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The heat capacity of dicesium monoxide (Cs2O) from 5 to 350 K and the Gibbs energy of formation to 763 K
J. Chem. Thermodynamics 1974, 6, 135-140
The heat capacity of dicesium monoxide (CsnO) from 5 to 350 K and the Gibbs energy of formation to 763 K” HOWARD
E. FLOTOW
and DARRELL
W. OSBORNE
Chemical Physics of Energy Systems Section, Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439, U.S.A. (Received 23 April
1973)
The heat capacity of dicesium monoxide, CsaO, was measured by adiabatic calorimetry from 5 to 350 K. The thermodynamic results at 298.15 K were as follows: Ci(298.15 K) = (76.00 & 0.23) J K-l mol-I; S"(298.15 K) = (146.87 & 0.44) J K-’ mol-I; (H"(298.15 K) - H”(O)] = (17678 * 53) J mol-I; {G”(298.15 K) - H”(O)f/298.15 K = - (87.58 & 0.29) J K-’ mol-‘. The standard Gibbs energy of formation, AC;, of CsaO was derived from experimental results to be AGr”(Cs,O, c, 298.15 K) = - (308.42 f 1.18) kJ mol-I. To permit practical thermodynamic calculations to be made at higher temperatures, the heat capacities were extrapolated to 763 K, the melting temperature of CsaO, and a table of extrapolated thermodynamic functions, including AC;(T) up to T = 763 K, is presented.
1. Introduction Experimental heat capacities and enthalpies are available for Li,0,(r9 ‘) and Na,0,(3-5) but no such data can be found in the literature for KzO, RbzO, and Cs,O. Precise thermodynamic data for Cs,O are of special interest to the Liquid Metal Fast Breeder Reactor program of the U.S. Atomic Energy Commission since cesium is a major fission product which may interact chemically with the uranium + plutonium oxide fuel and also with the fuel cladding alloy. This paper presents the measurement of the heat capacity of Cs,O from 5 to 350 K and some derived thermodynamic quantities, including the Gibbs energy of formation, AG,“, at 298.15 K. The heat capacity of Cs,O was extrapolated to 763 K, and calculated values of AGf” are given at selected temperatures up to 763 K. 2. Sample The Cs,O was prepared by the direct reaction of purified oxygen with cesium metal. The reaction was stopped after about 75 to 80 per cent of the stoichiometric amount of oxygen had reacted, and then the excess cesium metal was removed by distillation. The sample used for the measurements reported here was a part of a sample that was prepared to measure the enthalpy of the reaction of Cs,O with water.‘@ Immediately after preparation the sample was found by analysis(6’ to be 99.99 moles per cent Cs,O. a Work performed under the auspices of the U.S. Atomic Energy Commission,
136
H.
E. FLOTOW
AND
D.
W. OSBORNE
However, subsequent handling of the sample in a helium-filled glove box (H,O and O2 each 1 p.p.m. by mass) resulted in a significant amount of reaction of Cs,O with oxygen and water. Our sample was exposed to the glovebox atmosphere for a total of 10 h, and extensive analyses (6) showed the composition of this material to be 98.72 mass per cent Cs,O, 1.08 mass per cent CsZ02, and 0.19 mass per cent CsOH.
3. Heat capacity measurements CALORIMETER
The gold-plated OFHC copper calorimeter had a mass of 22.3 g and an internal volume of 5.98 cm3. A vacuum-tight seal was made by forcing a gold gasket down on a stainless steel knife edge by means of a threaded cover as described in a previous publication.(‘) The configuration of the calorimeter also included a re-entrant well for the containment of a bifilarly wound Evanohm heater and an encapsulated platinum resistance thermometer. A separate series of measurements was made to determine the heat capacity of the unloaded calorimeter. The Cs,O sample was prepared for loading into the calorimeter by containing and compressing the sample in two thin-walled (0.007 cm) OFHC copper capsules.(*) These capsules had a central hole sized to fit over the re-entrant well of the calorimeter. The mass of the sample was 22.1999 g and the mass of the copper and of the 50/50 lead + tin solder in the capsules was 2.5681 g and 0.0343 g, respectively. The sealed calorimeter also contained 0.0047 g of Apiezon T grease between the inner hole of the capsules and the wall of the re-entrant well and 1.841 x 10P6 mol of helium to promote rapid thermal equilibrium. Corrections were made for the copper in the capsules, (9, lo) for the SO/SO lead + tin solder, (*1,12) for the Apiezon T grease,(13’ and for the helium. The uncertainty caused by these corrections was less than 0.05 per cent of the heat capacity of the unloaded calorimeter. Also, a small correction for the finite temperature increments in each heat capacity determination, equal to - (d2CP/dT2)(AT)2/24, was applied. The heat capacity of the sample constituted 89 per cent of the total near 10 K, decreased to 67 per cent near 30 K, further decreased to 36 per cent at 100 K, and then fell gradually to 31 per cent at 350 K. APPARATUS
AND
METHOD
A rather complete description of the adiabatic-type calorimetric cryostat has been published.(r4) The method of operation, including automated data read-out equipment, details of the platinum temperature scale, and a listing of other associated equipment, has been given in a recent paper.‘15’ RESULTS
The experimental heat capacities are temperatures. For measurements with lower the AT for each result was about the AT was about 10 per cent of (T), 10 K. No correction was made for the
presented in table 1 in order of ascending a mean temperature (T) equal to 20 K or 2 K, for (T) above 20 K and less than 100 K and for (T) above 100 K the AT was about impurities in the sample. The heat capacities
HEAT TABLE
1. Experimental
K 5.58 5.73 7.25 7.58 9.39 9.63 11.70 11.76 13.75 14.06 15.82 16.24 17.87 18.37 19.83 20.55 21.86 22.74 24.93 27.14
G J K-l
137
OF CsaO
heat capacity Ci of dice&m monoxide Cs,O at constant pressure; M(Cs20) = 281.810 g mol-l
CT>
mol-1
1.130 1.239 2.399 2.648 4.205 4.446 6.612 6.672 9.070 9.447 11.69 12.22 14.27 14.89 16.65 17.50 19.04 20.00 22.44 24.79
CAPACITY
K 29.68 32.75 36.13 39.78 43.75 43.82 48.04 53.77 60.17 66.04 72.22 79.16 88.19 98.84 109.35 110.57 120.83 131.32 141.66 151.63
CT>
G .~ -
J
K-l mol-1 __-~-..--__ 27.33 30.11 32.88 35.57 38.26 38.30 40.84 44.00 47.10 49.63 52.01 54.41 57.17 59.66 61.82 62.09 63.98 65.59 66.97 67.85
K 161.56 171.49 181.44 191.42 201.39 211.32 221.23 231.16 241.14 251.18 261.00 271.12 281.25 291.34 301.38 311.35 321.27 331.16 341.06 348.04
c: J
K-’
molbl -~-
69.09 69.87 70.76 71.48 72.05 72.63 73.02 73.52 73.98 74.43 74.66 75.27 75.44 75.77 76.05 76.57 76.55 76.98 77.42 77.87
in table 1 have an estimated probable error of 5 per cent at 5 K, 1 per cent at 15 K, and 0.3 per cent at 20 K and higher temperatures. No unusual variation of the heat capacity as a function of temperature was evident. 4. Thermodynamic
properties
to 350 K
The heat capacity results in table 1 were fitted by the method of weighted least squares by two overlapping polynomials in T. The standard deviation of an individual result from the polynomial fitted to the data below 36.13 K was 0.24 per cent, and that from the polynomial fitted to the data from 20.55 K to 348.04 K was 0.11 per cent. The data between 250 and 348.04 K were also fitted within experimental error by the equation : C;/J K-’ moi-’ = 66.024+0.033461(T/K), (1) with a standard deviation of 0.18 per cent. Table 2 gives the heat capacity at constant pressure, C;(T), the entropy S"(T), the enthalpy {H"(T)-N"(O)}, and the function (G"(T)-H"(O)}JT at selected temperatures. Up to 285 K these entries were calculated from the two polynomials and the appropriate thermodynamic relations. Above 285 K equation (1) was used. The results at 5 K shown in parentheses in table 2 were calculated with the assumption that Ci varies as T3 below 5 K. No allowance has been made for the effect of isotopic mixing or for nuclear spin and consequently the values in table 2 are the ones to be used in practical thermodynamic calculations. 10
138
H. TABLE
G(T) J K-l mol-1
5 10
0.820 4.822 10.61 16.89 22.47 27.61 31.99 35.75 39.03 41.95 47.01 51.23 54.70 57.55 59.94 62.00 63.81 65.37 66.70 67.84 68.85 69.77 70.61 71.37 72.00 72.54 73.00 73.45 73.90 74.36 74.77 75.11 75.41 75.73 76.06 76.40 76.73 77.07 77.40 77.74 75.21 76.00 f0.23
1: 110 120 130 140 150 160 170 180 E 210 220 230 240 250 E 280 290 300 310 320 330 340 350 273.15 298.15
AND
D.
W.
OSBORNE
2. Thermodynamic functions for dicesium monoxide to 350 K; M(Cs20) = 281.810 g mol-’
T ii
ii 25 30 35 40 45 50 60 70 80
STANDARD
E. FLOTOW
ENTROPY,
S”(T) J K-’ mol-l
The standard entropy of formation
AND
-{G”(T)-W(O))/T J K-l mol-L
W(T)--H”(O)} J mol-’
(0.273) 1.947 4.947 8.871 13.25 17.81 22.41 26.93 31.33 35.60 43.71 51.28 58.36 64.97 71.16 76.97 82.45 87.62 92.51 97.15 101.56 105.77 109.78 113.62 117.30 120.82 124.21 127.46 130.60 133.62 136.55 139.38 142.11 144.76 147.34 149.84 152.27 154.63 156.94 159.19 140.25 146.87 *0.44
ENTHALPY,
Cs,O(c)
(0.068) 0.522 1.466 2.812 4.457 6.301 8.274 10.32 12.42 14.52 18.72 22.84 26.84 30.71 34.45 38.05 41.53 44.87 48.11 51.22 54.23 57.14 59.95 62.68 65.32 67.88 70.36 72.17 75.12 77.40 79.62 81.78 83.88 85.94 87.94 89.90 91.81 93.68 95.51 97.29 82.45 87.58 *0.29
(1.025) 14.25 52.21 121.2 219.8 345.3 494.6 664.2 851.3 1054 1500 1991 2522 3083 3611 4281 4910 5556 6217 6889 7573 8266 8968 9678 10395 11118 11846 12578 13315 14056 14802 15551 16304 17060 17819 18581 19346 20115 20888 21663 15788 17678 &53
GIBBS
ENERGY
OF
FORMATION
of Cs,O at 298.15 K was calculated to be
AS,“(Cs,O, c, 298.15 K) = -(125.94&-0.61)
J K-l
mol-‘.
The entropy of Cs,O was taken as (146.87+0&I) J K-’ mol-’ (see table 2), the entropy of O2 was taken as (205.033 ~0.04) J K-l mol- 1,(16) and the entropy of
HEAT
CAPACITY
139
OF CsaO
Cs was taken as (85.148kO.21) J K-’ mol- i .(I’) The standard enthalpy of formation at 298.15 K was determined by Settle, Johnson, and Hubbard’@ to be A,H,“(Cs,O, c, 298.15 K) = -(345.97+ 1.17) kJ mol-‘, and this result combined with the standard entropy of formation gives the standard Gibbs energy of formation at 298.15 K : AG,“(Cs,O, c, 298.15 K) = -(308.42+ 1.18) kJ mol-‘.
5. Extrapolation to 763 K Values of the thermodynamic properties of Cs,O, especially the standard Gibbs energy of formation, are needed for practical calculations at temperatures above 350 K. However, no experimental heat capacities or enthalpies for Cs,O above 350 K have been reported. Rengade (1**19) has determined that the melting temperature of Cs,O is about 763 K under N,, * under vacuum conditions Cs,O decomposed to Cs and CsO,. It therefore seemed appropriate to estimate the thermodynamic properties of Cs,O up to 763 K. As mentioned earlier our heat capacities between 250 and 350 K were fitted by equation (l), which is a linear function of T. Fredrickson and Chasanov”) found that the heat capacity of Na,O between 298 and 900 K was a linear function of T. Consequently, it was judged reasonable to use equation (1) in estimating the thermodynamic properties of Cs,O above 298.15 K. The results are given in table 3 at TABLE T ti 298.15 400 500 600 700 763
3. Thermodynamic properties of dicesium monoxide &O(c) above 350 K; M(Cs20) = 281.810 g mol-1
‘G(T) --.-__ J K-l mol-’ 76.00 79.4 82.8 86.1 89.4 91.6
wo J K-l mol-1 146.87 169.7 187.8 203.1 216.7 224.5
Q’W-)--H”(O)1 kJ mol-1
extrapolated
-PW’-F--HWJIT J K-l mol-1
17.678 25.59 33.70 42.14 50.92 56.62
87.58 105.7 120.4 132.9 143.9 150.3
AGXT) ____ kJ mol-’ -308.42 -294.2 -280.2 -266.3 -252.6 -244.0
selected temperatures to 763 K. The thermodynamic properties of 0, and of Cs used in the calculation of AG,“(T) were taken from the JANAF Thermochemical Tables.“6, “) The estimated uncertainty of AG,“(T) is 1.18 kJ mol-’ at 298.15 and rises to about 3 kJ mol-’ at 700 K. We thank J. L. Settle for preparation and analysis of the sample. We also acknowledge helpful discussions with G. K. Johnson, W. N. Hubbard, and Irving Johnson. REFERENCES 1. Johnston, H. L.; Batter, T. W. J. Amer. Chem. Sot. 1951, 73, 1119. 2. Shomate, C. H.; Cohen, A. J. J. Amer. Chem. Sot. 1955, 77, 285. 3. Furukawa, G. Private communication.
H. E. FLOTOW
140 4. 5. 6. 7. 8. 9. 10.
AND D. W. OSBORNE
Grimley, R. T.; Margrave, J. L. J. Phys. Chem. 1960, 64, 1763. Fredrickson, D. R.; Chasanov, M. G. J. Chem. Thermodynamics 1973, 5, 485. Settle, J. L.; Johnson, G. K.; Hubbard, W. N. J. Chem. Thermodynamics in the press. Flotow, H. E.; Klocek, E. E. Rev. Sci. Instrum. 1968, 39, 1578. Flotow, H. E.; Osborne, D. W. Rev. Sci. Znstrum. 1966, 37, 1414. Osborne, D. W.; Flotow, H. E.; Schreiner, F. Rev. Sri. Znstram. 1967, 38, 159. Furukawa, G. T.; Saba, W. G.; Reilly, M. L. Nat. Stand. Ref. Data Ser., Nat. Bur. Stand. 1968,
18.
11. Ziegler, W. T. ; Mullins, J. C. Cryogenics 1964, 4, 39. 12. De Nobel, J.; Du Chatenier, F. J. Physica (Utrecht) 1963, 29, 1231. These results, which cover the range 1.5 to 20 K, were all decreased by 4.3 per cent to bring them into agreement with the results in reference 11 at 20 K. 13. Westrum, E. F., Jr.; Chou, C.; Osborne, D. W.; Flotow, H. E. Cryogenics 1967, 7, 43. 14. Westrum, E. F., Jr.; Hatcher, J. B.; Osborne, D. W. J. Chem. Phys. 1953,21,419. 15. Osborne, D. W.; Schreiner, F.; Flotow, H. E.; Malm, J. G. J. Chem. Phys. 1972, 57, 3401. 16. JANAF Thermochemical Tables. The Dow Chemical Company: Midland, Mich. Sept. 30,1965. 17. JANAF ThermochemicaE Tables. The Dow Chemical Company: Midland, Mich. June 30, 1968. 18. Rengade, E. Bull. Sot. chim. France 1909, 5, 994. 19. Rengade, E. C. R. H. Acad. Sci. 1909, 148, 1199.
The heat capacity of dicesium monoxide (CsnO) from 5 to 350 K and the Gibbs energy of formation to 763 K” HOWARD
E. FLOTOW
and DARRELL
W. OSBORNE
Chemical Physics of Energy Systems Section, Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439, U.S.A. (Received 23 April
1973)
The heat capacity of dicesium monoxide, CsaO, was measured by adiabatic calorimetry from 5 to 350 K. The thermodynamic results at 298.15 K were as follows: Ci(298.15 K) = (76.00 & 0.23) J K-l mol-I; S"(298.15 K) = (146.87 & 0.44) J K-’ mol-I; (H"(298.15 K) - H”(O)] = (17678 * 53) J mol-I; {G”(298.15 K) - H”(O)f/298.15 K = - (87.58 & 0.29) J K-’ mol-‘. The standard Gibbs energy of formation, AC;, of CsaO was derived from experimental results to be AGr”(Cs,O, c, 298.15 K) = - (308.42 f 1.18) kJ mol-I. To permit practical thermodynamic calculations to be made at higher temperatures, the heat capacities were extrapolated to 763 K, the melting temperature of CsaO, and a table of extrapolated thermodynamic functions, including AC;(T) up to T = 763 K, is presented.
1. Introduction Experimental heat capacities and enthalpies are available for Li,0,(r9 ‘) and Na,0,(3-5) but no such data can be found in the literature for KzO, RbzO, and Cs,O. Precise thermodynamic data for Cs,O are of special interest to the Liquid Metal Fast Breeder Reactor program of the U.S. Atomic Energy Commission since cesium is a major fission product which may interact chemically with the uranium + plutonium oxide fuel and also with the fuel cladding alloy. This paper presents the measurement of the heat capacity of Cs,O from 5 to 350 K and some derived thermodynamic quantities, including the Gibbs energy of formation, AG,“, at 298.15 K. The heat capacity of Cs,O was extrapolated to 763 K, and calculated values of AGf” are given at selected temperatures up to 763 K. 2. Sample The Cs,O was prepared by the direct reaction of purified oxygen with cesium metal. The reaction was stopped after about 75 to 80 per cent of the stoichiometric amount of oxygen had reacted, and then the excess cesium metal was removed by distillation. The sample used for the measurements reported here was a part of a sample that was prepared to measure the enthalpy of the reaction of Cs,O with water.‘@ Immediately after preparation the sample was found by analysis(6’ to be 99.99 moles per cent Cs,O. a Work performed under the auspices of the U.S. Atomic Energy Commission,
136
H.
E. FLOTOW
AND
D.
W. OSBORNE
However, subsequent handling of the sample in a helium-filled glove box (H,O and O2 each 1 p.p.m. by mass) resulted in a significant amount of reaction of Cs,O with oxygen and water. Our sample was exposed to the glovebox atmosphere for a total of 10 h, and extensive analyses (6) showed the composition of this material to be 98.72 mass per cent Cs,O, 1.08 mass per cent CsZ02, and 0.19 mass per cent CsOH.
3. Heat capacity measurements CALORIMETER
The gold-plated OFHC copper calorimeter had a mass of 22.3 g and an internal volume of 5.98 cm3. A vacuum-tight seal was made by forcing a gold gasket down on a stainless steel knife edge by means of a threaded cover as described in a previous publication.(‘) The configuration of the calorimeter also included a re-entrant well for the containment of a bifilarly wound Evanohm heater and an encapsulated platinum resistance thermometer. A separate series of measurements was made to determine the heat capacity of the unloaded calorimeter. The Cs,O sample was prepared for loading into the calorimeter by containing and compressing the sample in two thin-walled (0.007 cm) OFHC copper capsules.(*) These capsules had a central hole sized to fit over the re-entrant well of the calorimeter. The mass of the sample was 22.1999 g and the mass of the copper and of the 50/50 lead + tin solder in the capsules was 2.5681 g and 0.0343 g, respectively. The sealed calorimeter also contained 0.0047 g of Apiezon T grease between the inner hole of the capsules and the wall of the re-entrant well and 1.841 x 10P6 mol of helium to promote rapid thermal equilibrium. Corrections were made for the copper in the capsules, (9, lo) for the SO/SO lead + tin solder, (*1,12) for the Apiezon T grease,(13’ and for the helium. The uncertainty caused by these corrections was less than 0.05 per cent of the heat capacity of the unloaded calorimeter. Also, a small correction for the finite temperature increments in each heat capacity determination, equal to - (d2CP/dT2)(AT)2/24, was applied. The heat capacity of the sample constituted 89 per cent of the total near 10 K, decreased to 67 per cent near 30 K, further decreased to 36 per cent at 100 K, and then fell gradually to 31 per cent at 350 K. APPARATUS
AND
METHOD
A rather complete description of the adiabatic-type calorimetric cryostat has been published.(r4) The method of operation, including automated data read-out equipment, details of the platinum temperature scale, and a listing of other associated equipment, has been given in a recent paper.‘15’ RESULTS
The experimental heat capacities are temperatures. For measurements with lower the AT for each result was about the AT was about 10 per cent of (T), 10 K. No correction was made for the
presented in table 1 in order of ascending a mean temperature (T) equal to 20 K or 2 K, for (T) above 20 K and less than 100 K and for (T) above 100 K the AT was about impurities in the sample. The heat capacities
HEAT TABLE
1. Experimental
G J K-l
137
OF CsaO
heat capacity Ci of dice&m monoxide Cs,O at constant pressure; M(Cs20) = 281.810 g mol-l
CT>
mol-1
1.130 1.239 2.399 2.648 4.205 4.446 6.612 6.672 9.070 9.447 11.69 12.22 14.27 14.89 16.65 17.50 19.04 20.00 22.44 24.79
CAPACITY
K 29.68 32.75 36.13 39.78 43.75 43.82 48.04 53.77 60.17 66.04 72.22 79.16 88.19 98.84 109.35 110.57 120.83 131.32 141.66 151.63
CT>
G .~ -
J
K-l mol-1 __-~-..--__ 27.33 30.11 32.88 35.57 38.26 38.30 40.84 44.00 47.10 49.63 52.01 54.41 57.17 59.66 61.82 62.09 63.98 65.59 66.97 67.85
K 161.56 171.49 181.44 191.42 201.39 211.32 221.23 231.16 241.14 251.18 261.00 271.12 281.25 291.34 301.38 311.35 321.27 331.16 341.06 348.04
c: J
K-’
molbl -~-
69.09 69.87 70.76 71.48 72.05 72.63 73.02 73.52 73.98 74.43 74.66 75.27 75.44 75.77 76.05 76.57 76.55 76.98 77.42 77.87
in table 1 have an estimated probable error of 5 per cent at 5 K, 1 per cent at 15 K, and 0.3 per cent at 20 K and higher temperatures. No unusual variation of the heat capacity as a function of temperature was evident. 4. Thermodynamic
properties
to 350 K
The heat capacity results in table 1 were fitted by the method of weighted least squares by two overlapping polynomials in T. The standard deviation of an individual result from the polynomial fitted to the data below 36.13 K was 0.24 per cent, and that from the polynomial fitted to the data from 20.55 K to 348.04 K was 0.11 per cent. The data between 250 and 348.04 K were also fitted within experimental error by the equation : C;/J K-’ moi-’ = 66.024+0.033461(T/K), (1) with a standard deviation of 0.18 per cent. Table 2 gives the heat capacity at constant pressure, C;(T), the entropy S"(T), the enthalpy {H"(T)-N"(O)}, and the function (G"(T)-H"(O)}JT at selected temperatures. Up to 285 K these entries were calculated from the two polynomials and the appropriate thermodynamic relations. Above 285 K equation (1) was used. The results at 5 K shown in parentheses in table 2 were calculated with the assumption that Ci varies as T3 below 5 K. No allowance has been made for the effect of isotopic mixing or for nuclear spin and consequently the values in table 2 are the ones to be used in practical thermodynamic calculations. 10
138
H. TABLE
G(T) J K-l mol-1
5 10
0.820 4.822 10.61 16.89 22.47 27.61 31.99 35.75 39.03 41.95 47.01 51.23 54.70 57.55 59.94 62.00 63.81 65.37 66.70 67.84 68.85 69.77 70.61 71.37 72.00 72.54 73.00 73.45 73.90 74.36 74.77 75.11 75.41 75.73 76.06 76.40 76.73 77.07 77.40 77.74 75.21 76.00 f0.23
1: 110 120 130 140 150 160 170 180 E 210 220 230 240 250 E 280 290 300 310 320 330 340 350 273.15 298.15
AND
D.
W.
OSBORNE
2. Thermodynamic functions for dicesium monoxide to 350 K; M(Cs20) = 281.810 g mol-’
T ii
ii 25 30 35 40 45 50 60 70 80
STANDARD
E. FLOTOW
ENTROPY,
S”(T) J K-’ mol-l
The standard entropy of formation
AND
-{G”(T)-W(O))/T J K-l mol-L
W(T)--H”(O)} J mol-’
(0.273) 1.947 4.947 8.871 13.25 17.81 22.41 26.93 31.33 35.60 43.71 51.28 58.36 64.97 71.16 76.97 82.45 87.62 92.51 97.15 101.56 105.77 109.78 113.62 117.30 120.82 124.21 127.46 130.60 133.62 136.55 139.38 142.11 144.76 147.34 149.84 152.27 154.63 156.94 159.19 140.25 146.87 *0.44
ENTHALPY,
Cs,O(c)
(0.068) 0.522 1.466 2.812 4.457 6.301 8.274 10.32 12.42 14.52 18.72 22.84 26.84 30.71 34.45 38.05 41.53 44.87 48.11 51.22 54.23 57.14 59.95 62.68 65.32 67.88 70.36 72.17 75.12 77.40 79.62 81.78 83.88 85.94 87.94 89.90 91.81 93.68 95.51 97.29 82.45 87.58 *0.29
(1.025) 14.25 52.21 121.2 219.8 345.3 494.6 664.2 851.3 1054 1500 1991 2522 3083 3611 4281 4910 5556 6217 6889 7573 8266 8968 9678 10395 11118 11846 12578 13315 14056 14802 15551 16304 17060 17819 18581 19346 20115 20888 21663 15788 17678 &53
GIBBS
ENERGY
OF
FORMATION
of Cs,O at 298.15 K was calculated to be
AS,“(Cs,O, c, 298.15 K) = -(125.94&-0.61)
J K-l
mol-‘.
The entropy of Cs,O was taken as (146.87+0&I) J K-’ mol-’ (see table 2), the entropy of O2 was taken as (205.033 ~0.04) J K-l mol- 1,(16) and the entropy of
HEAT
CAPACITY
139
OF CsaO
Cs was taken as (85.148kO.21) J K-’ mol- i .(I’) The standard enthalpy of formation at 298.15 K was determined by Settle, Johnson, and Hubbard’@ to be A,H,“(Cs,O, c, 298.15 K) = -(345.97+ 1.17) kJ mol-‘, and this result combined with the standard entropy of formation gives the standard Gibbs energy of formation at 298.15 K : AG,“(Cs,O, c, 298.15 K) = -(308.42+ 1.18) kJ mol-‘.
5. Extrapolation to 763 K Values of the thermodynamic properties of Cs,O, especially the standard Gibbs energy of formation, are needed for practical calculations at temperatures above 350 K. However, no experimental heat capacities or enthalpies for Cs,O above 350 K have been reported. Rengade (1**19) has determined that the melting temperature of Cs,O is about 763 K under N,, * under vacuum conditions Cs,O decomposed to Cs and CsO,. It therefore seemed appropriate to estimate the thermodynamic properties of Cs,O up to 763 K. As mentioned earlier our heat capacities between 250 and 350 K were fitted by equation (l), which is a linear function of T. Fredrickson and Chasanov”) found that the heat capacity of Na,O between 298 and 900 K was a linear function of T. Consequently, it was judged reasonable to use equation (1) in estimating the thermodynamic properties of Cs,O above 298.15 K. The results are given in table 3 at TABLE T ti 298.15 400 500 600 700 763
3. Thermodynamic properties of dicesium monoxide &O(c) above 350 K; M(Cs20) = 281.810 g mol-1
‘G(T) --.-__ J K-l mol-’ 76.00 79.4 82.8 86.1 89.4 91.6
wo J K-l mol-1 146.87 169.7 187.8 203.1 216.7 224.5
Q’W-)--H”(O)1 kJ mol-1
extrapolated
-PW’-F--HWJIT J K-l mol-1
17.678 25.59 33.70 42.14 50.92 56.62
87.58 105.7 120.4 132.9 143.9 150.3
AGXT) ____ kJ mol-’ -308.42 -294.2 -280.2 -266.3 -252.6 -244.0
selected temperatures to 763 K. The thermodynamic properties of 0, and of Cs used in the calculation of AG,“(T) were taken from the JANAF Thermochemical Tables.“6, “) The estimated uncertainty of AG,“(T) is 1.18 kJ mol-’ at 298.15 and rises to about 3 kJ mol-’ at 700 K. We thank J. L. Settle for preparation and analysis of the sample. We also acknowledge helpful discussions with G. K. Johnson, W. N. Hubbard, and Irving Johnson. REFERENCES 1. Johnston, H. L.; Batter, T. W. J. Amer. Chem. Sot. 1951, 73, 1119. 2. Shomate, C. H.; Cohen, A. J. J. Amer. Chem. Sot. 1955, 77, 285. 3. Furukawa, G. Private communication.
H. E. FLOTOW
140 4. 5. 6. 7. 8. 9. 10.
AND D. W. OSBORNE
Grimley, R. T.; Margrave, J. L. J. Phys. Chem. 1960, 64, 1763. Fredrickson, D. R.; Chasanov, M. G. J. Chem. Thermodynamics 1973, 5, 485. Settle, J. L.; Johnson, G. K.; Hubbard, W. N. J. Chem. Thermodynamics in the press. Flotow, H. E.; Klocek, E. E. Rev. Sci. Instrum. 1968, 39, 1578. Flotow, H. E.; Osborne, D. W. Rev. Sci. Znstrum. 1966, 37, 1414. Osborne, D. W.; Flotow, H. E.; Schreiner, F. Rev. Sri. Znstram. 1967, 38, 159. Furukawa, G. T.; Saba, W. G.; Reilly, M. L. Nat. Stand. Ref. Data Ser., Nat. Bur. Stand. 1968,
18.
11. Ziegler, W. T. ; Mullins, J. C. Cryogenics 1964, 4, 39. 12. De Nobel, J.; Du Chatenier, F. J. Physica (Utrecht) 1963, 29, 1231. These results, which cover the range 1.5 to 20 K, were all decreased by 4.3 per cent to bring them into agreement with the results in reference 11 at 20 K. 13. Westrum, E. F., Jr.; Chou, C.; Osborne, D. W.; Flotow, H. E. Cryogenics 1967, 7, 43. 14. Westrum, E. F., Jr.; Hatcher, J. B.; Osborne, D. W. J. Chem. Phys. 1953,21,419. 15. Osborne, D. W.; Schreiner, F.; Flotow, H. E.; Malm, J. G. J. Chem. Phys. 1972, 57, 3401. 16. JANAF Thermochemical Tables. The Dow Chemical Company: Midland, Mich. Sept. 30,1965. 17. JANAF ThermochemicaE Tables. The Dow Chemical Company: Midland, Mich. June 30, 1968. 18. Rengade, E. Bull. Sot. chim. France 1909, 5, 994. 19. Rengade, E. C. R. H. Acad. Sci. 1909, 148, 1199.