M-1477 J. Chem. Thermodynamics1983, 15, 231-243
Standard uranium
enthalpies compounds
X. Uranium
oxide
E. H. P. CORDFUNKE, P. VAN VLAANDEREN
of formation
of
chlorides
W. OUWELTJES,
and
NetherlandsEnergy ResearchFoundationECN, Petten, The Netherlands (Received9 July 1982; in revisedform 6 September1982) Enthalpies of solution of (UO&ls. U,O,Cl,, UOClr, and UOCI in H,SO,(aq) have been measured calorimetrically. In combination with the enthalpies of solution ofy-UO, and UCI, in this solvent the standard enthalpies of formation of these uranium oxide chlorides have been derived. The values obtained for A&& 298.15 K)/(kJ.mol-i) are: (UO,)rCI,, - (2404.5 * 1.9); U,O,Cl,, - (2197.4k4.2); UOCI,, - (1069.3* 2.7); LOCI, -(832.9*4.2).
1. Introduction In U,O,Cl, several oxide chlorides, in which the uranium valency varies from 3 to 6, have been described. Thus, the oxide chlorides UOCl, UOC12, UO,Cl, and U02CI, are known to exist, and their properties have been investigated to some extent.“) In addition, oxide chlorides with a mixed uranium valency have been found recently: (UO,)&l, in which the formal uranium valency is 5.5j2) and U,O,Cl,, in which the formal uranium valency is 4.5j3) The existence of other oxide chlorides, like UOC13, has been reported, (*) but not proved definitely. This might be due to the difficulty in isolating these oxide chlorides under the extreme conditions of high chlorine pressures and low oxygen pressures, which are apparently required. To support the experimental work, a better insight into the phase relations of this complicated system is necessary. In order to be able to calculate these phase relations we here present the enthalpies of formation of the oxide chlorides that could be prepared in a pure form: (U02)$13, U,O,CI,, UOCI,, and UOCl.
2. Experimental Starting materials for the preparation of the oxide chlorides are: anhydrous uranyl chloride, which is prepared as described before, -W the uranium chlorides UC13 and UCl,, prepared as published previously,‘5V6’ and the uranium oxides UOz or U30,. In the following we give for each oxide chloride a description of the “best” preparative method which is based on an extensive amount of experimental work. (U02)2C13 was prepared, as described before, (2) by the reaction at about 775 K of 0021-9614/83/030237 +07 %0200/O
0 1983 Academic Press Inc. (London) Limited
238
E. H. P. CORDFUNKE.
W. OUWELTJES.
AND
P.
VAN
VLAANDEREN
UOZ and anhydrous UO,Cl, in the ratio (n(UO&l,)/n(UO,) = 3:. The reaction was carried out in evacuated and sealed silica ampoules. U,O&l, was prepared according to Levet et al. (‘) by heating a stoichiometric mixture of UCI, and U,Os : (n(UCl,)/n(U,Os) = 5). Th e mixture was slowly heated in an evacuated ampoule to 725 K and kept at this temperature for about 20 h. A chocolate-brown powder was obtained with the X-ray pattern given by Levet et al. ; impurities were not detected on the X-ray films. However, when higher temperatures are used, as Levet et al. did, some decomposition into UOCIZ was observed. UOCl, has been prepared, according to Greenberg and Westrum,(7) by heating eq ual amounts of substance of UO, and UCl, at 875 K in an ampoule filled with purified argon. The resulting product was pale green and consisted of UOC12 which was, according to chemical and X-ray analysis, contaminated only by a small amount of UOZ. UOCl appears to be extremely difficult to prepare in a pure form. In fact, we did not succeed in it, but always obtained UOCl contaminated with UCl, and UO,. When a mixture of (UO, + U + UOCl,) (prepared as described before) in the ratio, corresponding to the stoichiometry of the reaction : 2UOC1, + UO, + U = 4UOC1, is heated in an argonfilled ampoule at 900 to 950 K for at least 20 h a product is obtained which contains mainly UOCl but also amounts of UCl, and UOd. It appeared possible to remove UCl, quantitatively by washing the product with cold water. Since UOCl does not decompose under these circumstances a mixture of UOCl and UOZ can be obtained in this way. The residue was dried in VUCUCJ over PZO,. UOCl is stable in dry air at room temperature. Its X-ray pattern agrees with values given by Shchukarev and Efimov.@’ Also in this case the reaction temnerature is an important variable : above 1050 K formation of UOCl was not observed. The various oxide chlorides were characterized by X-ray diffraction and chemical analysis. The total uranium was determined by a computer-controlled titration according to the procedure described by Lingerak et af.(9’The U(V1) content was found by titration with Fe(I1) in phosphoric acid solution”” and the U(IV) content by oxidation with potassium dichromate solution and back titration with Fe(I1). Chloride was determined by potentiometric titration with standard silver nitrate solution. The results of the analyses are given in table 1. All handlings of the very hygroscopic compounds were performed in a glove box filled with dry recirculating argon. The enthalpies of solution were measured in a calorimeter which was described TABLE Compound
M g,mol-’
wo,m3
646.415
U,O*Cl,-9 u,ozcl,-lo
685.322
UOCl,-7 UOCI,-12 UOCI-38
1. Analytical 102w(U) obs.
results;
molar
mass M and mass fraction
1o~w{u(Iv)j talc.
73.7kO.l g:i;$$;
73.65
324.934
;:.;5$;
73.25
289.481
82:35%0.05
82.23
69.46
obs. 18.69kO.06 :;;;;‘t8;;; 73’s9+_o.04
w
lo~w{u(vI)} talc.
obs.
18.41 52.10 73.25
o.25ko,o,
1o%v(c1) talc.
obs.
talc.
15.95+0.05 25.73 + 0.02
16.45 25.87
;:;;$;E
21.82
10.78?0.01
12.25
A&m FOR URANIUM
OXIDE CHLORIDES
239
previously, together with the calorimetric procedure and calibration method.“‘) Corrections to the temperature rise in the calorimeter were based on a complete calculation of the pre- and post-period according to Newton’s cooling law. The H,S04 solution was prepared from reagent-grade acid and analysed by potentiometric titration with standard NaOH solution.
3. Results Details of the measurements are given in tables 2 and 3, where m is mass of the sample dissolved, Eis the energy equivalent of the calorimeter, A8 is the temperature change, AJr is the specific enthatpy of solution, and ISAhl is the deviation of a measured Ah from the average value; 0 is an arbitrary unit of temperature. TABLE 2. Enthalpy of solution of a mass m of a specified solute in 200 cm3 of 1.505 mol edme H,SO,(aq) at 298.15 K m(soIute)/g
&0/J
A018
-Ah/(J.g-‘)
WWtJ g- 1)
(4 WO2LCb 0.6133
92.026
0.6003 0.6684
91.121 89.047
(b) L&O,&-9 0.65907 0.65520 0.66210 0.66568
1.9864 1.9609 2.2380
298.06
0.10
297.65
0.31 0.20
298.96 average: 297.96 twice standard error of mean : 0.31
2.9753
417.47
92.675 93.543
2.9152 2.9706 2.9592
418.64 415.80 415.84
93.333 94.717
2.8885 2.8630
92.475 94.092
0.53 1.71 1.14
1.10 average : 416.94 twice standard error of mean : 1.38
(c) U20,CI,-10 0.64993
0.64945
414.80 417.55
average: 416.18 (d) UOCI,-7 0.59226 0.61265 0.62684 0.59665
93.124 92.631 93.616 92.388
2.4382 2.5365 2.5718 2.4714
383.37 383.51 384.09 382.68
0.04
0.10 0.68 0.73
average: 383.41 twice standard error of mean: 0.58 (e) UOCl,-12 0.63679 0.62761 0.61836
0.63967
92.679 92.715 93.282 92.675
2.6317
2.5893 2.5433
2.6395
383.02 382.51 383.67 382.41
0.12
0.39 0.77
0.49 average : 382.90 twice standard error of mean : 0.58
240 TABLE
E. H. P. CORDFUNKE. 3. Enthalpy
ofsolution
W. OUWELTJES.
AND
ofa mass m of (UOCl-t-UOz) at 298.15 K
P.
VAN
VLAANDEREN
in 200 cm3 of 1.505 mol’dm-’
H,SO,(aq)
m(U021/g -
m(solute)/g
m(UOCI)/g
(dissolved)
1:0/J
At)/0
~ A,,,, M J g ’ ) _____
0.45988 0.48754 0.41531 0.47343 0.46351
0.41141 0.43615 0.42521 0.42353 0.41466
0.00322 0.00649
93.478 92.045 92.993 94.293 92.500
3.0119 3.2147 3.1886 3.1199 3.0605
679.03 668.48 697.34 694.60 676.10
0.0406
OF FORMATION
.rnol~
1)
198.06 196.32 201.87 201.07 197.58 Average
ENTHALPY
- A,,,,H,/(kJ
:
- 198.98 f 1.97
OF (UOzlaCl,
The reaction scheme to derive A,H‘,{(U02)2Cl,) is given in table 4. The sample used contains 2.45 mass per cent of an [in 1.505 mol.dme3 H,SO,(aq)] insoluble impurity which consists, according to the X-ray diffraction results, of UO, and an insoluble oxide chloride. The mass of (U02),Cl,, given in table 2, has been corrected for this impurity. The specific enthalpy of solution of (UOz),C1,. table 2(a), is equivalent to - (192.61 of:0.20) kJ. mol - ‘. This quantity has to be combined with the enthalpies of solution in 1.505 mol.dmm3 H,SO, of UCl,.‘“’ and y-UO,,“” and with the enthalpies of formation of UC1,,‘r3’ y-U0 3.‘11) H,O(sln). and HCl(sln). The latter quantities have been derived from the reaction scheme, given in table 4 of reference 5. The value ArH~{(U02),C13, s, 298.15 Ki = -(2404.462 1.88) kJ.mol-’ has been found. ENTHALPY
OF FORMATION
OF U,O,Cl,
Two different samples have been used for the determination of the enthalpy of solution of U,O,Cl, in 1.505 mol.dme3 H,S04. Sample-9 contains, according to the analyses in table 1, a small amount of U,O, which, however. could not be TABLE
4. Reaction
scheme for the enthalpy M,
~-..
2(UO,),Cl,ls)+SH,SO,(sln)= {3UO,SO,+U(SO,),+6HC1+2H,O)(sln) UCl,(s)+ZH,SO,(sln) = (U(SO,),+4HCl;(sln) U(s)+2Cl,(g) = UCl,(s) 3y-UO,(s)+3H,SO,(sln) = (3UOzS0,+3H,0) (sin) 3u(s)+~o,(g) = 3y-IJO, H,(g)+Cl,(g)+(sln) = ZHCl(sln)
H&)+9&)
8. 4U(s)+2O,(g)+3Cl,(g)
(sin) refers to 1.505 mol .dm-
=-M,+M,+M,+M,+M,+M,-M,
Reaction ^_________ 1. 2. 3. 4. 5. 6. 7.
of formation of (U02)zCI,: H,SO, in water:
= H,O(sln)
= 2(UO,),Cl,(s)
-.~_~
AH/(kJ.mol-’
)
- 385.222 0.40 -213.13?0.84 - 1018.8 k2.51 - 252.66 + 0.63 -3671.46k2.51 - 323.96kO.46 - 285.88 f 0.042 -4808.91
k3.75
3
AfHm FOR URANIUM TABLE
1. 2. 3. 4. 5. 6. 7.
5. Reaction
OXIDE CHLORIDES
scheme for the enthalpy of formation 1.511 mol.dn-3 H2S04 in water;
of U,OQ,;
241 (sln) refers to
Reaction
W/(kJ.mol-‘)
2U,O,Cl,(s) + IIH,SO,(sln) = { 3U(SO,), + UO,SO, + 1OHCl + 2H,O}(sIn) 3UCl,(s)+6HzS0,(sln) = {3U(SO,),+ 12HCI}(sln) 3U(s)+6Cl,(g) = 3UC14(s) y-UO,+H,SO,(sln) = {UO$04 +H,O)(sln) w)+m.d = Y-UWS) H,(g)+Cl,(g) = ZHCl(sln) Hkd+NMg) = WWn)
-570.96+ 2.76 -639.40+2.51 -3056.40+7.53 -84.22kO.21 -1223.82+0.84 -323.96kO.46 -285.88+0.042
8. 4U(s)+20,(g)+5Clz(g)
= 2U,O,Cl,(s)
-4394.80*
8.46
detected on the X-ray films. Based on the Cl-analysis we assume a mass fraction 0.0054 to be present. Because in this particular case, the specific enthalpies of solution of U308 and U,O,Cl, are almost identical, a correction for the presence of U,O, did not change the specific enthalpy of solution of U,O$l,, table 2(b), which corresponds to - (285.74kO.94) kJ*mol-‘. Sample-10 is, according to the chemical analysis very pure, and the value for the specific enthalpy of solution, table 2(c), which corresponds to -(285.22+0.94) kJ*mol-‘, need not be corrected. The mean value for the molar enthalpy of solution of U,02Cls in 1.505 mol .dmP3 -(285.48f 1.38) kJ*mol-‘, has been taken for the calculation of the H,SO,, enthalpy of formation of U,O&l~, according to the reaction scheme in table 5. When the same auxiliary quantities are used as in table 4, we obtain ArW,(U202Cls, s, 298.15 K} = -(2197.4+4.23) kJ.mol-‘. ENTHALPY
OF FORMATION
OF UOCI,
Again two different samples have been used for the determination of the enthalpy of solution of UOC12 in 1.505 mol.dm-3 HzS04. Sample-7 contains an insoluble residue which consists of UOz and SiOz (mass fractions 0.0463 and 0.0012, respectively). The masses, given in table 2, have been corrected for these impurities. No other corrections are necessary. The enthalpy of solution of UOCl,, table 2(d), corresponds to -(124.58+0.19) kJ.mol-‘. In sample-12 an insoluble residue, which consists of UOt, is present. The masses, given in table 2(e), have been corrected for a mass fraction 0.0048 of UOz. The enthalpy of solution of UOCl, - 12 in 1.505 mol +drnm3 H,SO,, table 2(e), has to be corrected for the presence of a mass fraction 0.0025 of U(W) in the sample. If we assume this to be present as UOzC12, of which the specific enthalpy of solution in 1.505 mol.dm-3 H,SO, is estimated as - 235 J *g-l, a correction for a mass fraction 0.0036 of U02Cl, gives for the specific enthalpy of solution of UOC12 the value -(383&l&0.58) J-g-’ which is equivalent to -(124.59$-0.19) kJ*mol-‘. The mean value of the two determinations is -(124.59+0.19) kJ*mol-‘.
242 TABLE
E. H. P. CORDFUNKE, 6.
Reactlon
scheme
W. OUWELTJES, for the enthalpy of 1.511 mol.dm-’ HzS04
AND
P.
formation in water;
VAN
VLAANDEREN of
UOCl,;
(sin)
refers
to
AH,=-AH+Atiz+AH3-AH4+AH5
1. 2. 3. 4. 5.
Reactlon
AJI/(kJ’mol-‘)
UOCl,(s)+ZH,SO,(sln) = {U(SO,),+3HCI+H,O}(sln) UCl,(s)+ ZH,SO,(sln) = ( U(S04)2 +4HClj(sln) U(s)+2Cl,(g) = UCl,(s) H,(g)+Cl,(g) = ZHCl(sln) HA)+@&) = H,O(slnf
- 124.59+0.19 -213.13+0.84 - 1018.8 k 2.51 - 323.962 0.46 - 285.88 f 0.042
6. U(s)++O&)+Cl,(g)
- 106Y.26+
= UOCl,(s)
2.69
When we combine the enthalpy of solution of UOCl, with the auxiliary quantities, as in table 4, the enthalpy of formation of UOC12 can be calculated according to the reaction scheme in table 6. The value AIIF(UOCl,, s, 298.15 K) = - (1069.3 + 2.7) kJ . mol - ’ has been found. ENTHALPY
OF FORMATION
OF UOCl
The mass fraction of UO, in the UOCl is 0.1054. The enthalpies of solution of UOCl in 1.505 mol.dme3 H,SO,(aq) have to be corrected for a small amount of UOz dissolved in the solvent. For this purpose the total amount of uranium in the solution (after separation from the insoluble part of the UO,) has been determined after each experiment. From the known amount of uranium originally present as UOCl, we find by subtracting the amount of UOz dissolved during the experiment (table 3). The enthalpy of solution of “UOCl” has to be corrected for the enthalpy of solution of UO, in 1.505 mol.dm-3 HzS04 solution. The latter value cannot be measured directly, but has been derived from a cycle in which the enthalpies of solution of yUO,(s) and UCl,(s) in the same solvent are combined with the known enthalpy of reaction of UO,(s)++O, = y-UO,(s). For the enthalpy of solution of UO,(s) in 1.505 mol.dmm3 H,SO, we then obtain -(70.87+3.23) kJ.mol-‘. This value has been taken for correcting the enthalpies of solution of “UOCl” to obtain the enthalpies of solution of pure UOCl in this solvent (table 3). The value - (198.83 f 1.97) kJ. mol-’ has been found. No evaporation correction due to escaping hydrogen has been applied. When we combine the enthalpy of solution of UOCl with the auxiliary quantities, as given in table 4, the enthalpy of formation of UOCl can be calculated according to the reaction scheme in table 7. The value AII,“(UOCl, s, 298.15 K) = -(833.1+4.2) kJ ’ mol-’ has been found. 4. Discussion The only value for the enthalpy of formation of (UO,)$l, available in the literature is the preliminary value - (2404.5 + 1.7) kJ. mol-‘, published by Cordfunke et al. in 1977.“) The present value is in perfect agreement with that. Values for the enthalpy of formation of U20,Cl, have not been published before.
A&& FOR URANIUM
OXIDE CHLORIDES
243
TABLE 7. Reaction scheme for the enthalpy of formation of UGCl ; (sln) refers to 1SO5 mol. dm - 3 H,SOI in water ;
1. 2. 3. 4. 5.
Reaction
m/(kJ.mol-‘)
UOCl(s)+ZH,SO&ln) = {U(S0,)2+H,0 +HCl}(sln)+fH,(g) UCl,(s)+ 2H$O,(sln) = {U(SO,), + 3HCl}(sln)+&HH2(g) U(s)+@,(g) = UCl,(s) H,(g)+Cl,(g) = SHCl(sln) H2W+t02k) = WXsln)
- 198.98 f 1.97 -207.9Ok1.75 -862.1 k3.2 - 323.96 f 0.46 -285.88kO.042
6. U(s)+to,(g)+tcl,(g)
= UOW)
-832.94k4.17
The enthalpy of formation of UOCl,(s) has been discussed recently by Parker.‘13’ From a number of independent measurements she selected the value -(1066.0+2.5) kJ.mol-i. Our value -(1069.3+2.7) kJ.mol-’ is in good agreement with the selected value. In the same assessment Parker discussed the situation for the enthalpy of formation of UOCl(s), and concluded that “no recommendations are made at this time”. Indeed, the value found here differs considerably from the enthalpy of formation of UOCl, -891 kJ*mol-‘, calculated by Parker from old equilibrium measurements. (r3) Although the uncertainty in our value is larger than customary, due to the complicated nature of the dissolution reaction, we prefer this value since it is based on direct calorimetric measurements. In addition to the oxide chlorides, described here, other compounds in UIO,Cl, have been described, in particular, the pentavalent oxide chlorides U02C1,(4*14) and UOC13.(4’ However, we did not succeed in isolating these compounds in a pure form. Levet’s UO,Cl appears to be a mixture of two compounds, one of them being insoluble in water, having an overall composition of U,0,.5Clo.6, whereas Eliseev et al.% UOC13 seems to be identical with our UzOzC15. The phase relations in this complicated system will be dealt with in a separate paper. REFERENCES 1. Gmelin Handbuch der Anorganischen Chemie. Uranium, Suppl. Vol. C9. Springer-Verlag: Berlin. 1979, p. 67-78. 2. Cordfunke, E. H. P.; Prins, G.; Vlaanderen, P. van J. Inorg. Nucl. Chem. 1977, 39, 2189. 3. Levet, J. C.; Potel, M.; Le Marouille, J. Y. J. Solid State Chem. 1980, 32, 297. 4. Eliseev, S. S.; Glukhov, J. A.; Vozhdaeva, E. E. Russ. J. Inorg. Chem. 1972, 17, 627. 5. Cordfunke, E. H. P.; Ouweltjes, W.; P’rins, G. J. Chem. Thermodynumics 1976, 8, 241. 6. Cordfunke, E. H. P.; Ouweltjes, W.; Prins, G. J. Chem. Thermodynamics 19112,14, 495. 7. Greenberg, E.; Westrum, E. F. Jr. /. Am. Chem. Sot. 1956, 78, 5144. 8. Shchukarev, S. A.; Elimov, A. I. Zh. Neorg. Khim. 1957, II (lo), 2304. 9. Lingerak, W. A.; Konijn, P. C.; Slanina, J. Proceedings of the 1st Annual Symposium on Safeguards and Nuclear Material Management, held by the European Safeguards Research and Development Association (ESARDA) at Brussels, Belgium, 1979. ESARDA Secretariat: Ispra. p. 157. 10. Tolk, A.; Lingerak, W. A. Report RCN127. 1970. 11. Cordfunke, E. H. P.; Ouweltjes, W.; Prins, G. J. Chem. Thermodynumics 1975, 7, 1137. 12. Cordfunke, E. H. P.; Ouweltjes, W. J. Chem. Thermodynamics 1977,9, 1057. 13. Parker, V. B. Nat1 Bw. Stand. U.S. interim Rep. NBSIR 8&2029. 1980. 14. Levet, J. C. Compt. Rend. 1969, (C) 268, 703. 15. Cordfunke, E. H. P.; Kubaschewski, 0. to be published.