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Thermochemistry of uranium compounds X. Standard enthalpies of formation of uranyl oxalate, uranyl acetate, and their hydrates. Thermodynamics of the UO2C2O4 + H2O and UO2(CH3COO)2 + H2O systems
M-779 J. Chem. Thermodynamics 1977,9, 1077-1086
Thermochemistry of uranium compounds X. Standard enthalpies of formation of uranyl oxalate, uranyl acetate, and their hydrates. Thermodynamics of the UOICzOl + Hz0 and UO,(CH,COO), + Hz0 systems’ b P. A. G. O’HAREC Chemical Engineering Division, Argonne National Laboratory, Illinois 60439, U.S.A.
Argonne,
(Received 21 April 1977) Solution-calorimetric studies of uranyl oxalate and acetate and their hydrates are described. The standard enthalpies of formation of the crystalline solids at 298.15 K, AH,“/kJ mol-’ are: UO&04, -(1796.7 i 2.5); UO&01.HzO, -(2111.8 f 2.4); U02CaO*.3Ha0, -(271X3 i 2.4); &UOa(CHaCOO)a, -0957.7 i 1.1); and UOa(CH&OO)a.2Hz0, -(2557.3 & 1.1). The calorimetric data yield the following enthalpy changes at 298.15 K: U02Ca04.3H,0(c)
= UO,C,O,*H,O(c)
+ 2HaO(g);
UWXb *H&(c) = UO&nOl(c) + HaO(g) ; UOz(CHaCOO)a .2H,O(c) = U02(CH,COO)a(c, 6) + 2HzO(g);
AH” = (119.9 & 3.4) kJ mol-I; AH’ = (73.3 i 3.5) kJ mol-I;
(1) (2)
AH” = (115.9 it 1.6) kJmol-I.
(3)
Equilibrium Hz0 vapor pressures are estimated to be (89 zb 30), (0.4 f 0.2), and (200 i 50) Pa for equilibria (l), (2), and (3), respectively.
1. Introduction A literature survey made in connection with a recent critical assessment of the thermodynamics of miscellaneous actinide compounds”’ revealed that thermochemical quantities for most organo-uranium compounds either are not available or are, at best, of marginal quality. For example, the enthalpy of formation of uranium oxalate, AH,“(U(C,O,),}, determined by oxygen bomb calorimetry(2) is uncertain by approximately 40 kJ mol-’ and disagrees seriously with the value that can be deduced from d.t.a.(3a*3b) Similarly, the value for anhydrous uranyl acetate is uncertain by 33 kJ nlol-1,(‘*2) and the only calorimetric results for uranyl acetate dihydrate and uranyl ammonium acetate date back to the last century. (4) One exception to the foregoing a This work was performed under the auspices of the U.S. Energy Research and Development Administration. b Presented in part at the VII’th All-Union Conference on Calorimetry, Moscow, U.S.S.R., 31 January to 3 February 1977. c Present address: Division of Research and Laboratories, International Atomic Energy Agency, A-101 1 Vienna, Austria.
P. A. G. O’HARE
1078
observations has been the recent careful study of many1 formate described by Bousquet et d(5) Under normal conditions of humidity and atmospheric pressure, uranyl oxalate exists as the trihydrate, UO&04*3H20, at room temperature. When this material is heated, the water of hydration is lost stepwise, a stable monohydrate is formed, and, ultimately, the anhydrous oxalate decomposes to carbon monoxide and dioxide and a mixture of uranium oxides.@) Uranyl acetate is normally obtained as the dihydrate, U0,(CH,C00)2*2H,0, which is stable in air at room temperature. Careful thermal dehydration of this material is supposed to yield the anhydride. There is no evidence of any stable intermediate hydrate. The anhydride decomposes thermally to a mixture of uranium and carbon oxides.(7) In the present study, we have determined standard enthalpies of formation AHf” for U02C20,*3Hz0, U02C204*Hz0, U02C204, U02(CH,COO),*2H,0, and UO,(CH,COO), by solution calorimetry. These results along with approximate entropy changes permit one to estimate the equilibrium water vapor pressures over the hydrates and give a qualitative measure of the ease of dehydration of the hydrates.
2. Experimental procedure and results PREPARATION
AND
ANALYSIS
OF SAMPLES
Uranyl oxalate trihydrate was prepared according to the method of Staritzky and Cramer.(8) An excess of oxalic acid was added to a dilute solution of pure uranyl nitrate in approximately 1 mol kg -’ HNO,, and this mixture was digested at 350 K. Precipitated oxalate was removed by filtration, washed with water until the washings were free of nitrate ion as indicated by the brucine test, and dried for one week at 333 K as described by Amphlett and Davidge. (9) This material rapidly gained mass when exposed to the atmosphere; it could therefore not be the trihydrate, which is stable in air. Analyses showed it to be the monohydrate; it was handled exclusively in a glovebox filled with pure recirculating helium. Part of the monohydrate preparation was exposed to the laboratory atmosphere at 292 K and a relative humidity of (50 rt 2) per cent and, within a few hours, attained constant mass. This product was the trihydrate; it could be handled in air but not in the glovebox because of rapid loss of water of hydration. The anhydrous oxalate was prepared by dehydration of the monohydrate under vacuum at 423 K, and was handled only in the glovebox. Uranyl acetate dihydrate was purchased from Apache Chemicals, Inc., Seward, Illinois. An attempt was made to prepare the anhydride by careful heating of the dihydrate at 350 K. When the mass of the specimen had decreased by an amount corresponding to the loss of 2 mol of H,O, the heating was discontinued. The uranium content of the resulting material was very close to the theoretical value for UOZ (CH,COO)l; however, the X-ray diffraction pattern indicated the presence not only of U0,(CHJC00)2, but also of some dihydrate and other unidentified decomposition products. Since additional heating at higher temperatures brought about further decomposition, our efforts to obtain UO,(CH,COO), by thermal decomposition of the dihydrate were discontinued.
THERMOCHEMISTRY
OF URANYL
OXALATES
1079
AND ACETATES
The calorimetric specimen was prepared by means of a dehydration procedure similar to that used by Giorgio et al. (lo) The dihydrate was refluxed with acetic anhydride for 6 h, and the resulting material was filtered in dry air. The filtrate was heated for several days at 318 K, pulverized in a glovebox containing dry recirculating helium, and heated again for several days in a drying oven at 350 K. Oxalic acid dihydrate was a “Baker analyzed” reagent; the oxalate content was greater than 99.8 moles per cent of CtO:- based on permanganate titrations. Analar reagent-grade nitric acid was diluted with CO,-free distilled water, titrated against Na,CO,, and further diluted to the desired concentration. Acetic acid, Baker “Ultrex” grade, was certified to have a purity of 99.8 moles per cent based on freezing-temperature analyses. The y-UOJ was part of the specimen used in earlier studies.(“) The X-ray powder diffraction patterns of the oxalates were in good agreement with the results of Staritzky and Cromer (8) for the trihydrate and in reasonable agreement with the results of Bressat, Claudel, and Trambouze(“) for the anhydride. However, although we agreed with the latter authors with regard to theposition of the diffraction arcs for the monohydrate, there were significant intensity discrepancies. The diffraction patterns for the acetates showed good agreement with the results reported by Mentzen and Giorgio (13) for the dihydrate and by Giorgio et a1.(“’ for the Z-form of the anhydride. However, the latter diffraction pattern contained two extraneous unidentified lines. TABLE
1. Analytical results for the calorimetric specimens; w denotes mass fraction theory
uo,c*o* UO&OI*HaO UOJZs0.j. 3HaO UOa(CHXOO)a *2H10 UO,(CH,COO), Y-U03
66.48 63.30 57.16 56.12 61.33 83.22
1OQcJ) found 4 66.65 63.28 57.72 56.11 61.32 83.22
30aw(Ha0) theory found * 4G9 13.11 -
4.79 13.11 -
a Analytical uncertainty about 0.1 per cent. b Analytical uncertainty about 0.15 per cent.
The oxalates were analyzed for H,O and U, and the acetates for U only. The H,O analysis was based on careful dehydration and, with the exception of UO,(CH,COO), *2H,O, the analysis for U was based on ignition in air to U30, at 1120 K. The y-UO, was also analyzed by ignition to U,Os. The acetate dihydrate was analyzed for U by potentiometric titration. The analytical results are summarized in table 1. Trace-metal impurities by emission-spectrochemical analyses amounted to approximately 10e4 by mass for each compound, and were judged to be thermochemically insignificant. CALORIMETRIC EQUIPMENT AND PROCEDURE The LKB-8700 Precision Calorimetric System and ancillary equipment have been described previously. (14) Calorimetric specimens were enclosed in glass ampoules
P. A. G. O’HARE
1080 TABLE Experiment No.
1
2 3 4 5 6
1
2 3 4 5 6
2. Calorimetric
results for the enthalpies of solution of the oxalates (L
-y k3
A0 ..A K
0.05656 0.05571 0.05650 0.05654 0.05612 0.05545
(--A&) J
b AH.,,, + AH,,,’ I
(COOH)2.2Hz0 -0.00591 2.411 -0.00255 1.040 o.ooo94 -0.383 0.00108 -0.441 o.ooo99 -0.404 0.00205 -0.836 :AH&M> = (284.11 f 1.04) J g-l d AH = (35.82 f 0.26) kJ mol-’
J
AHsodM .-_J g-l
13.467 14.969 16.471 16.471 16.472 16.472
280.73 287.36 284.74 283.52 286.32 28 1.98
11.965 10.964 10.964 10.964 10.964 10.964
60.68 59.26 61.19 60.39 61.91 62.20
e*f
0.18446 0.18447 0.18273 0.18359 0.18329 0.18443
UO&0,.3H,O 0.00189 -0.772 0.00908 -0.033 -0.00053 0.217 -0.00030 0.123 - 0.08094 0.384 -0.00124 0.507 (AH&M> = (60.94 xk 0.44) J g-’ ’ AH = (25.11 =t 0.36) kJ mol-’ e.r
0.16684 0.16994 0.16566 0.16459 0.16979 0.16725
UO,C,O,.H,O 0.00699 -2.852 -0.053 0.00736 -3.003 -0.053 0.00681 -2.778 -0.053 0.00710 -2.897 -0.053 0.00739 -3.015 -0.053 0.00676 - 2.758 -0.053 (AH,,,,,/M> = -(17.55 & 0.21) J g-’ ’ AH = -(6.60 ZII 0.16) kJ mol-’ o*f
0.15868 0.16143 0.15931 0.16015 0.16018 0.15883
uozc204 0.03820 - 15.593 0.03885 - 15.859 0.03883 - 15.850 0.03920 -16.001 0.03959 -16.161 0.03932 - 16.050
-0.053 -0.053 -0.053 -0.053 -0.053 -0.053
-17.41 -17.98 -17.09 -17.92 -18.07 -16.81
-98.60 -98.57 -99.82 - 100.24 - 101.22 - 101.39
e.r
4 Oxalic acid dihydrate was dissolved in a mixture of UOz(NO& and HNOB. The composition of this mixture corresponded to that denoted by “soln B” in table 6 (reference 11). The uranyl oxalates were dissolved in HNOa solution of approximate molality 2.8 mol kg-l. The mean temperature of each experiment was (298.15 i 0.01) K. b The values of
THERMOCHEMISTRY
OF URANYL
OXALATES
1081
AND ACETATES
with frangible end windows. Seven sets of calorimetric measurements were carried out. The three uranyl oxalates were dissolved in aqueous nitric acid of approximate molality 2.8 mol kg- I, and the oxalic acid dihydrate was dissolved in nitric acid to which an accurately weighed quantity of y-UO, had been added. This soIution corresponded to the product of a specific HNO, + y-UO, reaction, the enthalpy of which was reported in an earlier paper. (11) The uranyl acetates and y-UO, were dissolved in aqueous HF of approximate molality 0.5 mol kg- ‘. All of the above compounds dissolved readily; no unreacted material was observed. CALORIMETRIC
RESULTS
Calorimetric results for the four dissolution reactions of the oxalates are given in table 2. Similar results for y-UO3 and the acetates are given in table 3. In these tables, m denotes the mass of compound dissolved or reacted; A0, is the temperature change TABLE E.xperiment No. 2 3 4 5 6 8
3. Calorimetric results for y-UO, and the uranyl acetates (i
1’1 6
A& .-.K
(c(calor):p J K-’
AK,, -----1 J
y-uo, 0.49314 0.41774 439.00 -0.053 0.49522 0.41586 438.82 - 0.053 438.85 -0.053 0.49965 0.42143 438.77 -0.053 0.49464 0.42014 0.49892 0.42359 438.80 -0.053 0.50177 0.42511 437.76 -0.053 (AH,/M(y-U03)) = -(371.21 i 0.65) J g-’ ’ AH* = -(106.18 i 0.38) kJ mol-’ =sd
-371.99 -368.61 -370.26 -372.79 -372.65 -370.98
S-UOz(CH&OO)a 0.69246 0.22881 438.91 -0.043 0.67517 0.22377 438.67 -0.043 0.67401 0.22235 438.79 -0.043 0.68433 0.22561 438.49 -0.043 0.67176 0.22236 438.43 -0.043
- 145.09 - 145.45 - 144.82 - 144.62 -145.19
UO,(CH,COO),*2H,O 0.11521 437.48 -0.042 0.11149 438.22 -0.042 0.11330 439.78 -0.042 0.11147 439.24 -0.042 0.11301 439.29 -0.042 0.11167 439.52 -0.042 = -(66.99 zt 0.30) J g-’ b AH, = -(28.41 I& 0.25) kJmol-1 c.d
-67.99 -66.17 -67.41 - 66.46 -67.47 -66.44
2 0.74195 3 0.73917 4 0.73977 5 0.73732 6 0.73638 7 0.73935 (AH,/M{UO~(CH&00)~.2H,O))
n The mean temperature of each experiment was (298.15 i 0.01) K. b Uncertainty is the standard deviation of the mean. c MoIar masses (M/g mol-I) were taken to be 286.027, 388.117, and 424.147 for y-UO$, UOz(CH&00)2, and UOa(CH3COO)a.2Ha0, respectively. d Uncertainty is the uncertainty interval equal to twice the standard deviation of the mean.
1082
P. A. G. O’HARE
of the calorimeter corrected for heat exchange with the environment; (s(calor)) is the mean energy equivalent of the calorimetric system based on electrical calibrations performed before and after the enthalpy of solution and reaction measurements; AHelcc denotes the electrical energy supplied to the calorimeter to offset the cooling effect of the endothermal dissolution of (COOH),+2H,O and UO,C,O,*3H,O; AH,,, is a correction that takes into account the saturation with water vapor of the gas (air or helium) enclosed in the ampoules; AH&M is the specific enthalpy of solution; and AH,IM is the specific enthalpy of reaction. For the experiments with the oxalates, A0, was small and, therefore, (s(calor)) did not have to be known very precisely. The values given in the footnote to table 2 are based on separate calibrations and are accurate to better than 0.4 J K- ‘. For the calculation of AH,,, the densities of (COOH), *2H,O, UOzCz04*3Hz0, UO,(CH, COO)z *2Hz0, y-UOs, UOzCz04*Hz0, and UO,C,O, were taken to be 1.6,(r5’ 3 .,07 @) 2 .,92 (13) 8.03,‘16) 2.6, and 2.5 g cmW3, respectively. The last two values were calculated from the X-ray data (i2) by assuming that, like the trihydrate, the monohydrate and anhydride had four molecules per unit cell.~“~ The density of U0,(CH3C00)2 was estimated to be 3 g cmW3.
ENTHALPIES
OF FORMATION
The thermochemical cycle upon which AH,“(UOzCz04*3Hz0) is based is given in table 4. For the calculation of the enthalpies of formation of the other oxalates, the following equations were combined (it should be noted that “(sol@” indicates species TABLE
4. Calculation of AH,0(U02Ca04. 3Ha0) at 298.15 K
Reaction 1. y-UO&) + soln A = soln B 2. (COOH)z*2HzO(c) + soln B = 3. soln C = 4. y-UOs(c) + (COOH)a .2HzO(c) 5. U(C) + (3/2)wg) = Y-uom 6. ZC(graphite) + 3H& + 30,(g)
AH/kJ mol-’
soln C UOzCaOI. 3Ha0 + so111A = UO&Od
*3HaO(c)
= (COOH),.2HzO(c)
7. WC) + Wgraphite) + (9/2)0,(g) + 3Ha(g) = U02C201.3HzO(c)
-(75.5 * 0.7) a (35.8 31 0.3) b -(25.1 zt 0.4) b -(64.8 f 0.9) ’ -(1223.8 31 0.8) ’ -(1426.7 f 2.1) = -(2715.3
i 2.4) ’
a Here soln A is approximately 2.8 mol kg-l of HNOJ and AH is taken from reference 11. * AH from table 2. c AH for this reaction is the sum of AH for reactions 1, 2, and 3. d AHy(U03, c, y, 298.15 K) is taken from reference 18.. o AH,“{(COOH)2.2Hz0, c, 298.15 K} is the value selected by Domalski (reference 19). The uncertainty has been estimated. f This value is equivalent to AH,“(U0,C20,*3Hz0, c, 298.15 K), and is the sum of reactions 4, 5, and 6.
THERMOCHEMISTRY
OF URANYL
in a medium composed of 2.8 mol kg-’
OXALATES
1083
AND ACETATES
of HNO,):
UO,C,O,*3H,O(c)
= UO$+(soln)+C,Oi-(soln) AH = (25.1 & 0.4) kJ mol-‘, +3H,O(soln); (1) UOzC204*H,0(c) = UOg+(soln) +C,O:-(soln) +H,O(soln); AH = - (6.6 + 0.2) kJ mol-‘, (2) UO$,O,(c) = UO~+(soln)+C,O~-(soln); AH = -(35.8 + 0.4) kJ mol-‘. (3) The partial molar enthalpy of formation of H,O in this medium was calculated to be -(285.9 f 0.1) kJ mol-’ based on Parker’s #L values for HNO,(aq),(Zo) and AH;(H20, 1).(‘I) When this value, AH~(U02C20,*3H,0) from table 4, and the enthalpies of solution from equations (1) (2), and (3) are combined, we obtain: AH,“(U0&04*H20, c, 298.15 K) = -(2111.8 & 2.4) kJ mol-‘, AH,“(UOzCz04,
c, 298.15 K) = -(1796.7 + 2.5) kJ mol-‘.
The reaction scheme from which AH,“{U02(CH,C00),} was calculated is detailed in table 5. In that table, AH for reaction (2) is based upon our observation that, within the experimental uncertainty, the enthalpy of solution was indistinguishable from the enthalpy of solution in an equivalent amount of water.(23’ For the calculation of AH,“{U02(CH,C00)2 -2H,O}, the following equations were combined, where “(soln)” indicates species in a medium composed of 0.5 mol kg-’ of HF: UO,(CH,COO),(c) = UOi+(soln) +2CH,COO-(soln); UO,(CH,COO),*2H,O(c)
TABLE
AH = -(56.29 + 0.11) kJ mol-‘, = UO$+(soln)+2CH,COO-(soln) +2H,O(soln); AH = -(28.41 f 0.25) kJ mol-‘.
5. Calculation of AH,“{UO,(CH,COO),}
5. 6. 7. 8. 9.
y-UO,(c) i- soln D = soln E + H,O(soln) solnE + 2CH,COOH(I) = soln F solnF = UOa(CHaCOO)&, 8) + soln D HzO(soln) = H,O(l) -~ y-UOs(c) + 2CH&OOH(l) = UOz(CH,CO&(c, 8) + HaO(l) UC.3 + (3/2)0,(g) = y-UOs(c) 4C(c, graphite)+ 4Ha(g) + 202(g) = 2CH,COOH(l) &OO) = Ha(g) + U/W&) ..-___~~~~___ ._ _ U(c) + 4C(c, graphite)+ 30,(g) + 3Hz(g) = UOo(CH,COO),(c,
(5)
at 298.15 K
Reaction 1. 2. 3. 4.
(4)
AH/kJ mot-l -(106.18 -(1.23 (56.29 (0.00
8)
-(51.12 -(1223.82 -(968.60 (285.83 I.--(1957.71
f f i f
0.38) 0.05) 0.11) 0.00)
.a b D =
zt & f zt
0.406 0.84) e 0.58) ’ 0.04) P
i 1.10) h
a AH is taken from table 3, and “soln D” denotes HF. 115.3Ha0. b Calculation of AH discussed in text. c Based on dilution data from reference 20. dThis reactionisthe sumof reactions1 to 4. ’ AN,“(UOa, C, y. 298.15 K) is taken from reference 18. ’ AH:(CH&OOH, 1,298.15K) istakenfrom reference22. g AHi@%O,1,298.15K) is takenfrom reference21. ” This value is equivalent to AHP{UOQ(CH~COO)~, c, 6,298.15 Kj, and is the sum of reactions 5 to 8.
P. A.
1084
G.
O’HARE
The partial molar enthalpy of H,O in this medium was calculated to be -(285.83 kO.04) kJ mol-’ based on Parker’s +t values for HF(aq).‘20) Thus, we calculate AHf”(U02(CH,C00),*2H,0, VAPOR
PRESSURES
c, 298.15 K> = -(2557.25
OF THE
+ 1.14) kJ mol-‘.
HYDRATES
Although vapor-pressure measurements have been described for several hydrates of uranyl salts, e.g. uranyl nitratecz4’ and uranyl sulfate,(25) to date no such studies of the uranyl oxalates or acetates have been reported. However, the enthalpies of formation obtained by us, combined with approximate entropy changes AS”, permit estimates to be made of the Gibbs energies of dehydration and, thence, the equilibrium water vapor pressures. These quantities are in table 6. The values for AS” incorporate the assumption that the addition of each H,O molecule to the oxafate or acetate TABLE 6. Thermodynamic quantitiesat 298.15K for the UOzCaOa+ Ha0 and UO,(CH,COO), + H,O systems .--_-AH”
AS”
kJ mol-l
AG”
J K-l molri
kJ mol-’ ~- --__~ ~~ UO,C,O,~3H,O(c) = UO&O.S*H~O(C)+ 2H,O(g)
(119.9i 3.4)
(285 f
8)
(34.9 zk 4.2)
PUW) _-
Pa -.--
(89 x!x 30) b
UOXdh~ H,qc) = UO,C,O,(c) + H&(g) (73.3
f
3.5)
(142 3~ 4)
UOztCHKOO)~
(115.9i- 1.6)
(31.0 It 3.7)
.2H,O(c) = UO,(CH,COO,,(c, (285 zlc 8)
(30.9
f
(0.4 f
0.2) c
(200 f
50) b
6) $ 2H,O(g) 2.9)
a AH,“(H,O, g, 298.15K) wastakento be -(Ml.81 % 0.04)kJ mol-1 (reference21). bCalculatedfrom AG” = -2RTln{p(Hz0)/101325Pa}. c Calculatedfrom AC” = -RT ln{p(HzO)/lO1325Pa).
molecules increases the standard entropy S” by (46 f 4) J K-’ mol- ‘. This estimate was based on an inspection of literature data for various hydrates. In addition, we have used S”(H,O, g, 298.15 K) = (188.7 f 0.1) J K-r mol-I.(“) The enthalpies in table 6 indicate that, in the dehydration of the oxalate, the first ‘two Hz0 molecules are released somewhat more readily than the third H,O molecule. This result is in agreement with the findings of Bressat et .1.‘12’ The mean binding energies of the first two Hz0 moieties in the oxalate system also are close to the values for the acetate system. Discussion ENTHALPIES
OF FORMATION
Apart from a combined
d.t.a. and t.g.a. study by Padmanabhan, Saraiya, and Sundaramc6) which gave energies of activation for the decomposition of uranyl
oxalate, no thermal quantities have been previously reported for these materials.
THERMOCHEMISTRY
OF URANYL
OXALATES
AND
ACETATES
1085
In the case of the acetates, however, there have been earlier calorimetric studies. Athavale, Kalyanaraman, and Sundaresan”’ measured the energy of combustion in of their data yielded a value of oxygen of UO,(CH&OO),. A recalculation”) -(1632.6 + 33.0) kJ mol-’ for the enthalpy of the combustion reaction which, according to Athavale er al., was: UO,(CH,COO),(c)
+(23/6)0,(g)
= (I /3)U,O,(c)
+4CO,(g)
+3H,O(l).
(6)
This value differs significantly from that given in the original journal article and may be due to the use there of an incorrect molar mass for the acetate. Even so, the recalculated AH,“, -(1992 ) 33) kJmol- ’ ,(I) does not agree with the present determination. One possible explanation for the discrepancy could be that substoichiometric U308 was formed in the combustion experiments. Unfortunately, the authors gave no details of their postcombustion analytical procedures. Aloyt4’ reported the enthaipy of solution of U02(CH,C00)2 -2Hz0 in H,O to be 18 kJ mol-’ at an unspecified temperature between 291 and 293 K. In addition, only a very approximate idea of the concentration of the product solution was given. Consequently, conversion of Aloy’s result to refer to 298.15 K and infinite dilution is not possible. Because of this problem. the authors of a recent compilation took and thence, AH,“(UO,(CH,COO), -2H20,c. AH,“,,, to be (18 4 8) kJ mol-‘,(‘) 298.15 K) = -(2581 + 8) kJ mol-‘. This estimate does not agree with the present result.
THE
COMPLEXITY
OF AQUEOUS
URANYL
ACETATE
The complexity of the aqueous uranyl acetate system has been well documented. Three equilibria involving complexions such as U02CH,C00+ and U02(CH,C00); have been identified in a medium of ionic strength 1 mol kg-‘.(2G,27) Salman and White(2*’ found that the enthalpy of solution of U02(CH,C00), at infinite dilution could not be determined because of extensive hydrolysis of UO:+ and the formation of complex ions. These authors therefore had to refer the enthalpies of dilution to an arbitrary standard state. The present studies gave results that are consistent with the preceding observations. From our value for AH,“{UO,(CH,COO),} and literature data for AH: of UOZ +(aq),(*“) and CH,COO-(aq),(23’ one calculates the standard enthalpy of solution, AH& to be -(33.5 + 2.8) kJ mol-‘, for the reaction: UO,(CH,COO),(c)
+ a1H,0(1) = UO: ‘(aq) + 2CH,COO-(aq).
(7)
The enthalpy of solution of U02(CH,C00)2 in 0.002 mol kg-’ of HClO, was found by us to be -(6.2 f 0.2) kJ mol-‘. In this medium, hydrolysis ofUOz+ is minimized; we estimate the correction based on the work of Baes and Meyer(“” to be approximately -0.3 kJ mol-‘. If the solution behavior of uranyl acetate in HCIO, were the same as that depicted in equation (7), then the enthalpy of dilution by Debye-HGcke] theory would be about - 1.3 kJ mol-‘, and the total correction to the enthalpy of solution in HCIO, would be - 1.6 kJ mol- I. The large difference, approximately
P. A. G. O’HARE
-26 kJ mol-‘, between the hypothetical value for the enthalpy of solution and that estimated from the solution in acid, is a direct result of the formation of complex aqueous species in the uranyl acetate solution. We are grateful to Alice Essling, B. Tani, and J. P. Faris for performance of analyses. REFERENCES 1. Cordfunke, E. H. P.; O’Hare, P. A. G. The Chemical Thermodynamics of Actinide Elements and Compounds. Part 3. Miscellaneous Actinide Compounds, IAEA: Vienna, in press. 2. Athavale, V. T.; Kalyanaraman, R.; Sundaresan, M. Zndian J. Chem. 1969, 7, 386. 3a. Bharadwaj, D. S.; Murthy, A. R. V. Indian J. Chem. 1964, 2, 391. 3b. Subramanian, M. S.; Singh, R. N.; Sharma, H. D. J. Znorg. Nucf. Chem. 1969, 31, 3789. 4. Aloy, J. Compt. Rend. Is%, 122, 1541. 5. Bousquet, J.; Bonnetot, B.; Ciaudy, P.; Mathurin, D.; Turck, G. Thermochim. Acta 1976, 14, 357. 6. Padmanabhan, V. M. ; Saraiya, S. C.; Sundaram, A. K. J. Znorg. Nucl. Chem. 1960,12, 356. 7. Clough, P. S.; Dollimore, D.; Grundy, P. J. Znorg. Nucl. Chem. 1%9, 31, 361. 8. Staritzky. E.; Cromer, D. T. Anal. Chem. 1956, 28, 1353. 9. Amphlett, C. B.; Davidge, 0. T. J. Chem. Sot. 1952, 2938. 10. Giorgio, G.; Men&n, B.; Breysse, M.; Claude], B. J. Znorg. Nucl. Chem. 1970, 32, 1517. 11. O’Hare, P. A. G. ; Boerio, J.; Hoekstra, H. R. J. Chem. Thermodynamics 1976, 8, 845. 12. Bressat, R.; Claudel, B. ; Trambouze, Y. J. Chim. Phys. 1964, 61, 816. 13. Mentzen, B.; Giorgio, G. J. Znorg. Nucl. Chem. 1970, 32, 1509. 14. O’Hare, P. A. G. ; Hoekstra, H. R. J. Chem. Thermodynamics 1973, 5, 769. IS. Wyckoff, R. W. G. Crystal Structures 2nd edition., Vol. 5. Interscience: New York. 1966. 16. Donnay, J. D. H.; Ondik, H. M.; editors. Crystal Data Determinative Tables 3rd edition., Vol. 2. U.S. Dept. of Commerce, National Bureau of Standards and Joint Committee on Powder Diffraction Standards, U.S.A. 1973. 17. Boullb, A.; Jary, R. ; Domine-Berg&s, M. Compt. Rend. 1950, 230, 300. 18. Cordfunke, E. H. P.; Ouweltjes, W. ; Prins, G. J. J. Chem. Thermodynamics 1975, 7, 1137. 19. Domalski, E. S. J. Phys. Chem. Ref: Data 1972, 1, 211. 20. Parker, V. B. Thermal Properties of Aqueous W-univalent Electrolytes. NSRDS-NBS-2, 1965. 21. CODATA Recommended Key Values for Thermodynamics 1975. CODATA Bulletin No. 17, 1976.
22. Cox, J. D. ; Pilcher, G. Thermochemistry of Organic and Organometallic Compounds. Academic Press: New York. 1970. 23. 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. 24. Cordfunke, E. H. P. Thermodynamics, Vol. II. IAEA: Vienna, Austria. 1966, p. 483. 25. Cordfunke, E. H. P. J. Znorg. Nucl. Chew. 1972, 34, 1551. 26. Ahrland, S. Acta Chem. &and. 1951, 5, 199. 27. Ahrland, S.; Kullberg, L. Acta Chem. &and. 1971, 25, 3677. 28. Salman, B. C. L.; White, A. G. J. Chem. Sot. (London) 1957, 3197. 29. Parker, V. B.; Wagman, D. D.; Garvin, D. Report No. NBSIR 75-968 1976. 30. Baes, C. F., Jr.; Meyer, N. J. Znorg. Chem. 1962, 1, 780.
Thermochemistry of uranium compounds X. Standard enthalpies of formation of uranyl oxalate, uranyl acetate, and their hydrates. Thermodynamics of the UOICzOl + Hz0 and UO,(CH,COO), + Hz0 systems’ b P. A. G. O’HAREC Chemical Engineering Division, Argonne National Laboratory, Illinois 60439, U.S.A.
Argonne,
(Received 21 April 1977) Solution-calorimetric studies of uranyl oxalate and acetate and their hydrates are described. The standard enthalpies of formation of the crystalline solids at 298.15 K, AH,“/kJ mol-’ are: UO&04, -(1796.7 i 2.5); UO&01.HzO, -(2111.8 f 2.4); U02CaO*.3Ha0, -(271X3 i 2.4); &UOa(CHaCOO)a, -0957.7 i 1.1); and UOa(CH&OO)a.2Hz0, -(2557.3 & 1.1). The calorimetric data yield the following enthalpy changes at 298.15 K: U02Ca04.3H,0(c)
= UO,C,O,*H,O(c)
+ 2HaO(g);
UWXb *H&(c) = UO&nOl(c) + HaO(g) ; UOz(CHaCOO)a .2H,O(c) = U02(CH,COO)a(c, 6) + 2HzO(g);
AH” = (119.9 & 3.4) kJ mol-I; AH’ = (73.3 i 3.5) kJ mol-I;
(1) (2)
AH” = (115.9 it 1.6) kJmol-I.
(3)
Equilibrium Hz0 vapor pressures are estimated to be (89 zb 30), (0.4 f 0.2), and (200 i 50) Pa for equilibria (l), (2), and (3), respectively.
1. Introduction A literature survey made in connection with a recent critical assessment of the thermodynamics of miscellaneous actinide compounds”’ revealed that thermochemical quantities for most organo-uranium compounds either are not available or are, at best, of marginal quality. For example, the enthalpy of formation of uranium oxalate, AH,“(U(C,O,),}, determined by oxygen bomb calorimetry(2) is uncertain by approximately 40 kJ mol-’ and disagrees seriously with the value that can be deduced from d.t.a.(3a*3b) Similarly, the value for anhydrous uranyl acetate is uncertain by 33 kJ nlol-1,(‘*2) and the only calorimetric results for uranyl acetate dihydrate and uranyl ammonium acetate date back to the last century. (4) One exception to the foregoing a This work was performed under the auspices of the U.S. Energy Research and Development Administration. b Presented in part at the VII’th All-Union Conference on Calorimetry, Moscow, U.S.S.R., 31 January to 3 February 1977. c Present address: Division of Research and Laboratories, International Atomic Energy Agency, A-101 1 Vienna, Austria.
P. A. G. O’HARE
1078
observations has been the recent careful study of many1 formate described by Bousquet et d(5) Under normal conditions of humidity and atmospheric pressure, uranyl oxalate exists as the trihydrate, UO&04*3H20, at room temperature. When this material is heated, the water of hydration is lost stepwise, a stable monohydrate is formed, and, ultimately, the anhydrous oxalate decomposes to carbon monoxide and dioxide and a mixture of uranium oxides.@) Uranyl acetate is normally obtained as the dihydrate, U0,(CH,C00)2*2H,0, which is stable in air at room temperature. Careful thermal dehydration of this material is supposed to yield the anhydride. There is no evidence of any stable intermediate hydrate. The anhydride decomposes thermally to a mixture of uranium and carbon oxides.(7) In the present study, we have determined standard enthalpies of formation AHf” for U02C20,*3Hz0, U02C204*Hz0, U02C204, U02(CH,COO),*2H,0, and UO,(CH,COO), by solution calorimetry. These results along with approximate entropy changes permit one to estimate the equilibrium water vapor pressures over the hydrates and give a qualitative measure of the ease of dehydration of the hydrates.
2. Experimental procedure and results PREPARATION
AND
ANALYSIS
OF SAMPLES
Uranyl oxalate trihydrate was prepared according to the method of Staritzky and Cramer.(8) An excess of oxalic acid was added to a dilute solution of pure uranyl nitrate in approximately 1 mol kg -’ HNO,, and this mixture was digested at 350 K. Precipitated oxalate was removed by filtration, washed with water until the washings were free of nitrate ion as indicated by the brucine test, and dried for one week at 333 K as described by Amphlett and Davidge. (9) This material rapidly gained mass when exposed to the atmosphere; it could therefore not be the trihydrate, which is stable in air. Analyses showed it to be the monohydrate; it was handled exclusively in a glovebox filled with pure recirculating helium. Part of the monohydrate preparation was exposed to the laboratory atmosphere at 292 K and a relative humidity of (50 rt 2) per cent and, within a few hours, attained constant mass. This product was the trihydrate; it could be handled in air but not in the glovebox because of rapid loss of water of hydration. The anhydrous oxalate was prepared by dehydration of the monohydrate under vacuum at 423 K, and was handled only in the glovebox. Uranyl acetate dihydrate was purchased from Apache Chemicals, Inc., Seward, Illinois. An attempt was made to prepare the anhydride by careful heating of the dihydrate at 350 K. When the mass of the specimen had decreased by an amount corresponding to the loss of 2 mol of H,O, the heating was discontinued. The uranium content of the resulting material was very close to the theoretical value for UOZ (CH,COO)l; however, the X-ray diffraction pattern indicated the presence not only of U0,(CHJC00)2, but also of some dihydrate and other unidentified decomposition products. Since additional heating at higher temperatures brought about further decomposition, our efforts to obtain UO,(CH,COO), by thermal decomposition of the dihydrate were discontinued.
THERMOCHEMISTRY
OF URANYL
OXALATES
1079
AND ACETATES
The calorimetric specimen was prepared by means of a dehydration procedure similar to that used by Giorgio et al. (lo) The dihydrate was refluxed with acetic anhydride for 6 h, and the resulting material was filtered in dry air. The filtrate was heated for several days at 318 K, pulverized in a glovebox containing dry recirculating helium, and heated again for several days in a drying oven at 350 K. Oxalic acid dihydrate was a “Baker analyzed” reagent; the oxalate content was greater than 99.8 moles per cent of CtO:- based on permanganate titrations. Analar reagent-grade nitric acid was diluted with CO,-free distilled water, titrated against Na,CO,, and further diluted to the desired concentration. Acetic acid, Baker “Ultrex” grade, was certified to have a purity of 99.8 moles per cent based on freezing-temperature analyses. The y-UOJ was part of the specimen used in earlier studies.(“) The X-ray powder diffraction patterns of the oxalates were in good agreement with the results of Staritzky and Cromer (8) for the trihydrate and in reasonable agreement with the results of Bressat, Claudel, and Trambouze(“) for the anhydride. However, although we agreed with the latter authors with regard to theposition of the diffraction arcs for the monohydrate, there were significant intensity discrepancies. The diffraction patterns for the acetates showed good agreement with the results reported by Mentzen and Giorgio (13) for the dihydrate and by Giorgio et a1.(“’ for the Z-form of the anhydride. However, the latter diffraction pattern contained two extraneous unidentified lines. TABLE
1. Analytical results for the calorimetric specimens; w denotes mass fraction theory
uo,c*o* UO&OI*HaO UOJZs0.j. 3HaO UOa(CHXOO)a *2H10 UO,(CH,COO), Y-U03
66.48 63.30 57.16 56.12 61.33 83.22
1OQcJ) found 4 66.65 63.28 57.72 56.11 61.32 83.22
30aw(Ha0) theory found * 4G9 13.11 -
4.79 13.11 -
a Analytical uncertainty about 0.1 per cent. b Analytical uncertainty about 0.15 per cent.
The oxalates were analyzed for H,O and U, and the acetates for U only. The H,O analysis was based on careful dehydration and, with the exception of UO,(CH,COO), *2H,O, the analysis for U was based on ignition in air to U30, at 1120 K. The y-UO, was also analyzed by ignition to U,Os. The acetate dihydrate was analyzed for U by potentiometric titration. The analytical results are summarized in table 1. Trace-metal impurities by emission-spectrochemical analyses amounted to approximately 10e4 by mass for each compound, and were judged to be thermochemically insignificant. CALORIMETRIC EQUIPMENT AND PROCEDURE The LKB-8700 Precision Calorimetric System and ancillary equipment have been described previously. (14) Calorimetric specimens were enclosed in glass ampoules
P. A. G. O’HARE
1080 TABLE Experiment No.
1
2 3 4 5 6
1
2 3 4 5 6
2. Calorimetric
results for the enthalpies of solution of the oxalates (L
-y k3
A0 ..A K
0.05656 0.05571 0.05650 0.05654 0.05612 0.05545
b AH.,,, + AH,,,’ I
(COOH)2.2Hz0 -0.00591 2.411 -0.00255 1.040 o.ooo94 -0.383 0.00108 -0.441 o.ooo99 -0.404 0.00205 -0.836 :AH&M> = (284.11 f 1.04) J g-l d AH = (35.82 f 0.26) kJ mol-’
J
AHsodM .-_J g-l
13.467 14.969 16.471 16.471 16.472 16.472
280.73 287.36 284.74 283.52 286.32 28 1.98
11.965 10.964 10.964 10.964 10.964 10.964
60.68 59.26 61.19 60.39 61.91 62.20
e*f
0.18446 0.18447 0.18273 0.18359 0.18329 0.18443
UO&0,.3H,O 0.00189 -0.772 0.00908 -0.033 -0.00053 0.217 -0.00030 0.123 - 0.08094 0.384 -0.00124 0.507 (AH&M> = (60.94 xk 0.44) J g-’ ’ AH = (25.11 =t 0.36) kJ mol-’ e.r
0.16684 0.16994 0.16566 0.16459 0.16979 0.16725
UO,C,O,.H,O 0.00699 -2.852 -0.053 0.00736 -3.003 -0.053 0.00681 -2.778 -0.053 0.00710 -2.897 -0.053 0.00739 -3.015 -0.053 0.00676 - 2.758 -0.053 (AH,,,,,/M> = -(17.55 & 0.21) J g-’ ’ AH = -(6.60 ZII 0.16) kJ mol-’ o*f
0.15868 0.16143 0.15931 0.16015 0.16018 0.15883
uozc204 0.03820 - 15.593 0.03885 - 15.859 0.03883 - 15.850 0.03920 -16.001 0.03959 -16.161 0.03932 - 16.050
-0.053 -0.053 -0.053 -0.053 -0.053 -0.053
-17.41 -17.98 -17.09 -17.92 -18.07 -16.81
-98.60 -98.57 -99.82 - 100.24 - 101.22 - 101.39
e.r
4 Oxalic acid dihydrate was dissolved in a mixture of UOz(NO& and HNOB. The composition of this mixture corresponded to that denoted by “soln B” in table 6 (reference 11). The uranyl oxalates were dissolved in HNOa solution of approximate molality 2.8 mol kg-l. The mean temperature of each experiment was (298.15 i 0.01) K. b The values of
THERMOCHEMISTRY
OF URANYL
OXALATES
1081
AND ACETATES
with frangible end windows. Seven sets of calorimetric measurements were carried out. The three uranyl oxalates were dissolved in aqueous nitric acid of approximate molality 2.8 mol kg- I, and the oxalic acid dihydrate was dissolved in nitric acid to which an accurately weighed quantity of y-UO, had been added. This soIution corresponded to the product of a specific HNO, + y-UO, reaction, the enthalpy of which was reported in an earlier paper. (11) The uranyl acetates and y-UO, were dissolved in aqueous HF of approximate molality 0.5 mol kg- ‘. All of the above compounds dissolved readily; no unreacted material was observed. CALORIMETRIC
RESULTS
Calorimetric results for the four dissolution reactions of the oxalates are given in table 2. Similar results for y-UO3 and the acetates are given in table 3. In these tables, m denotes the mass of compound dissolved or reacted; A0, is the temperature change TABLE E.xperiment No. 2 3 4 5 6 8
3. Calorimetric results for y-UO, and the uranyl acetates (i
1’1 6
A& .-.K
(c(calor):p J K-’
AK,, -----1 J
y-uo, 0.49314 0.41774 439.00 -0.053 0.49522 0.41586 438.82 - 0.053 438.85 -0.053 0.49965 0.42143 438.77 -0.053 0.49464 0.42014 0.49892 0.42359 438.80 -0.053 0.50177 0.42511 437.76 -0.053 (AH,/M(y-U03)) = -(371.21 i 0.65) J g-’ ’ AH* = -(106.18 i 0.38) kJ mol-’ =sd
-371.99 -368.61 -370.26 -372.79 -372.65 -370.98
S-UOz(CH&OO)a 0.69246 0.22881 438.91 -0.043 0.67517 0.22377 438.67 -0.043 0.67401 0.22235 438.79 -0.043 0.68433 0.22561 438.49 -0.043 0.67176 0.22236 438.43 -0.043
- 145.09 - 145.45 - 144.82 - 144.62 -145.19
UO,(CH,COO),*2H,O 0.11521 437.48 -0.042 0.11149 438.22 -0.042 0.11330 439.78 -0.042 0.11147 439.24 -0.042 0.11301 439.29 -0.042 0.11167 439.52 -0.042 = -(66.99 zt 0.30) J g-’ b AH, = -(28.41 I& 0.25) kJmol-1 c.d
-67.99 -66.17 -67.41 - 66.46 -67.47 -66.44
2 0.74195 3 0.73917 4 0.73977 5 0.73732 6 0.73638 7 0.73935 (AH,/M{UO~(CH&00)~.2H,O))
n The mean temperature of each experiment was (298.15 i 0.01) K. b Uncertainty is the standard deviation of the mean. c MoIar masses (M/g mol-I) were taken to be 286.027, 388.117, and 424.147 for y-UO$, UOz(CH&00)2, and UOa(CH3COO)a.2Ha0, respectively. d Uncertainty is the uncertainty interval equal to twice the standard deviation of the mean.
1082
P. A. G. O’HARE
of the calorimeter corrected for heat exchange with the environment; (s(calor)) is the mean energy equivalent of the calorimetric system based on electrical calibrations performed before and after the enthalpy of solution and reaction measurements; AHelcc denotes the electrical energy supplied to the calorimeter to offset the cooling effect of the endothermal dissolution of (COOH),+2H,O and UO,C,O,*3H,O; AH,,, is a correction that takes into account the saturation with water vapor of the gas (air or helium) enclosed in the ampoules; AH&M is the specific enthalpy of solution; and AH,IM is the specific enthalpy of reaction. For the experiments with the oxalates, A0, was small and, therefore, (s(calor)) did not have to be known very precisely. The values given in the footnote to table 2 are based on separate calibrations and are accurate to better than 0.4 J K- ‘. For the calculation of AH,,, the densities of (COOH), *2H,O, UOzCz04*3Hz0, UO,(CH, COO)z *2Hz0, y-UOs, UOzCz04*Hz0, and UO,C,O, were taken to be 1.6,(r5’ 3 .,07 @) 2 .,92 (13) 8.03,‘16) 2.6, and 2.5 g cmW3, respectively. The last two values were calculated from the X-ray data (i2) by assuming that, like the trihydrate, the monohydrate and anhydride had four molecules per unit cell.~“~ The density of U0,(CH3C00)2 was estimated to be 3 g cmW3.
ENTHALPIES
OF FORMATION
The thermochemical cycle upon which AH,“(UOzCz04*3Hz0) is based is given in table 4. For the calculation of the enthalpies of formation of the other oxalates, the following equations were combined (it should be noted that “(sol@” indicates species TABLE
4. Calculation of AH,0(U02Ca04. 3Ha0) at 298.15 K
Reaction 1. y-UO&) + soln A = soln B 2. (COOH)z*2HzO(c) + soln B = 3. soln C = 4. y-UOs(c) + (COOH)a .2HzO(c) 5. U(C) + (3/2)wg) = Y-uom 6. ZC(graphite) + 3H& + 30,(g)
AH/kJ mol-’
soln C UOzCaOI. 3Ha0 + so111A = UO&Od
*3HaO(c)
= (COOH),.2HzO(c)
7. WC) + Wgraphite) + (9/2)0,(g) + 3Ha(g) = U02C201.3HzO(c)
-(75.5 * 0.7) a (35.8 31 0.3) b -(25.1 zt 0.4) b -(64.8 f 0.9) ’ -(1223.8 31 0.8) ’ -(1426.7 f 2.1) = -(2715.3
i 2.4) ’
a Here soln A is approximately 2.8 mol kg-l of HNOJ and AH is taken from reference 11. * AH from table 2. c AH for this reaction is the sum of AH for reactions 1, 2, and 3. d AHy(U03, c, y, 298.15 K) is taken from reference 18.. o AH,“{(COOH)2.2Hz0, c, 298.15 K} is the value selected by Domalski (reference 19). The uncertainty has been estimated. f This value is equivalent to AH,“(U0,C20,*3Hz0, c, 298.15 K), and is the sum of reactions 4, 5, and 6.
THERMOCHEMISTRY
OF URANYL
in a medium composed of 2.8 mol kg-’
OXALATES
1083
AND ACETATES
of HNO,):
UO,C,O,*3H,O(c)
= UO$+(soln)+C,Oi-(soln) AH = (25.1 & 0.4) kJ mol-‘, +3H,O(soln); (1) UOzC204*H,0(c) = UOg+(soln) +C,O:-(soln) +H,O(soln); AH = - (6.6 + 0.2) kJ mol-‘, (2) UO$,O,(c) = UO~+(soln)+C,O~-(soln); AH = -(35.8 + 0.4) kJ mol-‘. (3) The partial molar enthalpy of formation of H,O in this medium was calculated to be -(285.9 f 0.1) kJ mol-’ based on Parker’s #L values for HNO,(aq),(Zo) and AH;(H20, 1).(‘I) When this value, AH~(U02C20,*3H,0) from table 4, and the enthalpies of solution from equations (1) (2), and (3) are combined, we obtain: AH,“(U0&04*H20, c, 298.15 K) = -(2111.8 & 2.4) kJ mol-‘, AH,“(UOzCz04,
c, 298.15 K) = -(1796.7 + 2.5) kJ mol-‘.
The reaction scheme from which AH,“{U02(CH,C00),} was calculated is detailed in table 5. In that table, AH for reaction (2) is based upon our observation that, within the experimental uncertainty, the enthalpy of solution was indistinguishable from the enthalpy of solution in an equivalent amount of water.(23’ For the calculation of AH,“{U02(CH,C00)2 -2H,O}, the following equations were combined, where “(soln)” indicates species in a medium composed of 0.5 mol kg-’ of HF: UO,(CH,COO),(c) = UOi+(soln) +2CH,COO-(soln); UO,(CH,COO),*2H,O(c)
TABLE
AH = -(56.29 + 0.11) kJ mol-‘, = UO$+(soln)+2CH,COO-(soln) +2H,O(soln); AH = -(28.41 f 0.25) kJ mol-‘.
5. Calculation of AH,“{UO,(CH,COO),}
5. 6. 7. 8. 9.
y-UO,(c) i- soln D = soln E + H,O(soln) solnE + 2CH,COOH(I) = soln F solnF = UOa(CHaCOO)&, 8) + soln D HzO(soln) = H,O(l) -~ y-UOs(c) + 2CH&OOH(l) = UOz(CH,CO&(c, 8) + HaO(l) UC.3 + (3/2)0,(g) = y-UOs(c) 4C(c, graphite)+ 4Ha(g) + 202(g) = 2CH,COOH(l) &OO) = Ha(g) + U/W&) ..-___~~~~___ ._ _ U(c) + 4C(c, graphite)+ 30,(g) + 3Hz(g) = UOo(CH,COO),(c,
(5)
at 298.15 K
Reaction 1. 2. 3. 4.
(4)
AH/kJ mot-l -(106.18 -(1.23 (56.29 (0.00
8)
-(51.12 -(1223.82 -(968.60 (285.83 I.--(1957.71
f f i f
0.38) 0.05) 0.11) 0.00)
.a b D =
zt & f zt
0.406 0.84) e 0.58) ’ 0.04) P
i 1.10) h
a AH is taken from table 3, and “soln D” denotes HF. 115.3Ha0. b Calculation of AH discussed in text. c Based on dilution data from reference 20. dThis reactionisthe sumof reactions1 to 4. ’ AN,“(UOa, C, y. 298.15 K) is taken from reference 18. ’ AH:(CH&OOH, 1,298.15K) istakenfrom reference22. g AHi@%O,1,298.15K) is takenfrom reference21. ” This value is equivalent to AHP{UOQ(CH~COO)~, c, 6,298.15 Kj, and is the sum of reactions 5 to 8.
P. A.
1084
G.
O’HARE
The partial molar enthalpy of H,O in this medium was calculated to be -(285.83 kO.04) kJ mol-’ based on Parker’s +t values for HF(aq).‘20) Thus, we calculate AHf”(U02(CH,C00),*2H,0, VAPOR
PRESSURES
c, 298.15 K> = -(2557.25
OF THE
+ 1.14) kJ mol-‘.
HYDRATES
Although vapor-pressure measurements have been described for several hydrates of uranyl salts, e.g. uranyl nitratecz4’ and uranyl sulfate,(25) to date no such studies of the uranyl oxalates or acetates have been reported. However, the enthalpies of formation obtained by us, combined with approximate entropy changes AS”, permit estimates to be made of the Gibbs energies of dehydration and, thence, the equilibrium water vapor pressures. These quantities are in table 6. The values for AS” incorporate the assumption that the addition of each H,O molecule to the oxafate or acetate TABLE 6. Thermodynamic quantitiesat 298.15K for the UOzCaOa+ Ha0 and UO,(CH,COO), + H,O systems .--_-AH”
AS”
kJ mol-l
AG”
J K-l molri
kJ mol-’ ~- --__~ ~~ UO,C,O,~3H,O(c) = UO&O.S*H~O(C)+ 2H,O(g)
(119.9i 3.4)
(285 f
8)
(34.9 zk 4.2)
PUW) _-
Pa -.--
(89 x!x 30) b
UOXdh~ H,qc) = UO,C,O,(c) + H&(g) (73.3
f
3.5)
(142 3~ 4)
UOztCHKOO)~
(115.9i- 1.6)
(31.0 It 3.7)
.2H,O(c) = UO,(CH,COO,,(c, (285 zlc 8)
(30.9
f
(0.4 f
0.2) c
(200 f
50) b
6) $ 2H,O(g) 2.9)
a AH,“(H,O, g, 298.15K) wastakento be -(Ml.81 % 0.04)kJ mol-1 (reference21). bCalculatedfrom AG” = -2RTln{p(Hz0)/101325Pa}. c Calculatedfrom AC” = -RT ln{p(HzO)/lO1325Pa).
molecules increases the standard entropy S” by (46 f 4) J K-’ mol- ‘. This estimate was based on an inspection of literature data for various hydrates. In addition, we have used S”(H,O, g, 298.15 K) = (188.7 f 0.1) J K-r mol-I.(“) The enthalpies in table 6 indicate that, in the dehydration of the oxalate, the first ‘two Hz0 molecules are released somewhat more readily than the third H,O molecule. This result is in agreement with the findings of Bressat et .1.‘12’ The mean binding energies of the first two Hz0 moieties in the oxalate system also are close to the values for the acetate system. Discussion ENTHALPIES
OF FORMATION
Apart from a combined
d.t.a. and t.g.a. study by Padmanabhan, Saraiya, and Sundaramc6) which gave energies of activation for the decomposition of uranyl
oxalate, no thermal quantities have been previously reported for these materials.
THERMOCHEMISTRY
OF URANYL
OXALATES
AND
ACETATES
1085
In the case of the acetates, however, there have been earlier calorimetric studies. Athavale, Kalyanaraman, and Sundaresan”’ measured the energy of combustion in of their data yielded a value of oxygen of UO,(CH&OO),. A recalculation”) -(1632.6 + 33.0) kJ mol-’ for the enthalpy of the combustion reaction which, according to Athavale er al., was: UO,(CH,COO),(c)
+(23/6)0,(g)
= (I /3)U,O,(c)
+4CO,(g)
+3H,O(l).
(6)
This value differs significantly from that given in the original journal article and may be due to the use there of an incorrect molar mass for the acetate. Even so, the recalculated AH,“, -(1992 ) 33) kJmol- ’ ,(I) does not agree with the present determination. One possible explanation for the discrepancy could be that substoichiometric U308 was formed in the combustion experiments. Unfortunately, the authors gave no details of their postcombustion analytical procedures. Aloyt4’ reported the enthaipy of solution of U02(CH,C00)2 -2Hz0 in H,O to be 18 kJ mol-’ at an unspecified temperature between 291 and 293 K. In addition, only a very approximate idea of the concentration of the product solution was given. Consequently, conversion of Aloy’s result to refer to 298.15 K and infinite dilution is not possible. Because of this problem. the authors of a recent compilation took and thence, AH,“(UO,(CH,COO), -2H20,c. AH,“,,, to be (18 4 8) kJ mol-‘,(‘) 298.15 K) = -(2581 + 8) kJ mol-‘. This estimate does not agree with the present result.
THE
COMPLEXITY
OF AQUEOUS
URANYL
ACETATE
The complexity of the aqueous uranyl acetate system has been well documented. Three equilibria involving complexions such as U02CH,C00+ and U02(CH,C00); have been identified in a medium of ionic strength 1 mol kg-‘.(2G,27) Salman and White(2*’ found that the enthalpy of solution of U02(CH,C00), at infinite dilution could not be determined because of extensive hydrolysis of UO:+ and the formation of complex ions. These authors therefore had to refer the enthalpies of dilution to an arbitrary standard state. The present studies gave results that are consistent with the preceding observations. From our value for AH,“{UO,(CH,COO),} and literature data for AH: of UOZ +(aq),(*“) and CH,COO-(aq),(23’ one calculates the standard enthalpy of solution, AH& to be -(33.5 + 2.8) kJ mol-‘, for the reaction: UO,(CH,COO),(c)
+ a1H,0(1) = UO: ‘(aq) + 2CH,COO-(aq).
(7)
The enthalpy of solution of U02(CH,C00)2 in 0.002 mol kg-’ of HClO, was found by us to be -(6.2 f 0.2) kJ mol-‘. In this medium, hydrolysis ofUOz+ is minimized; we estimate the correction based on the work of Baes and Meyer(“” to be approximately -0.3 kJ mol-‘. If the solution behavior of uranyl acetate in HCIO, were the same as that depicted in equation (7), then the enthalpy of dilution by Debye-HGcke] theory would be about - 1.3 kJ mol-‘, and the total correction to the enthalpy of solution in HCIO, would be - 1.6 kJ mol- I. The large difference, approximately
P. A. G. O’HARE
-26 kJ mol-‘, between the hypothetical value for the enthalpy of solution and that estimated from the solution in acid, is a direct result of the formation of complex aqueous species in the uranyl acetate solution. We are grateful to Alice Essling, B. Tani, and J. P. Faris for performance of analyses. REFERENCES 1. Cordfunke, E. H. P.; O’Hare, P. A. G. The Chemical Thermodynamics of Actinide Elements and Compounds. Part 3. Miscellaneous Actinide Compounds, IAEA: Vienna, in press. 2. Athavale, V. T.; Kalyanaraman, R.; Sundaresan, M. Zndian J. Chem. 1969, 7, 386. 3a. Bharadwaj, D. S.; Murthy, A. R. V. Indian J. Chem. 1964, 2, 391. 3b. Subramanian, M. S.; Singh, R. N.; Sharma, H. D. J. Znorg. Nucf. Chem. 1969, 31, 3789. 4. Aloy, J. Compt. Rend. Is%, 122, 1541. 5. Bousquet, J.; Bonnetot, B.; Ciaudy, P.; Mathurin, D.; Turck, G. Thermochim. Acta 1976, 14, 357. 6. Padmanabhan, V. M. ; Saraiya, S. C.; Sundaram, A. K. J. Znorg. Nucl. Chem. 1960,12, 356. 7. Clough, P. S.; Dollimore, D.; Grundy, P. J. Znorg. Nucl. Chem. 1%9, 31, 361. 8. Staritzky. E.; Cromer, D. T. Anal. Chem. 1956, 28, 1353. 9. Amphlett, C. B.; Davidge, 0. T. J. Chem. Sot. 1952, 2938. 10. Giorgio, G.; Men&n, B.; Breysse, M.; Claude], B. J. Znorg. Nucl. Chem. 1970, 32, 1517. 11. O’Hare, P. A. G. ; Boerio, J.; Hoekstra, H. R. J. Chem. Thermodynamics 1976, 8, 845. 12. Bressat, R.; Claudel, B. ; Trambouze, Y. J. Chim. Phys. 1964, 61, 816. 13. Mentzen, B.; Giorgio, G. J. Znorg. Nucl. Chem. 1970, 32, 1509. 14. O’Hare, P. A. G. ; Hoekstra, H. R. J. Chem. Thermodynamics 1973, 5, 769. IS. Wyckoff, R. W. G. Crystal Structures 2nd edition., Vol. 5. Interscience: New York. 1966. 16. Donnay, J. D. H.; Ondik, H. M.; editors. Crystal Data Determinative Tables 3rd edition., Vol. 2. U.S. Dept. of Commerce, National Bureau of Standards and Joint Committee on Powder Diffraction Standards, U.S.A. 1973. 17. Boullb, A.; Jary, R. ; Domine-Berg&s, M. Compt. Rend. 1950, 230, 300. 18. Cordfunke, E. H. P.; Ouweltjes, W. ; Prins, G. J. J. Chem. Thermodynamics 1975, 7, 1137. 19. Domalski, E. S. J. Phys. Chem. Ref: Data 1972, 1, 211. 20. Parker, V. B. Thermal Properties of Aqueous W-univalent Electrolytes. NSRDS-NBS-2, 1965. 21. CODATA Recommended Key Values for Thermodynamics 1975. CODATA Bulletin No. 17, 1976.
22. Cox, J. D. ; Pilcher, G. Thermochemistry of Organic and Organometallic Compounds. Academic Press: New York. 1970. 23. 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. 24. Cordfunke, E. H. P. Thermodynamics, Vol. II. IAEA: Vienna, Austria. 1966, p. 483. 25. Cordfunke, E. H. P. J. Znorg. Nucl. Chew. 1972, 34, 1551. 26. Ahrland, S. Acta Chem. &and. 1951, 5, 199. 27. Ahrland, S.; Kullberg, L. Acta Chem. &and. 1971, 25, 3677. 28. Salman, B. C. L.; White, A. G. J. Chem. Sot. (London) 1957, 3197. 29. Parker, V. B.; Wagman, D. D.; Garvin, D. Report No. NBSIR 75-968 1976. 30. Baes, C. F., Jr.; Meyer, N. J. Znorg. Chem. 1962, 1, 780.