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The enthalpy of formation of dicesium monoxide (Cs2O)
J. Chem. Thermodynamics 1974,6,263-269
The enthalpy of formation monoxide (Cs20) a
of dicesium
JACK L. SETTLE, GERALD K. JOHNSON, and WARD N. HUBBARD Chemical Engineering Division, Argonne National Argonne, Illinois 60439, U.S.A.
Laboratory,
(Received 12 June 1973) A sample of high-purity CszO(c) was especially prepared for a solution-calorimetric study of its enthalpy of reaction with excess water to form CsOH(aq). The standard enthalpy of formation, AHI”(Cs20,c, 298.15 K), was derived to be -(82.69 f 0.28) kcal,, moF.
1. Introduction A part of the overall program of this laboratory is to obtain those basic thermodynamic data that will aid in the understanding of the chemical interactions that occur in reactor fuels. Cesium, a high yield fission product,“) has been found in postirradiation studiesc2’ of Urania + plutonia fuel pins to be present in high concentrations, along with molybdenum, in the region of the oxide fuel adjacent to the fuel-cladding interface. These observations suggest that the formation of cesium and molybdenum oxides and/or cesium molybdate could be important factors in determining the chemical interactions that occur between irradiated oxide fuel and its cladding. The enthalpies of formation of the molybdenum oxides(3’ and CS,MOO,(~) have been reliably determined. However, for Cs,O the available datat5*@ are discordant. This paper describes the careful preparation of a sample of high-purity Cs,O and the measurement of its enthalpy of reaction with water according to 0,0(c) + H,O(l) = ZCsOH(aq). (1) Combination of the enthalpy of reaction (I), AH,, with the enthalpies of formation of H20(l),(‘) and CsOH(aq), (*) both of which are well known, yields the standard enthalpy of formation of dicesium monoxide, AHF(Cs20, c, 298.15 K).
2. Experimental CALORIMETRIC
SYSTEM
The measurements of the enthalpy of reaction were carried out in an LKB-8700-I Precision Calorimetric System. The glass reaction vessel was modified to accommodate the probe of a quartz crystal thermometer (Hewlett-Packard Model 2801-A) which was used to measure the calorimetric temperatures. The quartz crystal thermometer was compared with a calibrated platinum resistance thermometer at 298.15 K. a This work was performed under the auspices of the U.S. Atomic Energy Commission.
264
J. L. SETTLE,
G. K. JOHNSON,
The mean energy equivalent of the the results of electrical calibrations experiment. The mean temperature reaction measurements was (298.15
AND W. N. HUBBARD
calorimetric system was determined by averaging performed before and after each Cs,O reaction of the calorimetric system for all calibration and + 0.01) K.
MATERIALS
The sample of Cs,O was prepared by incomplete oxidation of high-purity cesium, followed by vacuum distillation to remove the excess cesium. All operations involving the handling of the cesium metal or oxide were conducted in a helium atmosphere glovebox (H20, 1 p.p.m.; 02, 1 p.p.m.) or in sealed containers. The preparative procedure was as follows: 75 g of high-purity (greater than 99.999 mass per cent with respect to metallic impurities) cesium metal (Kawecki Berylco Industries, Inc.) was placed in an outgassed crucible made from recrystallized alumina. The crucible was placed in a Pyrex vessel which was sealed and attached to a manifold. The vessel containing the crucible was thoroughly evacuated. Oxygen, which had been purified by passage through heated copper oxide, Ascarite, magnesium perchlorate, and P,O,, was admitted to a calibrated volume in the manifold, and then very slowly introduced into the Pyrex vessel. The cesium metal was stirred by magnetic induction while the reaction with oxygen was taking place. During the preparation of the oxide, it was considered desirable to maintain an exposed liquid cesium phase in the crucible. By heating the crucible to 473 K, a temperature at which solid Cs,O is unstable, the exposed liquid cesium phase could be maintained for a longer period of time than in an unheated crucible. When about 75 per cent of the stoichiometric amount of oxygen had been introduced, the cesium could no longer be stirred and the oxygen supply was shut off. The system was then evacuated at 473 K to distill the excess cesium which condensed on to the colder surfaces of the Pyrex vessel. When the coating of cesium on the walls of the Pyrex vessel made it impossible to see the sample, the assembly was cooled to room temperature and taken into the glovebox. The sample was removed and ground in an agate mortar. At this point in the preparation, the sample was a scarlet color with flecks of dark green and was probably composed of mostly Cs,O with isolated particles of Cs,O. The condensed cesium was removed from the Pyrex vessel by reaction with butanol, the vessel outgassed at 573 K, the sample reintroduced, and the assembly returned to the manifold. The cleaning of the vessel and grinding of the sample were done five times before all the excess cesium was removed. The temperature at which the vessel was heated was increased after each grinding procedure by approximately 25 K until the sample was finally being evacuated at 593 K. After cesium ceased to condense on the glass surfaces of the vessel heating was continued for an additional three days at 593 K. At this point the preparation, which was a uniform scarlet color, was returned to the glovebox and stored in a sealed container. The yield of CszO was approximately 55 g. Previous experience in handling CszO had shown that the samples underwent a linear increase in mass with time when exposed to even the highIy purified atmosphere of the glovebox. The increase in mass was shown by analysis to be due to reaction with O2 to form Cs,O, and with water to form CsOH. Therefore, in handling the calorimetric sample, it was decided that the following procedures would be necessary:
ENTHALPY
OF FORMATION
OF CszO
265
(1) immediately analyze for C&O, and CsOH; (2) load the ampoules for the calorimetric experiments as rapidly as possible, noting for each ampoule how long the sample had been exposed to the glovebox atmosphere; and (3) repeat the analyses for Cs20, and CsOH. Thus, by interpolation, the amounts of Cs,O, and CsOH in each ampoule could be determined. In addition, spark source mass-spectrometric and X-ray diffraction analyses, as well as chemical analyses for free cesium, carbonate, and total cesium were performed on the preparation. The impurities found in the sample are given in table 1. TABLE
1. Analyses of the Cs,O sample; w is the mass fraction of the species and t the time of exposure to glovebox atmosphere
Cs(free) cog Al Na Si Rb Ca t/h
(70 It 20) n.d. n 6 3 2 2 1 0
H(as OHH(asOHO(as Ok O(as Oz-
at I = 0) at r = 1Oh) at t = 0) at t = 10 h)
0 13 112
10 102w
csao csaoz CsOH Other
99.99 0.00 0.00 0.01
98.72 1.08 0.19 0.01
a n.d. = not detected.
The analysis for peroxide was conducted as follows: a weighed sample of Cs,O was dissolved in water and the solution acidified with concentrated HzS04. A measured excess of Fe’+ was added to the solution and back-titrated with MnO,. The difference between Fe2+ added to the solution and the amount determined by the permanganate titration is a measure of the O”,- content of the solution. The appropriate reactions on which the calculation of peroxide content is based are Cs202(c) + 2H,O = 2CsOH(aq) + H,O,(aq), H202(aq)+2Fe2+(aq)+2H*(aq) = 2Fe3+(aq)+2H20(1).
(2)
(3) The analysis for CsOH in the Cs,O was conducted as follows: a weighed sample of Cs,O was put into a vessel containing calcium-amalgam, and boiled under argon at 633 K. The decomposition of CsOH by calcium-amalgam liberated H, which was collected, along with the argon, in a calibrated vessel. A portion of this gas was then analyzed for H2 by gas chromatography. This method of analysis had been found to be reliable ($- 10 per cent at the CsOH concentration of interest) by performing analyses on small weighed samples of CsOH. The analyses for free cesium and carbonate in the sample were performed on weighed amounts of Cs,O placed in small bottles which were sealed with serum caps.
266
J. L. SETTLE,
G. K. JOHNSON,
AND W. N. HUBBARD
For free cesium, water was introduced into the bottle with a syringe and the evolved hydrogen gas was determined gas chromatographically. For carbonate, excess HCl was introduced into the bottle and the gas phase was analyzed chromatographically. No CO, was found. The method was judged to be capable of detecting 10 p.p.m. by mass of CO;-’ in the sample. The Cs,O was also analyzed for total cesium by taking weighed samples in platinum dishes, dissolving in water, acidifying with HCl, evaporating to dryness, and weighing the CsCl formed. The analytical method had been found to be accurate on samples of the high-purity cesium used in the Cs,O preparation. The samples taken for the total cesium analyses had been exposed to the glovebox atmosphere for approximately 18 h, and the result of the analyses was (94.150.1) mass per cent Cs. The theoretical amount of Cs in pure Cs,O is 94.32 mass per cent. The calculated amount of cesium in the sample after 10 h exposure to the glovebox atmosphere based on the composition given in table 1 is 94.25 mass per cent. Therefore, the analyzed cesium content would be in accord with the estimated sample composition at the time the analyses were performed. The X-ray diffraction pattern of the sample was in excellent agreement with the pattern for Cs,O reported by Tsai et aLc9’ The CsOH solution used as the reaction medium was prepared by dissolving commercial CsOH in boiled distilled water. PROCEDURES
The Cs,O (approximately 0.36 g) was placed into thin-walled Pyrex ampoules which were sealed with silicone-rubber stoppers and wax. The ampoule was placed in the stirrer assembly of the caIorimeter, the reaction vessel was loaded with 99.54cm’ of approximately 0.01 mol dme3 CsOH, and the experiments were performed. The solution of CsOH was used instead of pure water so that any dissolved COZ could be neutralized before the calorimetric measurements. Following several of the experiments, portions of the calorimetric solutions were analyzed for peroxide content, which was found to be only slightly lower than the calculated value based on the Cs,O, impurity. Therefore it was concluded that very little of the unstable peroxide had decomposed before the end of the experiment, and no correction was made for the corresponding decomposition of the hydrogen peroxide in solution.
3. Results The results of six measurements of the enthalpy of reaction of &O(c) with water according to Cs,O(c) +0.74O(CsOH*5555.4H,O)(aq) = 2.74O(CsOH* lSOOH,O)(aq), (4) are presented in table 2. In the table, m”(Cs,O,) and m”‘(CsOH) are the calculated masses of those impurities in the sample at the time the ampoule was sealed; AHVap is the correction for the vaporization of water into the free volume of the ampoule and is based on a density of 4.71 g cm -3(9) for Cs,O and an enthalpy of vaporization of water of 10519 Cal,, mol-‘;(‘) AHcS202 and AHcsOH are the corrections for the
g- I Average =
g mol-l.
AH,
AH,/M(C&O) i
0.28)
= -(291.59 kcal,,
*
0.38279 0.00124 0.00021 0.38134 106.674 1 Ml41 -111.091 -0.012 0.097 0.024 -0.001 -291.03
mol-l
0.47)
results for the reaction (calth = 4.184 J)
-(82.17
0.37636 0.00107 0.00019 0.37510 106.637 1.02676 - 109.491 -0.012 0.084 0.022 0.000 -291.65
2. Calorimetric
to be 281.8102
0.35321 OSKJO76 0.00013 0.35232 106.598 0.96624 - 102.999 -0.012 0.059 0.015 0.004 -292.16
n Standard deviation of the mean. b Uncertainty interval. c The molar mass of Cs;O was taken
IAWMV320);lcalt~
m”(CsOH)/g m’(CsL%(AT&al,, M-L&&, AfLx9*0./4h gcso~~ dlln th
m(samt-W/g m”(CsKh)/g
TABLE
cal h g-l b.C
with
Ha0
L1
0.41090 0.00249 0.00043 0.40798 106.744 1.12401 -119.981 -0.012 0.194 0.049 -0.006 -293.53
of Cs,O
0.36679 0.00224 0.00038 0.36417 106.695 0.99289 - - 105.936 -0.012 0.175 0.043 0.002 - -290.33
0.37191 0.0025 1 0.00043 0.36897 106.616 1.00860 -107.533 -0.012 0.196 0.049 0.001 -290.81
268
J. L. SETTLE,
G. K. JOHNSON,
AND W. N. HUBBARD
reaction of Cs,O, with water according to reaction (2), and the solution of CsOH in 1500H,O, respective1y.f The specific enthalpies of reaction and solution were taken to be -78.08 and - 113.6 Cal,, g-l, respectively. The AZ&,, term is the enthalpy change for adjusting the composition of the final solution to CsOH. 1500Hz0. Except for Cs,Oz and CsOH, the thermal correction for impurities in the sample was negligible. The standard enthalpy of formation of Cs,O(c) according to 2c44 +twd = C%O(C), (5) is obtained by combination of AH4 with the enthalpy changes for the following processes : 0.74O(CsOH~15OOH,O)+3001H,0(1) = 0.740(CsOH*5555.4H,G), (6) 2Cs(c) + 3002H,O(l) = 2(CsOH * 1500Hz0) + H,(g), (7) H,(g) +tWg) = H,W (8) The value AH, = - (0.020f0.005) kcal,, mol- ’ was based on QL for CsOH(aq) as given by Gunn, *(a) AH, = -(96.564$0.022) kcal,, mol-r was derived from the measurements of Gunn;(*) and AH*, the enthalpy of formation of water, was taken tobe -(68.315+0.015) kcal,, mol- ‘.(‘) The value obtained for AH,“(Cs,O, c, 298.15 K) is - (82.69 kO.28) kcal,, mol-‘. Values for the standard entropy of formation, AS,“(Cs,O, c, 298.15 K) = - 30.10 Cal,, K-’ mol- ‘, and standard Gibbs energy of formation, AG,“(Cs,O, c, 298.15 K) = - (73.72 kO.28) kcal,, mol-’ were derived based on the AHf” result and values of S”(298.15 K) of 35.10, 20.35, and 49.00 Cal,, K-r mol-r for Cs,O,(“) Cs,(‘l) and 02,(‘) respectively. The uncertainties given in the derived results are uncertainty interval@ equal to twice the combined standard deviations arising from all known sources. 4. Discussion Previous studies of the enthalpy of reaction of Cs,O with excess water by Beketo@ and Rengade@’ yielded for AH, values of - 72.2 kcalth mol-’ and - 83.2 kcal,, mol- l, respectively. Our result, AH, = -(82.17+0.28) kcal,, mol-‘, is much closer to the result obtained by Rengade than to that obtained by Beketov, who, according to Rengade,@) used a very impure sample. We believe that the result reported herein for AHF(CszO, c), - (82.69 f0.28) kcal, mol-I, is the most reliable value reported to date because of the care taken in the preparation, handling, and analysis of the sample. The authors gratefully acknowledge K. J. Jensen, R. V. Schablaske, and M. I, Homa for the performance of analyses, D. V. Steidl for advice on the preparation of the CszO sample, and A. D. Tevebaugh for administrative assistance. REFERENCES 1. Lisman, F. L.; Abernathey, R. M. ; Maeck, W. J. ; Rein, J. E. Nucl. Sci. Eng. 1970, 42, 191. 2. Johnson, C. E.; Seils, C. A.; Anderson, K. E. U.S. At. Energy Comm. Rep. No. ANL-7115 1971, p. 92. 7 Throughout
this paper caltb = 4.184 J.
ENTHALPY
OF
FORMATION
OF
CsaO
269
3. Wagman, D. D.; Evans, W. H. ; Parker, V. B.; Halow, I. ; Bailey, S. M. ; Schumm, R. H. Nut. Bur. Stand. (U.S.) Tech. Note 270-4 1%9. 4. O’Hare, P. A. G.; Hoekstra, H. R. J. Chem. Thermodynamics 1973, 5, 851. 5. Beketov, N. Bull. Acad. Sci. Russ. 1894, 35, 541. 6. Rengade, E. Ann. Chim. Phys. 1908, 14, 540. 7. Wagman, D. D.; Evans, W. H.; Parker, V. B.; Halow, 1.; Bailey, S. M.; Schumm, R. H. Nat. Bur. Stand. (U.S.) Tech. Note 270-3 1968. 8. Gunn, S. R. .I. Phys. Chem. 1967, 71, 1386. 9. Tsai, K. R.; Harris, P. M.; Lassettre, E. N. J. Phys. Chem. 1956, 60, 338. 10. Flotow, H. E.; Osborne, D. W. J. Chem. Thermodynamics 1974, 6, 135. 11. JANAF Thermochemical Tables, The Dow Chemical Co., Midland, Mich. 1968. 12. Rossini, F. D. In Experimental Thermochemistry. F. D. Rossini; editor. Interscience: New York. 1956, Chapter 14.
The enthalpy of formation monoxide (Cs20) a
of dicesium
JACK L. SETTLE, GERALD K. JOHNSON, and WARD N. HUBBARD Chemical Engineering Division, Argonne National Argonne, Illinois 60439, U.S.A.
Laboratory,
(Received 12 June 1973) A sample of high-purity CszO(c) was especially prepared for a solution-calorimetric study of its enthalpy of reaction with excess water to form CsOH(aq). The standard enthalpy of formation, AHI”(Cs20,c, 298.15 K), was derived to be -(82.69 f 0.28) kcal,, moF.
1. Introduction A part of the overall program of this laboratory is to obtain those basic thermodynamic data that will aid in the understanding of the chemical interactions that occur in reactor fuels. Cesium, a high yield fission product,“) has been found in postirradiation studiesc2’ of Urania + plutonia fuel pins to be present in high concentrations, along with molybdenum, in the region of the oxide fuel adjacent to the fuel-cladding interface. These observations suggest that the formation of cesium and molybdenum oxides and/or cesium molybdate could be important factors in determining the chemical interactions that occur between irradiated oxide fuel and its cladding. The enthalpies of formation of the molybdenum oxides(3’ and CS,MOO,(~) have been reliably determined. However, for Cs,O the available datat5*@ are discordant. This paper describes the careful preparation of a sample of high-purity Cs,O and the measurement of its enthalpy of reaction with water according to 0,0(c) + H,O(l) = ZCsOH(aq). (1) Combination of the enthalpy of reaction (I), AH,, with the enthalpies of formation of H20(l),(‘) and CsOH(aq), (*) both of which are well known, yields the standard enthalpy of formation of dicesium monoxide, AHF(Cs20, c, 298.15 K).
2. Experimental CALORIMETRIC
SYSTEM
The measurements of the enthalpy of reaction were carried out in an LKB-8700-I Precision Calorimetric System. The glass reaction vessel was modified to accommodate the probe of a quartz crystal thermometer (Hewlett-Packard Model 2801-A) which was used to measure the calorimetric temperatures. The quartz crystal thermometer was compared with a calibrated platinum resistance thermometer at 298.15 K. a This work was performed under the auspices of the U.S. Atomic Energy Commission.
264
J. L. SETTLE,
G. K. JOHNSON,
The mean energy equivalent of the the results of electrical calibrations experiment. The mean temperature reaction measurements was (298.15
AND W. N. HUBBARD
calorimetric system was determined by averaging performed before and after each Cs,O reaction of the calorimetric system for all calibration and + 0.01) K.
MATERIALS
The sample of Cs,O was prepared by incomplete oxidation of high-purity cesium, followed by vacuum distillation to remove the excess cesium. All operations involving the handling of the cesium metal or oxide were conducted in a helium atmosphere glovebox (H20, 1 p.p.m.; 02, 1 p.p.m.) or in sealed containers. The preparative procedure was as follows: 75 g of high-purity (greater than 99.999 mass per cent with respect to metallic impurities) cesium metal (Kawecki Berylco Industries, Inc.) was placed in an outgassed crucible made from recrystallized alumina. The crucible was placed in a Pyrex vessel which was sealed and attached to a manifold. The vessel containing the crucible was thoroughly evacuated. Oxygen, which had been purified by passage through heated copper oxide, Ascarite, magnesium perchlorate, and P,O,, was admitted to a calibrated volume in the manifold, and then very slowly introduced into the Pyrex vessel. The cesium metal was stirred by magnetic induction while the reaction with oxygen was taking place. During the preparation of the oxide, it was considered desirable to maintain an exposed liquid cesium phase in the crucible. By heating the crucible to 473 K, a temperature at which solid Cs,O is unstable, the exposed liquid cesium phase could be maintained for a longer period of time than in an unheated crucible. When about 75 per cent of the stoichiometric amount of oxygen had been introduced, the cesium could no longer be stirred and the oxygen supply was shut off. The system was then evacuated at 473 K to distill the excess cesium which condensed on to the colder surfaces of the Pyrex vessel. When the coating of cesium on the walls of the Pyrex vessel made it impossible to see the sample, the assembly was cooled to room temperature and taken into the glovebox. The sample was removed and ground in an agate mortar. At this point in the preparation, the sample was a scarlet color with flecks of dark green and was probably composed of mostly Cs,O with isolated particles of Cs,O. The condensed cesium was removed from the Pyrex vessel by reaction with butanol, the vessel outgassed at 573 K, the sample reintroduced, and the assembly returned to the manifold. The cleaning of the vessel and grinding of the sample were done five times before all the excess cesium was removed. The temperature at which the vessel was heated was increased after each grinding procedure by approximately 25 K until the sample was finally being evacuated at 593 K. After cesium ceased to condense on the glass surfaces of the vessel heating was continued for an additional three days at 593 K. At this point the preparation, which was a uniform scarlet color, was returned to the glovebox and stored in a sealed container. The yield of CszO was approximately 55 g. Previous experience in handling CszO had shown that the samples underwent a linear increase in mass with time when exposed to even the highIy purified atmosphere of the glovebox. The increase in mass was shown by analysis to be due to reaction with O2 to form Cs,O, and with water to form CsOH. Therefore, in handling the calorimetric sample, it was decided that the following procedures would be necessary:
ENTHALPY
OF FORMATION
OF CszO
265
(1) immediately analyze for C&O, and CsOH; (2) load the ampoules for the calorimetric experiments as rapidly as possible, noting for each ampoule how long the sample had been exposed to the glovebox atmosphere; and (3) repeat the analyses for Cs20, and CsOH. Thus, by interpolation, the amounts of Cs,O, and CsOH in each ampoule could be determined. In addition, spark source mass-spectrometric and X-ray diffraction analyses, as well as chemical analyses for free cesium, carbonate, and total cesium were performed on the preparation. The impurities found in the sample are given in table 1. TABLE
1. Analyses of the Cs,O sample; w is the mass fraction of the species and t the time of exposure to glovebox atmosphere
Cs(free) cog Al Na Si Rb Ca t/h
(70 It 20) n.d. n 6 3 2 2 1 0
H(as OHH(asOHO(as Ok O(as Oz-
at I = 0) at r = 1Oh) at t = 0) at t = 10 h)
0 13 112
10 102w
csao csaoz CsOH Other
99.99 0.00 0.00 0.01
98.72 1.08 0.19 0.01
a n.d. = not detected.
The analysis for peroxide was conducted as follows: a weighed sample of Cs,O was dissolved in water and the solution acidified with concentrated HzS04. A measured excess of Fe’+ was added to the solution and back-titrated with MnO,. The difference between Fe2+ added to the solution and the amount determined by the permanganate titration is a measure of the O”,- content of the solution. The appropriate reactions on which the calculation of peroxide content is based are Cs202(c) + 2H,O = 2CsOH(aq) + H,O,(aq), H202(aq)+2Fe2+(aq)+2H*(aq) = 2Fe3+(aq)+2H20(1).
(2)
(3) The analysis for CsOH in the Cs,O was conducted as follows: a weighed sample of Cs,O was put into a vessel containing calcium-amalgam, and boiled under argon at 633 K. The decomposition of CsOH by calcium-amalgam liberated H, which was collected, along with the argon, in a calibrated vessel. A portion of this gas was then analyzed for H2 by gas chromatography. This method of analysis had been found to be reliable ($- 10 per cent at the CsOH concentration of interest) by performing analyses on small weighed samples of CsOH. The analyses for free cesium and carbonate in the sample were performed on weighed amounts of Cs,O placed in small bottles which were sealed with serum caps.
266
J. L. SETTLE,
G. K. JOHNSON,
AND W. N. HUBBARD
For free cesium, water was introduced into the bottle with a syringe and the evolved hydrogen gas was determined gas chromatographically. For carbonate, excess HCl was introduced into the bottle and the gas phase was analyzed chromatographically. No CO, was found. The method was judged to be capable of detecting 10 p.p.m. by mass of CO;-’ in the sample. The Cs,O was also analyzed for total cesium by taking weighed samples in platinum dishes, dissolving in water, acidifying with HCl, evaporating to dryness, and weighing the CsCl formed. The analytical method had been found to be accurate on samples of the high-purity cesium used in the Cs,O preparation. The samples taken for the total cesium analyses had been exposed to the glovebox atmosphere for approximately 18 h, and the result of the analyses was (94.150.1) mass per cent Cs. The theoretical amount of Cs in pure Cs,O is 94.32 mass per cent. The calculated amount of cesium in the sample after 10 h exposure to the glovebox atmosphere based on the composition given in table 1 is 94.25 mass per cent. Therefore, the analyzed cesium content would be in accord with the estimated sample composition at the time the analyses were performed. The X-ray diffraction pattern of the sample was in excellent agreement with the pattern for Cs,O reported by Tsai et aLc9’ The CsOH solution used as the reaction medium was prepared by dissolving commercial CsOH in boiled distilled water. PROCEDURES
The Cs,O (approximately 0.36 g) was placed into thin-walled Pyrex ampoules which were sealed with silicone-rubber stoppers and wax. The ampoule was placed in the stirrer assembly of the caIorimeter, the reaction vessel was loaded with 99.54cm’ of approximately 0.01 mol dme3 CsOH, and the experiments were performed. The solution of CsOH was used instead of pure water so that any dissolved COZ could be neutralized before the calorimetric measurements. Following several of the experiments, portions of the calorimetric solutions were analyzed for peroxide content, which was found to be only slightly lower than the calculated value based on the Cs,O, impurity. Therefore it was concluded that very little of the unstable peroxide had decomposed before the end of the experiment, and no correction was made for the corresponding decomposition of the hydrogen peroxide in solution.
3. Results The results of six measurements of the enthalpy of reaction of &O(c) with water according to Cs,O(c) +0.74O(CsOH*5555.4H,O)(aq) = 2.74O(CsOH* lSOOH,O)(aq), (4) are presented in table 2. In the table, m”(Cs,O,) and m”‘(CsOH) are the calculated masses of those impurities in the sample at the time the ampoule was sealed; AHVap is the correction for the vaporization of water into the free volume of the ampoule and is based on a density of 4.71 g cm -3(9) for Cs,O and an enthalpy of vaporization of water of 10519 Cal,, mol-‘;(‘) AHcS202 and AHcsOH are the corrections for the
g- I Average =
g mol-l.
AH,
AH,/M(C&O) i
0.28)
= -(291.59 kcal,,
*
0.38279 0.00124 0.00021 0.38134 106.674 1 Ml41 -111.091 -0.012 0.097 0.024 -0.001 -291.03
mol-l
0.47)
results for the reaction (calth = 4.184 J)
-(82.17
0.37636 0.00107 0.00019 0.37510 106.637 1.02676 - 109.491 -0.012 0.084 0.022 0.000 -291.65
2. Calorimetric
to be 281.8102
0.35321 OSKJO76 0.00013 0.35232 106.598 0.96624 - 102.999 -0.012 0.059 0.015 0.004 -292.16
n Standard deviation of the mean. b Uncertainty interval. c The molar mass of Cs;O was taken
IAWMV320);lcalt~
m”(CsOH)/g m’(CsL%
m(samt-W/g m”(CsKh)/g
TABLE
cal h g-l b.C
with
Ha0
L1
0.41090 0.00249 0.00043 0.40798 106.744 1.12401 -119.981 -0.012 0.194 0.049 -0.006 -293.53
of Cs,O
0.36679 0.00224 0.00038 0.36417 106.695 0.99289 - - 105.936 -0.012 0.175 0.043 0.002 - -290.33
0.37191 0.0025 1 0.00043 0.36897 106.616 1.00860 -107.533 -0.012 0.196 0.049 0.001 -290.81
268
J. L. SETTLE,
G. K. JOHNSON,
AND W. N. HUBBARD
reaction of Cs,O, with water according to reaction (2), and the solution of CsOH in 1500H,O, respective1y.f The specific enthalpies of reaction and solution were taken to be -78.08 and - 113.6 Cal,, g-l, respectively. The AZ&,, term is the enthalpy change for adjusting the composition of the final solution to CsOH. 1500Hz0. Except for Cs,Oz and CsOH, the thermal correction for impurities in the sample was negligible. The standard enthalpy of formation of Cs,O(c) according to 2c44 +twd = C%O(C), (5) is obtained by combination of AH4 with the enthalpy changes for the following processes : 0.74O(CsOH~15OOH,O)+3001H,0(1) = 0.740(CsOH*5555.4H,G), (6) 2Cs(c) + 3002H,O(l) = 2(CsOH * 1500Hz0) + H,(g), (7) H,(g) +tWg) = H,W (8) The value AH, = - (0.020f0.005) kcal,, mol- ’ was based on QL for CsOH(aq) as given by Gunn, *(a) AH, = -(96.564$0.022) kcal,, mol-r was derived from the measurements of Gunn;(*) and AH*, the enthalpy of formation of water, was taken tobe -(68.315+0.015) kcal,, mol- ‘.(‘) The value obtained for AH,“(Cs,O, c, 298.15 K) is - (82.69 kO.28) kcal,, mol-‘. Values for the standard entropy of formation, AS,“(Cs,O, c, 298.15 K) = - 30.10 Cal,, K-’ mol- ‘, and standard Gibbs energy of formation, AG,“(Cs,O, c, 298.15 K) = - (73.72 kO.28) kcal,, mol-’ were derived based on the AHf” result and values of S”(298.15 K) of 35.10, 20.35, and 49.00 Cal,, K-r mol-r for Cs,O,(“) Cs,(‘l) and 02,(‘) respectively. The uncertainties given in the derived results are uncertainty interval@ equal to twice the combined standard deviations arising from all known sources. 4. Discussion Previous studies of the enthalpy of reaction of Cs,O with excess water by Beketo@ and Rengade@’ yielded for AH, values of - 72.2 kcalth mol-’ and - 83.2 kcal,, mol- l, respectively. Our result, AH, = -(82.17+0.28) kcal,, mol-‘, is much closer to the result obtained by Rengade than to that obtained by Beketov, who, according to Rengade,@) used a very impure sample. We believe that the result reported herein for AHF(CszO, c), - (82.69 f0.28) kcal, mol-I, is the most reliable value reported to date because of the care taken in the preparation, handling, and analysis of the sample. The authors gratefully acknowledge K. J. Jensen, R. V. Schablaske, and M. I, Homa for the performance of analyses, D. V. Steidl for advice on the preparation of the CszO sample, and A. D. Tevebaugh for administrative assistance. REFERENCES 1. Lisman, F. L.; Abernathey, R. M. ; Maeck, W. J. ; Rein, J. E. Nucl. Sci. Eng. 1970, 42, 191. 2. Johnson, C. E.; Seils, C. A.; Anderson, K. E. U.S. At. Energy Comm. Rep. No. ANL-7115 1971, p. 92. 7 Throughout
this paper caltb = 4.184 J.
ENTHALPY
OF
FORMATION
OF
CsaO
269
3. Wagman, D. D.; Evans, W. H. ; Parker, V. B.; Halow, I. ; Bailey, S. M. ; Schumm, R. H. Nut. Bur. Stand. (U.S.) Tech. Note 270-4 1%9. 4. O’Hare, P. A. G.; Hoekstra, H. R. J. Chem. Thermodynamics 1973, 5, 851. 5. Beketov, N. Bull. Acad. Sci. Russ. 1894, 35, 541. 6. Rengade, E. Ann. Chim. Phys. 1908, 14, 540. 7. Wagman, D. D.; Evans, W. H.; Parker, V. B.; Halow, 1.; Bailey, S. M.; Schumm, R. H. Nat. Bur. Stand. (U.S.) Tech. Note 270-3 1968. 8. Gunn, S. R. .I. Phys. Chem. 1967, 71, 1386. 9. Tsai, K. R.; Harris, P. M.; Lassettre, E. N. J. Phys. Chem. 1956, 60, 338. 10. Flotow, H. E.; Osborne, D. W. J. Chem. Thermodynamics 1974, 6, 135. 11. JANAF Thermochemical Tables, The Dow Chemical Co., Midland, Mich. 1968. 12. Rossini, F. D. In Experimental Thermochemistry. F. D. Rossini; editor. Interscience: New York. 1956, Chapter 14.