1023 to 1073 K. The stoichiometry of the U,N, sample is dependent on the temperature of the reaction and on the nitrogen partial pressure. Two samples of a-U,N, having the designations a-U,N,-10 and a-U,N,-12 were prepared for the calorimetric measurements. The UN was prepared by heating a-U2N, in argon at 1673 K for several hours. The calorimetric sample was designated UN-36 X-ray diffraction analysis, using procedures described previously, (‘r showed the sample to have a cubic structure with a = (0.48897 + 0.00004) nm. The j3-U,N, was prepared by heating a-U,N, for seven hours in an induction furnace at 1523 K under a nitrogen pressure of 50 kPa. The sample was then cooled very rapidly, while the nitrogen atmosphere was simultaneously pumped off. The X-ray diffraction pattern of this material indicated that approximately 4 mass per cent of the B-U,N, had been converted to a-U,N, during the quenching procedure. The lattice parameters of the hexagonal B-U,N, preparation were u = (0.3698 f 0.0001) nm and c= (0.5832 + 0.0002) nm. Following preparation, portions of the calorimetric samples were sealed under helium and shipped to Argonne National Laboratory where they were stored and handled exclusively in a high-purity helium-atmosphere glovebox. Complete analyses of the sample compositions using procedures described previously”’ are given in table 1. It is assumed that the non-metal impurities are substituted for nitrogen in the nitrogen sublattice and the metal impurities for uranium in the uranium sublattice.
ENTHALPIES TABLE
OF FORMATION
OF URANIUM
1. Analytical results for the uranium nitrides; w denotes mass fraction UN-36
a-U,N,-10
element a
a-U,N,-12
P-UP,
lo2 )I‘
U N 0 C Si Al Ca co Cr Fe Mg Mn
94.20 5.41 0.120 0.008 0.0025 0.0015
tie V
0.0010
Non-metal-tometal
275
NITRIDES
0.0010
0.997 + 0.004
90.84 8.87 0.070 0.017 0.0025 0.0020 0.0010 0.0008 0.0015 0.0002 0.0004 o.OcK32 0.0012 o.c0O7 1.674 f 0.005
91.20 8.49 0.121 0.020 0.0025 0.0015 0.0020 0.0005 0.0005 0.0015 0.0002 0.0001 0.0008 0.0004 1606~0.005
91.63 7.83 0.038 0.014 0.0025 0.0025 0.0010 OS!018 0.0010 0.0015 0.0002 0.0002 0.0005 0.0005 1.466 f 0.005
a The uncertainties on the U, N, 0, and C mass fractions are +2x 10m4, + 2 x 10e4, & 1.5 x 10. 4. and It_2 x lo- ‘, respectively; the remaining spectrochemically determined impurities were assumed to have uncertainties of + the value determined.
Hence, when corrected for impurities, the derived results will be for uranium nitrides having N-to-U mass ratios which are the same as the non-metal-to-metal mass ratios of the samples. Tungsten foil (0.025 mm), which was used as an auxiliary combustion aid, was purchased from Schwarzkopf Development Corporation. Five combustion experiments in fluorine were conducted with the tungsten foils, and a value and standard deviation of the mean of - (9382.24+ 0.84) J g- ’ were obtained for the standard energy of combustion, AVz/M(sample), of this tungsten sample. Sulfur, used in the combustions as an ignition aid, was taken from the same lot used to determine AJ$(SF,,g).‘*) Purified fluorine (99.99 moles per cent) was prepared by distillation of commercial fluorine in a low-temperature still.“’ PROCEDURES Preliminary experiments established that the uranium nitride powders were attacked by high-pressure fluorine and, hence, that it would be necessary to use a twochambered bomb for these combustions so that the sample could be isolated from the fluorine until reaction was desired. It was further established that the combustion of the uranium nitrides without an auxiliary combustion aid usually resulted in a large residue of U,F, ,. It was found, however, that by using several pieces of tungsten foil under the sample, the amount of the U,F,, residue could be significantly reduced.
276
G. K. JOHNSON
AND
E. H. P. CORDFUNKE
The procedures used in the combustions were as follows. The bomb was taken into the glovebox and opened. Two pieces of the 0.025 mm tungsten foil, one of which had been formed into a saucer shape and the other a square piece which had the four corners turned down to form legs, were weighed. An approximately 2 g sample of uranium nitride was placed in the saucer-shaped foil and the tungsten plus uranium nitride were then weighed. A small amount of sulfur. used to ignite the sample, was then placed on top of the uranium nitride and the saucer and the contents were reweighed. The one square tungsten piece was placed on its legs on the bottom of a weighed 32 g nickel crucible, and the other saucer-shaped foil containing the sample was placed on top of the crucible, with the lip of the saucer resting on the top edge of the crucible. The bomb was assembled, and the tank, which had been charged to 1997 kPa pressure with fluorine, was attached. The assembled reaction vessel was removed from the glovebox, evacuated, placed in the calorimeter, and the experiment preformed. Following each combustion the bomb was evacuated, returned to the glovebox, and opened. In most experiments a small amount of residue, identified as U,F,, by X-ray diffraction, was found in the crucible. The amount of U,F,, was determined by weighing the crucible plus residue, removing the residue by gentle scraping and then reweighing the crucible. The mass of the crucible usually increased slightly from experiment to experiment and was attributed to the formation of NiF,. 3. Results The results of the combustions of UN,,,,,, E-UN,,,,,, cl-UN,,,,,, and P-UN,.,,, at 298.15 K are presented in tables 2 to 5, respectively. The general combustion reaction is UN,(c)+3F,(g)
= UF,(c)+txN,k).
(1)
The corrections to standard states were applied in the usual manner.(“) Most of the entries in the tables are self-explanatory or have their usual significance. The following, however, require some additional explanation: AU(U,F,,) is the thermal correction for the hypothetical conversion of the U4F1, combustion residue to UF,(c); and AU(condense UFs) is the thermal correction for the hypothetical condensation of that portion of the UF, product which is gaseous. For the calculation of AU,,,, the following values were used: cp/J K- ’ g- ’ for UN,(3’ a-U,N3,‘3’ p-U,N3,(3’ W,‘“’ So2 Ni,“” and UF,(c),‘i3’ of 0.189, 0.209, 0.208, 0.132, 0.706, 0.444, and 0.473, respectively, and C,/J K-’ mall’ for gaseous F,,(i2’ N2,(12’ WF,,“” SFs,(12’ and UF, ‘14’ of 22.98, 20.81, 110.7, 88.96, and 121.3, respectively. For the calculation of AU,,, p in the equation of state pV = nRT( 1-pp) and (aU/ap)r were estimated by the method of Hirschfelder et a/.“” from the intermolecular-force constants for UFs,(16’ WF6,‘16’ F,,“” SF,,“s and N2.(ls’ The densities of UN, U2N3, W, Ni, and UF,(c) were taken to be 14.32,8.94, 19.3,8.9, and 4.68 g cme3, respectively. The volumes of the empty bomb and tank were 0.3059 and 0.2361 dm3, respectively. For the calculation of AUsu,rurr the specific energy of combustion of the sulfur sample was taken to be - 37917.1 J g-l.@’ AUblank is the
ENTHALPIES
OF FORMATION TABLE
mWh9,
reacted)/g
m(W reacted)/g m(S reacted)/g m(LJ,F,, formed j/g A&/K e(calor)( - AO,)iJ AU,.,IJ AU&J
A(J,,,,,lJ Au,,,,,,dJ Au,,,JJ AL’W,F,
,)/J
AU,,,&J
AU(condense UF, )/J ~AL’~/M(sample)}/J g- ’
1.99918 1.14448 0.00313 0.00071 1.83187 - 25776.55 -58.15 -0.75 118.68 10737.79 - 10.29 -0.51 0.00 -85.17 -7540.57
OF URANIUM
2. Results
2.00442 1.14413 0.00482 0.01724 1.83818 - 25865.34 - 58.32 -0.71 182.76 10734.50 - 10.29 - 12.44 6.38 -85.17 -7537.66
of UN,,,,,
2.00336 1.12144 0.00487 0.00128 1.82270 -- 25647.52 58.06 -0.75 184.66 10521.62 - 10.29 -0.92 0.00 -85.17 - 7535.56
277
NITRIDES
combustions 2.00621 1.10253 0.00308 0.01179 1.80653 - 25419.99 - 57.29 - 0.75 116.78 10344.20 - 10.29 -8.51 0.00 -85.17 -7537.11
1.01189 1.08832 0.00280 O.ooO25 1.26687 - 17826.34 - 23.85 0.54 106.17 10210.88 - 10.29 -0.18 3.96 -86.12 - 7535.63
2.99877 1.10502 0.00346 0.00454 2.33947 - 32919.08 -75.51 - 2.26 131.19 10367.56 - 10.29 - 3.28 0.00 - 85.08 - 7535.34
1.01164 1.11962 0.00384 0.0025 I 1.29007 - 18152.79 - 24.28 0.59 145.60 10504.54 - 10.29 -1.81 0.00 - 86.12 - 7536.83
(AU:/M(sample)): - (7536.96kO.69) J g-’ a Impurity correction: -(7.6* 1.8) J g-i” (AU;/M(UN,.,,, f 0.0004)) : - (7544.6 f 4.6) J g ’ * 0 f Standard
deviation
of the mean.
TABLE
WJNl.674
reacted)/g
m(W reacted)/g m(S reacted)/g m(U,F,, formed)/g A&/K E(calor)( - AO,)lJ
AU,JJ AU&J AU,,,,lJ AU,,,,,/J AUtiurJJ LIUWZI,
AU,,,IIJ
j/J
AU(condense UF,)/J I AUP/M(sample)}/J g-- ’
b*
Uncertainty
3. Results of
2.01615 1.12304 0.00340 0.00037 1.73638 - 24432.90 - 34.43 -0.44 128.92 10536.63 - 10.29 -0.27 4.48 -86.12 -6891.56
interval.
a-UN,.,,,
2.00767 1.07405 0.00383 0.00049 1.70091 - 23933.79 - 34.02 - 0.48 145.22 10076.99 - 10.29 -0.35 4.14 - 86.12 - 6892.92
combustions. 1.99972 1.08422 0.00348 0.00158 1.70244 - 23955.32 - 34.03 - 0.47 131.95 10172.41 - 10.29 -1.14 0.00 -86.12 - 6892.47
1.99838 1.15196 0.00375 0.00005 1.74840 - 24602.03 - 34.97 -0.43 142.19 10807.97 - 10.29 -0.04 0.00 - 86.12 - 6897.45
2.00889 1.15413 0.00296 0.00401 1.75238 - 24658.04 - 34.76 -0.42 112.23 10828.32 - 10.29 - 2.89 0.00 -X6.12 - 6895.34
(AU,‘/M(sample)): -(6893,9+1.l)Jg-‘” Impurity correction :(6.8 k 2.1) J gg’ ’ (AU:/M(UN,,,,*,,,)):-(6887.1+5.1) J g-’ b ” + Standard
deviation
of the mean.
bf
Uncertainty
interval.
combined thermal correction for the expansion of F, from the tank into the combustion chamber and for any subsequent reaction of F, with the bomb surfaces. This was determined to be (10.29 kO.21) J in a series of seven experiments in which fluorine was expanded into an empty bomb. These experiments were interspersed among the uranium nitride experiments. For the calculation of AUNi, the specific energy of combustion of Ni to form NiF, was taken to be - 11158.3 J g-‘.(19’ For the
278
G. K. JOHNSON TABLE
m(UN1.606reactedVg
4. Results
2.02767 1.17208 0.00398 0.00082 1.78486 -25115.07 - 35.71 -0.45 150.91 10996.74 - 10.29 -0.59 5.69 -86.12 -6951.27
m(W reacted)/g m(S reacted)/g m(U4F17formed)/g WK s(calor)( - Ae,)/J
AtJ,JJ AtJ,JJ AU,W/J AU,,.,dJ Au,,lJJ AuW,F,,)/J AU.,MIJ
AU(condense UF,)/J (AU~/M(sample)}/J g-i
AND
E. H. P. CORDFUNKE
of x-UN,.,a6
2.01269 1.13208 0.00317 0.00074 1.74730 - 24586.56 - 34.65 - 0.49 120.20 10621.45 - 10.29 -0.53 0.00 -86.12 - 6944.43
combustions 2.00829 1.16294 0.00333 0.01624 1.76650 - 24856.72 - 35.35 -0.47 126.26 10910.98 - 10.29 -11.72 6.38 - 86.12 - 6949.72
2.02101 1.18177 0.00402 0.00064 1.78739 - 25150.67 - 35.78 - 0.45 152.43 11087.65 - 10.29 - 0.46 6.90 -86.12 - 6945.43
1.99984 1.14937 0.00377 0.00083 1.75484 - 25692.65 - 35.10 - 0.48 142.95 10783.67 - 10.29 -0.60 0.00 -86.12 - 6949.87
(AU,“/M(sample)): -(6948.1+ 1.3) J g-’ ’ Impurity correction :(0.0*2.1) J gg” (Au~/M(UN,.,,,,)):(6948.1 k5.3) J g-t * a k Standard
deviation
of the mean.
TABLE
WNl.466 reacted)/g m( W reacted)/g m(S reacted)/g mW.J~, fomed)/g WK s(calor)( - A&)/J AU,“t/J AUs.,,‘J
AU,,,,,IJ AU,,,dJ W,.,,t/J AWJ,F,,)/J AU&J
AU(condense UF,)/J {AU,“/M(sample)}/J g-t
b + Uncertainty
5. Results
2.00169 1.10946 0.00446 O.OOOOO 1.75062 - 24633.27 - 34.7 1 -0.55 169.11 10409.22 - 10.29 0.00 7.93 -86.12 - 7083.35
of
1.99918 1.06496 0.00643 0.00016 1.72456 - 24266.58 - 34.49 - 0.58 243.81 9991.71 - 10.29 -0.12 7.93 -86.12 - 7080.27
B-UN1,466
interval.
combustions
2.00871 1.09176 0.00433 O.Oc088 1.74162 - 24506.63 - 34.52 -0.56 164.18 10243.15 - 10.29 -0.63 1.38 -86.12 -7084.17
2.00723 1.05576 0.00594 0.00334 1.72194 - 24229.7 1 - 34.43 -0.59 225.23 9905.39 - 10.29 -2.41 7.93 - 86.12 - 7086.88
2.00534 1.11669 0.00288 O.OWOO 1.75264 - 24661.70 - 34.75 - 0.55 109.20 10477.05 - 10.29 0.00 5.86 86.12 - 7081.74
2.00594 1.05480 0.00466 0.00025 1.71654 - 24153.73 - 34.31 -0.59 176.69 9896.39 - 10.29 -0.18 5.17 -86.12 - 7082.45
(AUg/M(sample)) : - (7083.14+0.93) J g-’ ’ Impurity correction :(8.6 + 2.3) J g- ’ ’ (AU:/M(UN,.,,,,,)): - (7074.5 k5.5) J g-i b 4 Standard
deviation
of the mean.
b + Uncertainty
interval.
calculation of ALr(condense UF,), the energy of sublimation of UF6 was taken to be 47.10 kJ mol-’ based on AH,, = (49.58kO.42) kJ mol-‘.“3J The vapor pressure of UF,(c) at 298.15 K was taken to be 14.93 kPa. (“J For the calculation of AU(U,F,,), the specific energy of combustion of U4F,, to form UF,(c) was derived to be -(722-L-63) J g-’ based on AZ-ZfO(UqF17,c) = - (7861+ 80) kJ mol- ‘. The latter value is based on the preliminary value AIf,“(UF,,c) = - 1921 kJ mol-‘,‘21)
ENTHALPIES
OF FORMATION
OF
A&‘(UF,,g) = -(2148.1 + 1.8) kJ mol-1,‘4) reaction :(’ 3, 2U,F,,(c)
URANIUM
279
NITRIDES
and the enthalpy,
126 kJ mol-‘,
of the
= 7UF,(c)+UF,(g).
(2)
The impurity corrections are based on the assumptions stated previously, namely, that the non-metals occupy positions in the nitrogen sublattice and the metals positions in the uranium sublattice. For the uranium mononitride sample the impurities were taken to be UO, UC, USi, and metal mononitrides and for the uranium sesquinitrides they were taken to be U,O,, U,C,, U,Si,, and metal sesquinitrides. The impurities were assumed to form O,, CF,, SiF4, and the most stable fluoride of the metal during combustion. The major corrections are due to the oxygen and carbon impurities; the other impurities make only a minor contribution. Potter and Rand(j) have estimated the enthalpy of the r-to-p transition in UN,,, to be 2.5 kJ mol- ‘. Based on this value, the correction for 4 mass per cent of a-U,N, in the /3-U,N, sample is 0.4 J g-r. The derived results for the uranium nitrides are given in table 6. The molar masses of uranium and nitrogen were taken to be 238.029 and 14.0067 g mol-‘, respectively. The enthalpy of formation of UF,(c) used in calculating AI$(UN,) was - (2197.7 + 1.8) kJ mol-‘. The uncertainties given in the table are uncertainty intervals’22’ equal to twice the combined standard deviations arising from a)) known sources. The uncertainties include an appreciable contribution from the uncertainties in the N-to-U mass ratio. In addition to the combustions described above, a series of combustions of a sample of uranium sesquinitride used in the previous study of O’Hare et al.,“’ was also conducted. The sample, UN1.69,(1) was reacted using the same combustion procedures described herein, i.e. using high fluorine pressure and a tungsten auxiliary, as well as using procedures similar to those described by O’Hare et al.,“’ i.e. using diluted fluorine and no auxiliary. Two combustions (designated Series A), using procedures identical to those described herein gave results for AUz/M(sample) of - 6876.3 and -6871.6 J g-‘. Five combustions (designated Series B), without tungsten auxiliary gave results for AU,“/M(sample) of - 6866.0, - 6886.9, - 6811.4, -6866.8, -6792.8 J g-r. The average and standard deviation of the mean was -(6844.8& 18.1) J g-l. O’Hare et al. (I) had obtained for AU,“/M(sample): -6797.7. -6874.3, -6823.3, -6830.8, -6830.8, and -6847.1 J gg’, with the average and standard deviation of the mean : - (6834.1+ 10.5) J g-r. The difference between the
TABLE
6. Derived
UN 0997*0.N!-$ A&/kJ AfQkJ Aff;/kJ
molt ’ mol - ’ mol ’
-(1901.2+1.2) -(1907.4+1.2) - (290.3 f 2.2)
results
for the uranium a-UN
1.671*0005
-(1800.8k 1.4) - (1806.2 + 1.4) -(391.5*2.3)
nitrides
at 298.15
ci-UN 1.606*0.CKX -(1810.1+ 1.5) -(1815.5+ 1.5) - (382.2 + 2.3)
K &UN
I466*o.w5
-(1829.2* 1.5) -(1834.8+1.5) - (362.9 f 2.3)
280
G. K. JOHNSON
AND
E. H. P. CORDFUNKE
Series A results and those of O’Hare et al. is 10.4 kJ mol ‘, which is almost the same difference previously found (4) between uranium combustions, 11 .O kJ mol- ‘, when similarly different combustion procedures were employed. Hence, it appears that there is a systematic difference which is in some way related to the combustion procedure. The Series B experiments were conducted to see if this difference could be explained. The average of the Series B results is in agreement with the results of O’Hare et al. within the relatively large uncertainties. However, three of the five results would be in good agreement with the Series A results and hence, the results are inconclusive. Without an auxiliary combustion aid, there tended to be a very large combustion residue, between 0.07 and 0.37 g, which adds to the uncertainty. Hence. we cannot unequivocally account for the difference between our results and those obtained previously. We feel, however, that the combustion procedure employed in this study, which resulted in almost complete reaction of the samples, should yield more reliable results. The enthalpy of formation of UN has been determined calorimetrically by several other investigators. Neumann et aI.‘23’ measured the enthalpy of reaction of very impure uranium with nitrogen at 2533 kPa pressure and 973 K. Under should be formed. Their result, these conditions some UN,,, M,“(UN, c, 298.15 K) = -(286.6+8.4) kJ mol-‘, is, however, in good agreement with the present value. Gross et al. (24’ have also studied the direct reaction of finely divided uranium with nitrogen in a hot-zone calorimeter to obtain A&‘(UN, c, 298.15 K) = - (291.2f3.3) kJ mol-‘. This result is also in good agreement with the present value. O’Hare et al.“’ have determined AEZ,“(UN) by fluorine bomb calorimetry. The reported value, A&‘(UN,,965, c, 298.15 K) = -(299.2f4.6) kJ mol-‘, was subsequently changed to M,“(UN,,,,,, c) = - (296.2 f 4.6) kJ mol- ’ by Oetting and Leitnaker”’ to account for a suspected analytical error in the uranium content of the sample. The latter, but not the original, result would be in agreement with the present AH,“. However, with the more recent valueC4) for AiY;(UF,, c, 298.15 K), -(2197.7_f1.8) kJ mol-’ rather than the value used by O’Hare et al.: -(2186.7+ 1.8) kJ mol-‘, mF(UN,,,,,) becomes -(307.2+4.6) kJ mol.-‘, in severe disagreement with the present value. Because of these difficulties it seems prudent to give that study little or no weight. In addition to the calorimetric studies, Oetting and Leitnakert2’ have reassessed the high-temperature vaporization study of Inouye and LeitnakerC2” and deduced A&‘(UN, c, 298.15 K) = - (295.4f4.2) kJ mol-‘, also in agreement with the present result. The enthalpy of formation of a-uranium sesquinitride has been determined calorimetrically by Gross et al .(24) and O’Hare et al.“’ Gross et al. measured the enthalpy of the reaction UN(c)++
N,(g) = (w-UN,,,,
(3)
-(61.0+2.1) kJ mol-‘, from which we derive A&(u-UN,,,, to be c, 298.15 K) = - (352.2+ 3.9) kJ mol-‘. It is difficult to compare this result with ours
ENTHALPIES
OF
FORMATION
OF LJRANIUM
NlTRIDES
2x1
because of the composition differences. This result would, however, appear to be much too positive when compared with a reasonable extrapolation of our results. O’Hare et al.“’ report AIf,“(u-UN 1.510) = -(377.0f7.1) kJ mall’ from fluorine bomb-calorimetric measurements. If we assume that the uranium analysis has a bias as discussed by Oetting and Leitnaker”’ and correct for it, the composition becomes UN i ,54 and M,“(a-UN 1,54) = - (374.5 f 7.1) kJ mol- ‘. In this instance, both values appear to be in agreement within the combined uncertainties with reasonable extrapolation of our results. However, because of the problem discussed previously concerning the proper value of A&(UF,) to use with this combustion work, we feel that these values should be given no weight. For P-UN,.,, there are apparently no previous determinations of AH,” with which we can compare our results. In conclusion, we feel that the values derived herein for the enthalpies of formation of the uranium nitrides are the best values currently available. We realize that the fluorine bomb-calorimetric results of O’Hare et al. and ourselves yield enthalpies of combustion which are different by approximately 11 kJ mall’, and that this difference can be removed by using A&‘(UF,) based on combustion procedures similar to that used in the earlier uranium nitride combustion. Clearly, an offsetting systematic bias is indicated in one of the procedures. We have, as yet, been unable to identify the source of this problem. However, we believe that the more negative results for the enthalpies of combustion of both uranium and uranium nitride are the more correct. The more negative value for AH,“(UF), has been shown to be compatible with the accepted result for LW,“(y-UO,).“.“’ The authors wish to thank B. S. Tani, H. T. Goodspeed, and the Analytical Group of the Chemistry Department of ECN for analytical services. REFERENCES I. O’Hare, P. A. G .. Settle, J. L.; Feder, H. M.; Hubbard, W. N. Thermodyn. Nucl. Mater., Proc. .YLwz~. Vienna, 1967. IAEA: Vienna. 1968, p.265. 2. Oetting, F. L.; Leitnaker, J. M. J. Chem. Thermodynamics 1972, 4, 199. 3. Potter, P. E.; Rand, M. H. 4th International Conference on Chemkal Thermodynamics, Montpellier. France. 1975, Paper III/31. 4. Johnson, G. K. J. Chem. Thermodynamics 1979, 11. 483. 5. Nuttall, R. L.; Wise, S.; Hubbard, W. N. Rev. Sci. Instrum. 1961, 32, 1402. 6. Hubbard, W. N.: Katz, C.; Waddington, G. J. Phys. Chem. 1954, 58. 142. 7. Cordfunke, E. H. P. J. Nuci. Mater. 1975, 56. 319. 8. O’Hare. P. A. G.; Settle, J. L.; Hubbard, W. N. Trans. Far&v Sot. 1966, 62, 558. 9. Stein, L.; Rudzitis, E.; Settle, J. L. Purification of Fluorine by Distillation. USAEC Report No. ANL--6364. June 1961. IO. Hubbard, W. N. In Experimental Thermochemktry Vol. 1 I. Chap. 6. Skinner, H. A.: editor. Interscience: New York. 1%2. 11. Wagman, D. D.; Evans, W. H.; Parker, V. B.; Halow, I.; Bailey, S. M.: Schumm. R. H. Nat/. Bur. Stand., U.S. Tech. Note 27&l. 1969. 12. Wagman, D. D.; Evans, W. H.; Parker, V. B.; Halow, I.: Bailey. S. M. ; Schumm. R. H. Nat/. Bur. Stand., U.S. Tech. Note 270-3. 1968. 13. Parker, V. B. The Thermochemical Properties qf the Uranium-Halogen Containing Compounds NBSIR 80-2029. July 1980.
282
G. K. JOHNSON
AND
E. H. P. CORDFUNKE
14. Gaunt, J. Trans. Faraday Sot. 1953, 49, 1122. 15. Hirschfelder, J. 0.; Curtiss, C. F.: Bird, R. B. Molecular Theory q/ Gases and Liquid,u. Wiley: New York. 1954. 16. Morizot, P.; Ostorero, J.; Plurien, P. J. Chim. Phys. 1973, 70, 1582. 17. White, D.; Hu, J. H.; Johnston, H. L. J. Chem. Phys. 1953, 21, 1149. 18. Ueda, K.; Kigoshi, K. J. Inorg. Nucl. Chem. 1974, 36, 989. 19. Rudzitis, E.; Van Deventer, E. H.; Hubbard, W. N. J. Chem. Eng. Data 1967, 12, 133. 20. Oliver, G. D.; Milton, H. T.; Grisard, J. W. J. Am. Chem. Sot. 1953, 75, 2827. 21. Wijbenga, G.; Johnson, G. K.: Cordfunke. E. H. P. Unpublished measurements. 22. Rossini, F. D. In E.uperimenta/ Thermochemistry Chap. 14. Rossini, F. D. : editor. Interscience: New York. 1956. 23. Neumann, B.; Kroger, C.; Haebler, H. Z. Anorg. A&em. Chem. 1932, 207, 148. 24. Gross, P.; Hayman, C.; Clayton, H. Thermodyn. Nwl. Marer.. Proc. Symp. Vienna, 1962. IAEA: Vienna. 1962, p. 653. 25. Inouye, H.; Leitnaker, J. M. J. Am. Ceram. Sot. 1968, 51, 6. 26. Johnson, G. K.; O’Hare, P. A. G. J. Chem. Thermodynamics 1978. 10, 577.
thermodynamics
1981, 13, 273-282
The enthalpies of formation of uranium mononitride and oc- and p-uranium sesquinitride by fluorine bomb calorimetrya GERALD
K. JOHNSON
Chemical Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, U.S.A. and E. H. P. CORDFUNKE Netherlands Energy Research Foundation ECN, Petten, The Netherlands (Received 1I August 1980) The energies of combustion in fluorine of uranium mononitride and a- and g-uranium sesquinitride were measured in a bomb calorimeter. The derived standard enthalpies of formation, AH,“(UN,, c, 298.15 K), are - (290.3+ 2.2), - (391.5+ 2.3), - (382.2+2.3), and -(362.9+2.3) kJ mol-’ for UN,,,,,, a-UN,.h74r a-UN,,,,,, and b-UN,.466. respectively.
1. Introduction
Previous fluorine bomb-calorimetric measurements of the enthalpies of formation of uranium mononitride and u-uranium sesquinitride have been reported”’ from this laboratory. These results have subsequently been revised by Oetting and Leitnaker”’ because of a suspected bias in the analytical procedures used for the determination of uranium. These revised results are in general agreement with results from other techniques and have been regarded as the most reliable values available.‘3’ However, a recent revision (4) of AHF(UF,, 298.15 K) by approximately 11 kJ mol- ‘, would presumably affect by a similar amount the enthalpies of formation of the uranium nitrides based on fluorine bomb calorimetry. Such a change, however, thus causes serious disagreement between the fluorine bomb results and those from other methods. In order to resolve these difficulties, a re-determination of the enthalpies of formation of the uranium nitrides by fluorine bomb calorimetry was undertaken. The current availability of well-characterized samples of UN and a-U,N, as well as l3U,N, and procedures which yielded nearly complete combustion, gave us confidence that reliable and definitive results for the enthalpies of formation were obtainable. 0 This work
was performed
under
the auspices
of the U.S. Department
of Energy.
274
G. K. JOHNSON AND E. H. P. CORDFUNKE
2. Experimental CALORIMETRIC
SYSTEM
The calorimetric system consisted of a two-chambered nickel reaction vessel similar to that described by Nuttall et ~1.‘~’ and a rotating bomb calorimeter, laboratory designation ANL-R-3, similar to that described by Hubbard et al.‘@ Calorimetric temperatures were measured with a quartz-crystal thermometer (Hewlett-Packard Model 2801-A). The calorimetric system was calibrated by combustion in oxygen of National Bureau of Standards benzoic acid (Sample 39i), whose certified specific energy of combustion under prescribed conditions is - (26.434 f 0.003) kJ g- ‘. Ten calibration experiments, five performed before and five after the uranium nitride combustions, gave an average value and standard deviation of the mean for .s(calor), the energy equivalent of the calorimetric system, of (14071.17 kO.54) J K-i. MATERIALS
The uranium nitride samples were prepared at the Netherlands Energy Research Foundation ECN, Petten, by the reaction of uranium metal and nitrogen gas. The a-U,N, was prepared as follows: National Bureau of Standards uranium metal (Sample 960), with a purity of (99.975 kO.017) mass per cent, was surface cleaned by mechanical polishing and converted to UH, by reaction with hydrogen at 523 K. The material was then dehydrided at 723 K. Successive hydriding and dehydriding procedures produced a very finely divided uranium powder. The powder was then converted to a-U,N, by reaction with argon-diluted nitrogen (approximately 9 moles per cent of N2) at 1023 to 1073 K. The stoichiometry of the U,N, sample is dependent on the temperature of the reaction and on the nitrogen partial pressure. Two samples of a-U,N, having the designations a-U,N,-10 and a-U,N,-12 were prepared for the calorimetric measurements. The UN was prepared by heating a-U2N, in argon at 1673 K for several hours. The calorimetric sample was designated UN-36 X-ray diffraction analysis, using procedures described previously, (‘r showed the sample to have a cubic structure with a = (0.48897 + 0.00004) nm. The j3-U,N, was prepared by heating a-U,N, for seven hours in an induction furnace at 1523 K under a nitrogen pressure of 50 kPa. The sample was then cooled very rapidly, while the nitrogen atmosphere was simultaneously pumped off. The X-ray diffraction pattern of this material indicated that approximately 4 mass per cent of the B-U,N, had been converted to a-U,N, during the quenching procedure. The lattice parameters of the hexagonal B-U,N, preparation were u = (0.3698 f 0.0001) nm and c= (0.5832 + 0.0002) nm. Following preparation, portions of the calorimetric samples were sealed under helium and shipped to Argonne National Laboratory where they were stored and handled exclusively in a high-purity helium-atmosphere glovebox. Complete analyses of the sample compositions using procedures described previously”’ are given in table 1. It is assumed that the non-metal impurities are substituted for nitrogen in the nitrogen sublattice and the metal impurities for uranium in the uranium sublattice.
ENTHALPIES TABLE
OF FORMATION
OF URANIUM
1. Analytical results for the uranium nitrides; w denotes mass fraction UN-36
a-U,N,-10
element a
a-U,N,-12
P-UP,
lo2 )I‘
U N 0 C Si Al Ca co Cr Fe Mg Mn
94.20 5.41 0.120 0.008 0.0025 0.0015
tie V
0.0010
Non-metal-tometal
275
NITRIDES
0.0010
0.997 + 0.004
90.84 8.87 0.070 0.017 0.0025 0.0020 0.0010 0.0008 0.0015 0.0002 0.0004 o.OcK32 0.0012 o.c0O7 1.674 f 0.005
91.20 8.49 0.121 0.020 0.0025 0.0015 0.0020 0.0005 0.0005 0.0015 0.0002 0.0001 0.0008 0.0004 1606~0.005
91.63 7.83 0.038 0.014 0.0025 0.0025 0.0010 OS!018 0.0010 0.0015 0.0002 0.0002 0.0005 0.0005 1.466 f 0.005
a The uncertainties on the U, N, 0, and C mass fractions are +2x 10m4, + 2 x 10e4, & 1.5 x 10. 4. and It_2 x lo- ‘, respectively; the remaining spectrochemically determined impurities were assumed to have uncertainties of + the value determined.
Hence, when corrected for impurities, the derived results will be for uranium nitrides having N-to-U mass ratios which are the same as the non-metal-to-metal mass ratios of the samples. Tungsten foil (0.025 mm), which was used as an auxiliary combustion aid, was purchased from Schwarzkopf Development Corporation. Five combustion experiments in fluorine were conducted with the tungsten foils, and a value and standard deviation of the mean of - (9382.24+ 0.84) J g- ’ were obtained for the standard energy of combustion, AVz/M(sample), of this tungsten sample. Sulfur, used in the combustions as an ignition aid, was taken from the same lot used to determine AJ$(SF,,g).‘*) Purified fluorine (99.99 moles per cent) was prepared by distillation of commercial fluorine in a low-temperature still.“’ PROCEDURES Preliminary experiments established that the uranium nitride powders were attacked by high-pressure fluorine and, hence, that it would be necessary to use a twochambered bomb for these combustions so that the sample could be isolated from the fluorine until reaction was desired. It was further established that the combustion of the uranium nitrides without an auxiliary combustion aid usually resulted in a large residue of U,F, ,. It was found, however, that by using several pieces of tungsten foil under the sample, the amount of the U,F,, residue could be significantly reduced.
276
G. K. JOHNSON
AND
E. H. P. CORDFUNKE
The procedures used in the combustions were as follows. The bomb was taken into the glovebox and opened. Two pieces of the 0.025 mm tungsten foil, one of which had been formed into a saucer shape and the other a square piece which had the four corners turned down to form legs, were weighed. An approximately 2 g sample of uranium nitride was placed in the saucer-shaped foil and the tungsten plus uranium nitride were then weighed. A small amount of sulfur. used to ignite the sample, was then placed on top of the uranium nitride and the saucer and the contents were reweighed. The one square tungsten piece was placed on its legs on the bottom of a weighed 32 g nickel crucible, and the other saucer-shaped foil containing the sample was placed on top of the crucible, with the lip of the saucer resting on the top edge of the crucible. The bomb was assembled, and the tank, which had been charged to 1997 kPa pressure with fluorine, was attached. The assembled reaction vessel was removed from the glovebox, evacuated, placed in the calorimeter, and the experiment preformed. Following each combustion the bomb was evacuated, returned to the glovebox, and opened. In most experiments a small amount of residue, identified as U,F,, by X-ray diffraction, was found in the crucible. The amount of U,F,, was determined by weighing the crucible plus residue, removing the residue by gentle scraping and then reweighing the crucible. The mass of the crucible usually increased slightly from experiment to experiment and was attributed to the formation of NiF,. 3. Results The results of the combustions of UN,,,,,, E-UN,,,,,, cl-UN,,,,,, and P-UN,.,,, at 298.15 K are presented in tables 2 to 5, respectively. The general combustion reaction is UN,(c)+3F,(g)
= UF,(c)+txN,k).
(1)
The corrections to standard states were applied in the usual manner.(“) Most of the entries in the tables are self-explanatory or have their usual significance. The following, however, require some additional explanation: AU(U,F,,) is the thermal correction for the hypothetical conversion of the U4F1, combustion residue to UF,(c); and AU(condense UFs) is the thermal correction for the hypothetical condensation of that portion of the UF, product which is gaseous. For the calculation of AU,,,, the following values were used: cp/J K- ’ g- ’ for UN,(3’ a-U,N3,‘3’ p-U,N3,(3’ W,‘“’ So2 Ni,“” and UF,(c),‘i3’ of 0.189, 0.209, 0.208, 0.132, 0.706, 0.444, and 0.473, respectively, and C,/J K-’ mall’ for gaseous F,,(i2’ N2,(12’ WF,,“” SFs,(12’ and UF, ‘14’ of 22.98, 20.81, 110.7, 88.96, and 121.3, respectively. For the calculation of AU,,, p in the equation of state pV = nRT( 1-pp) and (aU/ap)r were estimated by the method of Hirschfelder et a/.“” from the intermolecular-force constants for UFs,(16’ WF6,‘16’ F,,“” SF,,“s and N2.(ls’ The densities of UN, U2N3, W, Ni, and UF,(c) were taken to be 14.32,8.94, 19.3,8.9, and 4.68 g cme3, respectively. The volumes of the empty bomb and tank were 0.3059 and 0.2361 dm3, respectively. For the calculation of AUsu,rurr the specific energy of combustion of the sulfur sample was taken to be - 37917.1 J g-l.@’ AUblank is the
ENTHALPIES
OF FORMATION TABLE
mWh9,
reacted)/g
m(W reacted)/g m(S reacted)/g m(LJ,F,, formed j/g A&/K e(calor)( - AO,)iJ AU,.,IJ AU&J
A(J,,,,,lJ Au,,,,,,dJ Au,,,JJ AL’W,F,
,)/J
AU,,,&J
AU(condense UF, )/J ~AL’~/M(sample)}/J g- ’
1.99918 1.14448 0.00313 0.00071 1.83187 - 25776.55 -58.15 -0.75 118.68 10737.79 - 10.29 -0.51 0.00 -85.17 -7540.57
OF URANIUM
2. Results
2.00442 1.14413 0.00482 0.01724 1.83818 - 25865.34 - 58.32 -0.71 182.76 10734.50 - 10.29 - 12.44 6.38 -85.17 -7537.66
of UN,,,,,
2.00336 1.12144 0.00487 0.00128 1.82270 -- 25647.52 58.06 -0.75 184.66 10521.62 - 10.29 -0.92 0.00 -85.17 - 7535.56
277
NITRIDES
combustions 2.00621 1.10253 0.00308 0.01179 1.80653 - 25419.99 - 57.29 - 0.75 116.78 10344.20 - 10.29 -8.51 0.00 -85.17 -7537.11
1.01189 1.08832 0.00280 O.ooO25 1.26687 - 17826.34 - 23.85 0.54 106.17 10210.88 - 10.29 -0.18 3.96 -86.12 - 7535.63
2.99877 1.10502 0.00346 0.00454 2.33947 - 32919.08 -75.51 - 2.26 131.19 10367.56 - 10.29 - 3.28 0.00 - 85.08 - 7535.34
1.01164 1.11962 0.00384 0.0025 I 1.29007 - 18152.79 - 24.28 0.59 145.60 10504.54 - 10.29 -1.81 0.00 - 86.12 - 7536.83
(AU:/M(sample)): - (7536.96kO.69) J g-’ a Impurity correction: -(7.6* 1.8) J g-i” (AU;/M(UN,.,,, f 0.0004)) : - (7544.6 f 4.6) J g ’ * 0 f Standard
deviation
of the mean.
TABLE
WJNl.674
reacted)/g
m(W reacted)/g m(S reacted)/g m(U,F,, formed)/g A&/K E(calor)( - AO,)lJ
AU,JJ AU&J AU,,,,lJ AU,,,,,/J AUtiurJJ LIUWZI,
AU,,,IIJ
j/J
AU(condense UF,)/J I AUP/M(sample)}/J g-- ’
b*
Uncertainty
3. Results of
2.01615 1.12304 0.00340 0.00037 1.73638 - 24432.90 - 34.43 -0.44 128.92 10536.63 - 10.29 -0.27 4.48 -86.12 -6891.56
interval.
a-UN,.,,,
2.00767 1.07405 0.00383 0.00049 1.70091 - 23933.79 - 34.02 - 0.48 145.22 10076.99 - 10.29 -0.35 4.14 - 86.12 - 6892.92
combustions. 1.99972 1.08422 0.00348 0.00158 1.70244 - 23955.32 - 34.03 - 0.47 131.95 10172.41 - 10.29 -1.14 0.00 -86.12 - 6892.47
1.99838 1.15196 0.00375 0.00005 1.74840 - 24602.03 - 34.97 -0.43 142.19 10807.97 - 10.29 -0.04 0.00 - 86.12 - 6897.45
2.00889 1.15413 0.00296 0.00401 1.75238 - 24658.04 - 34.76 -0.42 112.23 10828.32 - 10.29 - 2.89 0.00 -X6.12 - 6895.34
(AU,‘/M(sample)): -(6893,9+1.l)Jg-‘” Impurity correction :(6.8 k 2.1) J gg’ ’ (AU:/M(UN,,,,*,,,)):-(6887.1+5.1) J g-’ b ” + Standard
deviation
of the mean.
bf
Uncertainty
interval.
combined thermal correction for the expansion of F, from the tank into the combustion chamber and for any subsequent reaction of F, with the bomb surfaces. This was determined to be (10.29 kO.21) J in a series of seven experiments in which fluorine was expanded into an empty bomb. These experiments were interspersed among the uranium nitride experiments. For the calculation of AUNi, the specific energy of combustion of Ni to form NiF, was taken to be - 11158.3 J g-‘.(19’ For the
278
G. K. JOHNSON TABLE
m(UN1.606reactedVg
4. Results
2.02767 1.17208 0.00398 0.00082 1.78486 -25115.07 - 35.71 -0.45 150.91 10996.74 - 10.29 -0.59 5.69 -86.12 -6951.27
m(W reacted)/g m(S reacted)/g m(U4F17formed)/g WK s(calor)( - Ae,)/J
AtJ,JJ AtJ,JJ AU,W/J AU,,.,dJ Au,,lJJ AuW,F,,)/J AU.,MIJ
AU(condense UF,)/J (AU~/M(sample)}/J g-i
AND
E. H. P. CORDFUNKE
of x-UN,.,a6
2.01269 1.13208 0.00317 0.00074 1.74730 - 24586.56 - 34.65 - 0.49 120.20 10621.45 - 10.29 -0.53 0.00 -86.12 - 6944.43
combustions 2.00829 1.16294 0.00333 0.01624 1.76650 - 24856.72 - 35.35 -0.47 126.26 10910.98 - 10.29 -11.72 6.38 - 86.12 - 6949.72
2.02101 1.18177 0.00402 0.00064 1.78739 - 25150.67 - 35.78 - 0.45 152.43 11087.65 - 10.29 - 0.46 6.90 -86.12 - 6945.43
1.99984 1.14937 0.00377 0.00083 1.75484 - 25692.65 - 35.10 - 0.48 142.95 10783.67 - 10.29 -0.60 0.00 -86.12 - 6949.87
(AU,“/M(sample)): -(6948.1+ 1.3) J g-’ ’ Impurity correction :(0.0*2.1) J gg” (Au~/M(UN,.,,,,)):(6948.1 k5.3) J g-t * a k Standard
deviation
of the mean.
TABLE
WNl.466 reacted)/g m( W reacted)/g m(S reacted)/g mW.J~, fomed)/g WK s(calor)( - A&)/J AU,“t/J AUs.,,‘J
AU,,,,,IJ AU,,,dJ W,.,,t/J AWJ,F,,)/J AU&J
AU(condense UF,)/J {AU,“/M(sample)}/J g-t
b + Uncertainty
5. Results
2.00169 1.10946 0.00446 O.OOOOO 1.75062 - 24633.27 - 34.7 1 -0.55 169.11 10409.22 - 10.29 0.00 7.93 -86.12 - 7083.35
of
1.99918 1.06496 0.00643 0.00016 1.72456 - 24266.58 - 34.49 - 0.58 243.81 9991.71 - 10.29 -0.12 7.93 -86.12 - 7080.27
B-UN1,466
interval.
combustions
2.00871 1.09176 0.00433 O.Oc088 1.74162 - 24506.63 - 34.52 -0.56 164.18 10243.15 - 10.29 -0.63 1.38 -86.12 -7084.17
2.00723 1.05576 0.00594 0.00334 1.72194 - 24229.7 1 - 34.43 -0.59 225.23 9905.39 - 10.29 -2.41 7.93 - 86.12 - 7086.88
2.00534 1.11669 0.00288 O.OWOO 1.75264 - 24661.70 - 34.75 - 0.55 109.20 10477.05 - 10.29 0.00 5.86 86.12 - 7081.74
2.00594 1.05480 0.00466 0.00025 1.71654 - 24153.73 - 34.31 -0.59 176.69 9896.39 - 10.29 -0.18 5.17 -86.12 - 7082.45
(AUg/M(sample)) : - (7083.14+0.93) J g-’ ’ Impurity correction :(8.6 + 2.3) J g- ’ ’ (AU:/M(UN,.,,,,,)): - (7074.5 k5.5) J g-i b 4 Standard
deviation
of the mean.
b + Uncertainty
interval.
calculation of ALr(condense UF,), the energy of sublimation of UF6 was taken to be 47.10 kJ mol-’ based on AH,, = (49.58kO.42) kJ mol-‘.“3J The vapor pressure of UF,(c) at 298.15 K was taken to be 14.93 kPa. (“J For the calculation of AU(U,F,,), the specific energy of combustion of U4F,, to form UF,(c) was derived to be -(722-L-63) J g-’ based on AZ-ZfO(UqF17,c) = - (7861+ 80) kJ mol- ‘. The latter value is based on the preliminary value AIf,“(UF,,c) = - 1921 kJ mol-‘,‘21)
ENTHALPIES
OF FORMATION
OF
A&‘(UF,,g) = -(2148.1 + 1.8) kJ mol-1,‘4) reaction :(’ 3, 2U,F,,(c)
URANIUM
279
NITRIDES
and the enthalpy,
126 kJ mol-‘,
of the
= 7UF,(c)+UF,(g).
(2)
The impurity corrections are based on the assumptions stated previously, namely, that the non-metals occupy positions in the nitrogen sublattice and the metals positions in the uranium sublattice. For the uranium mononitride sample the impurities were taken to be UO, UC, USi, and metal mononitrides and for the uranium sesquinitrides they were taken to be U,O,, U,C,, U,Si,, and metal sesquinitrides. The impurities were assumed to form O,, CF,, SiF4, and the most stable fluoride of the metal during combustion. The major corrections are due to the oxygen and carbon impurities; the other impurities make only a minor contribution. Potter and Rand(j) have estimated the enthalpy of the r-to-p transition in UN,,, to be 2.5 kJ mol- ‘. Based on this value, the correction for 4 mass per cent of a-U,N, in the /3-U,N, sample is 0.4 J g-r. The derived results for the uranium nitrides are given in table 6. The molar masses of uranium and nitrogen were taken to be 238.029 and 14.0067 g mol-‘, respectively. The enthalpy of formation of UF,(c) used in calculating AI$(UN,) was - (2197.7 + 1.8) kJ mol-‘. The uncertainties given in the table are uncertainty intervals’22’ equal to twice the combined standard deviations arising from a)) known sources. The uncertainties include an appreciable contribution from the uncertainties in the N-to-U mass ratio. In addition to the combustions described above, a series of combustions of a sample of uranium sesquinitride used in the previous study of O’Hare et al.,“’ was also conducted. The sample, UN1.69,(1) was reacted using the same combustion procedures described herein, i.e. using high fluorine pressure and a tungsten auxiliary, as well as using procedures similar to those described by O’Hare et al.,“’ i.e. using diluted fluorine and no auxiliary. Two combustions (designated Series A), using procedures identical to those described herein gave results for AUz/M(sample) of - 6876.3 and -6871.6 J g-‘. Five combustions (designated Series B), without tungsten auxiliary gave results for AU,“/M(sample) of - 6866.0, - 6886.9, - 6811.4, -6866.8, -6792.8 J g-r. The average and standard deviation of the mean was -(6844.8& 18.1) J g-l. O’Hare et al. (I) had obtained for AU,“/M(sample): -6797.7. -6874.3, -6823.3, -6830.8, -6830.8, and -6847.1 J gg’, with the average and standard deviation of the mean : - (6834.1+ 10.5) J g-r. The difference between the
TABLE
6. Derived
UN 0997*0.N!-$ A&/kJ AfQkJ Aff;/kJ
molt ’ mol - ’ mol ’
-(1901.2+1.2) -(1907.4+1.2) - (290.3 f 2.2)
results
for the uranium a-UN
1.671*0005
-(1800.8k 1.4) - (1806.2 + 1.4) -(391.5*2.3)
nitrides
at 298.15
ci-UN 1.606*0.CKX -(1810.1+ 1.5) -(1815.5+ 1.5) - (382.2 + 2.3)
K &UN
I466*o.w5
-(1829.2* 1.5) -(1834.8+1.5) - (362.9 f 2.3)
280
G. K. JOHNSON
AND
E. H. P. CORDFUNKE
Series A results and those of O’Hare et al. is 10.4 kJ mol ‘, which is almost the same difference previously found (4) between uranium combustions, 11 .O kJ mol- ‘, when similarly different combustion procedures were employed. Hence, it appears that there is a systematic difference which is in some way related to the combustion procedure. The Series B experiments were conducted to see if this difference could be explained. The average of the Series B results is in agreement with the results of O’Hare et al. within the relatively large uncertainties. However, three of the five results would be in good agreement with the Series A results and hence, the results are inconclusive. Without an auxiliary combustion aid, there tended to be a very large combustion residue, between 0.07 and 0.37 g, which adds to the uncertainty. Hence. we cannot unequivocally account for the difference between our results and those obtained previously. We feel, however, that the combustion procedure employed in this study, which resulted in almost complete reaction of the samples, should yield more reliable results. The enthalpy of formation of UN has been determined calorimetrically by several other investigators. Neumann et aI.‘23’ measured the enthalpy of reaction of very impure uranium with nitrogen at 2533 kPa pressure and 973 K. Under should be formed. Their result, these conditions some UN,,, M,“(UN, c, 298.15 K) = -(286.6+8.4) kJ mol-‘, is, however, in good agreement with the present value. Gross et al. (24’ have also studied the direct reaction of finely divided uranium with nitrogen in a hot-zone calorimeter to obtain A&‘(UN, c, 298.15 K) = - (291.2f3.3) kJ mol-‘. This result is also in good agreement with the present value. O’Hare et al.“’ have determined AEZ,“(UN) by fluorine bomb calorimetry. The reported value, A&‘(UN,,965, c, 298.15 K) = -(299.2f4.6) kJ mol-‘, was subsequently changed to M,“(UN,,,,,, c) = - (296.2 f 4.6) kJ mol- ’ by Oetting and Leitnaker”’ to account for a suspected analytical error in the uranium content of the sample. The latter, but not the original, result would be in agreement with the present AH,“. However, with the more recent valueC4) for AiY;(UF,, c, 298.15 K), -(2197.7_f1.8) kJ mol-’ rather than the value used by O’Hare et al.: -(2186.7+ 1.8) kJ mol-‘, mF(UN,,,,,) becomes -(307.2+4.6) kJ mol.-‘, in severe disagreement with the present value. Because of these difficulties it seems prudent to give that study little or no weight. In addition to the calorimetric studies, Oetting and Leitnakert2’ have reassessed the high-temperature vaporization study of Inouye and LeitnakerC2” and deduced A&‘(UN, c, 298.15 K) = - (295.4f4.2) kJ mol-‘, also in agreement with the present result. The enthalpy of formation of a-uranium sesquinitride has been determined calorimetrically by Gross et al .(24) and O’Hare et al.“’ Gross et al. measured the enthalpy of the reaction UN(c)++
N,(g) = (w-UN,,,,
(3)
-(61.0+2.1) kJ mol-‘, from which we derive A&(u-UN,,,, to be c, 298.15 K) = - (352.2+ 3.9) kJ mol-‘. It is difficult to compare this result with ours
ENTHALPIES
OF
FORMATION
OF LJRANIUM
NlTRIDES
2x1
because of the composition differences. This result would, however, appear to be much too positive when compared with a reasonable extrapolation of our results. O’Hare et al.“’ report AIf,“(u-UN 1.510) = -(377.0f7.1) kJ mall’ from fluorine bomb-calorimetric measurements. If we assume that the uranium analysis has a bias as discussed by Oetting and Leitnaker”’ and correct for it, the composition becomes UN i ,54 and M,“(a-UN 1,54) = - (374.5 f 7.1) kJ mol- ‘. In this instance, both values appear to be in agreement within the combined uncertainties with reasonable extrapolation of our results. However, because of the problem discussed previously concerning the proper value of A&(UF,) to use with this combustion work, we feel that these values should be given no weight. For P-UN,.,, there are apparently no previous determinations of AH,” with which we can compare our results. In conclusion, we feel that the values derived herein for the enthalpies of formation of the uranium nitrides are the best values currently available. We realize that the fluorine bomb-calorimetric results of O’Hare et al. and ourselves yield enthalpies of combustion which are different by approximately 11 kJ mall’, and that this difference can be removed by using A&‘(UF,) based on combustion procedures similar to that used in the earlier uranium nitride combustion. Clearly, an offsetting systematic bias is indicated in one of the procedures. We have, as yet, been unable to identify the source of this problem. However, we believe that the more negative results for the enthalpies of combustion of both uranium and uranium nitride are the more correct. The more negative value for AH,“(UF), has been shown to be compatible with the accepted result for LW,“(y-UO,).“.“’ The authors wish to thank B. S. Tani, H. T. Goodspeed, and the Analytical Group of the Chemistry Department of ECN for analytical services. REFERENCES I. O’Hare, P. A. G .. Settle, J. L.; Feder, H. M.; Hubbard, W. N. Thermodyn. Nucl. Mater., Proc. .YLwz~. Vienna, 1967. IAEA: Vienna. 1968, p.265. 2. Oetting, F. L.; Leitnaker, J. M. J. Chem. Thermodynamics 1972, 4, 199. 3. Potter, P. E.; Rand, M. H. 4th International Conference on Chemkal Thermodynamics, Montpellier. France. 1975, Paper III/31. 4. Johnson, G. K. J. Chem. Thermodynamics 1979, 11. 483. 5. Nuttall, R. L.; Wise, S.; Hubbard, W. N. Rev. Sci. Instrum. 1961, 32, 1402. 6. Hubbard, W. N.: Katz, C.; Waddington, G. J. Phys. Chem. 1954, 58. 142. 7. Cordfunke, E. H. P. J. Nuci. Mater. 1975, 56. 319. 8. O’Hare. P. A. G.; Settle, J. L.; Hubbard, W. N. Trans. Far&v Sot. 1966, 62, 558. 9. Stein, L.; Rudzitis, E.; Settle, J. L. Purification of Fluorine by Distillation. USAEC Report No. ANL--6364. June 1961. IO. Hubbard, W. N. In Experimental Thermochemktry Vol. 1 I. Chap. 6. Skinner, H. A.: editor. Interscience: New York. 1%2. 11. Wagman, D. D.; Evans, W. H.; Parker, V. B.; Halow, I.; Bailey, S. M.: Schumm. R. H. Nat/. Bur. Stand., U.S. Tech. Note 27&l. 1969. 12. Wagman, D. D.; Evans, W. H.; Parker, V. B.; Halow, I.: Bailey. S. M. ; Schumm. R. H. Nat/. Bur. Stand., U.S. Tech. Note 270-3. 1968. 13. Parker, V. B. The Thermochemical Properties qf the Uranium-Halogen Containing Compounds NBSIR 80-2029. July 1980.
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14. Gaunt, J. Trans. Faraday Sot. 1953, 49, 1122. 15. Hirschfelder, J. 0.; Curtiss, C. F.: Bird, R. B. Molecular Theory q/ Gases and Liquid,u. Wiley: New York. 1954. 16. Morizot, P.; Ostorero, J.; Plurien, P. J. Chim. Phys. 1973, 70, 1582. 17. White, D.; Hu, J. H.; Johnston, H. L. J. Chem. Phys. 1953, 21, 1149. 18. Ueda, K.; Kigoshi, K. J. Inorg. Nucl. Chem. 1974, 36, 989. 19. Rudzitis, E.; Van Deventer, E. H.; Hubbard, W. N. J. Chem. Eng. Data 1967, 12, 133. 20. Oliver, G. D.; Milton, H. T.; Grisard, J. W. J. Am. Chem. Sot. 1953, 75, 2827. 21. Wijbenga, G.; Johnson, G. K.: Cordfunke. E. H. P. Unpublished measurements. 22. Rossini, F. D. In E.uperimenta/ Thermochemistry Chap. 14. Rossini, F. D. : editor. Interscience: New York. 1956. 23. Neumann, B.; Kroger, C.; Haebler, H. Z. Anorg. A&em. Chem. 1932, 207, 148. 24. Gross, P.; Hayman, C.; Clayton, H. Thermodyn. Nwl. Marer.. Proc. Symp. Vienna, 1962. IAEA: Vienna. 1962, p. 653. 25. Inouye, H.; Leitnaker, J. M. J. Am. Ceram. Sot. 1968, 51, 6. 26. Johnson, G. K.; O’Hare, P. A. G. J. Chem. Thermodynamics 1978. 10, 577.