YOICHITAKAHASHI AND EDGAR F. WESTRUM, JR.
3618
the spectral value. Heats of solution in water have been measured for 9-methyladenine and l-methylthymine'; a heat of solution for adenine can also be calculated from the data of T'so, et ala8 These values (Table 11)lead to a large negative (--20 kcal./ for the process: purine Or pyrimidine in gas phase --t purine or pyrimidine in aqueous solution.
Knowledge of these numbers is necessary to an understanding of the forces which determine the stability of nucleic acids in s o l ~ t i o n . ~ (7) s. J. Gill, D. B. Martin, and M. Downing, J. Am. Chem. SOC., 85, 706 (1963). (8) P. 0. P. T'so, I. S. Melvin, and A. C. Olson, ibid., 85,1289 (1963). (9) H. DeVoe and I. ~ i n o c oJr., , J. M ~ Z~. i ~ 4,500 z . , (1962).
Uranium Monoselenide. Heat Capacity and Thermodynamic Properties from 5 to 350°K.l*
by Yoichi Takahashi and Edgar F. Westrum, Jr.lb Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48104
(Received M a y 80, 1966)
The low temperature heat capacity of Use was determined by adiabatic calorimetry and found to have a normal sigmoid temperature dependence except near the 160.5'K. transition from the ferromagnetic t o the paramagnetic state. The heat capacity (Cp), entropy ( S O ) , enthalpy function ([HO - H0o]/T),and Gibbs energy function (- [ G O - Hoo]/T) at 298.15'K. in cal./(g.f.m. OK.) are 13.10, 23.07, 10.39, and 12.68, respectively.
Introduction The presence of several phases, such as Use, UaSe4, U2Ses,U3Ses, a-,p-, and y-USe2, and USe3, has been recognized in. the uranium-selenium system,2 but few thermodynamic properties have been reported. Recent investigations of magnetic3 and electrical4 properties have shown uranium monoselenide (Use) t o be of particular interest because of its ferromagnetism below 200°K. and low electrical resistivity and high thermoelectric power at room temperature. Its thermal properties are of interest also in comparison with those of other uranium compounds such as US, UC, UN, and UP, all of which also possess the NaCl structure.
Experimental Section Preparation and Characterization of the Sample. The sample of uranium monoselenide was prepared at the Battelle Memorial Institute4by allowing uranium turnings (containing 0.01 wt. yospectrographically detected The Journal of Physical Chemistry
impurities) t o react with the vapor of rectifier grade (99.999% pure) selenium in evacuated, sealed quartz capsules. The resulting finely divided selenide preparations were consolidated, melted, and homogenized to monoselenide under an argon at'mosphere at about 2000" in a tantalum crucible. Determination of uranium and selenium gave values within =t0.295 of the stoichiometric ratio. Tantalum WELSnot detected by spectrochemical analysis sensitive to O.OOl%, and oxygen contamination was less than 0.1% by weight. (1) (a) This research was supported in part by the United States Atomic Energy Commission and by the Selenium-Tellurium Development Association, Inc. (b) To whom correspondence concerning this work should be addressed. (2) (a) R. Ferro, 2. anorg. allgem. Chem., 275, 320 (1954); (b) P. Khodadad, Bull. SOC. chim. France, 133 (1961). (3) W. Trzebiatowski and W. Suski, Bull. Acad. Polon. Sei., Sera sci. chim., 10, 399 (1962). (4)L. K.Matson, J. W. Moody, and R. C. Himes, J. Inorg. Nuel. Chem., 25,795 (1963).
HEATCAPACITY AND THERMODYNAMIC PROPERTIES OF Use
Cryostat and Calorimeter. Measurements were made in the Mark 111 vacuum cryostat6 by the quasi-adiabatic technique.6 The gold-plated copper calorimeter (laboratory designation W-38) used has a capacity of 13.8 cma3and is similar to one previously described.' The heat capacity of the calorimeter-heater-thermometer assembly mas determined in a separate series of measurements. Minor adjustments were applied for the dzerences (between these runs and those on the loaded calorimeter) in the amounts of Cerroseal (indium-tin) solder for sealing the calorimeter, Apiezon-T grease for thermal contact with the heater-thermometer assembly, and helium gas for thermal conductivity in the sample space. The mass of the calorimetric sample was 25.026 g. in vacuo, and its heat capacity ranged from 80% of the total at 5OK. to 32% a t 350°K. Buoyancy corrections were made using the reported4 density of 10.91 g . / ~ m . ~ A . helium pressure of 173 torr at, 300°K. was used to facilitate thermal % "K
7; O
K
Figure 1. Heat capacity of uranium monoselenide.
3619
Results and Discussion Heat Capacities and Thermal Properties. The experimental heat capacities at the mean temperatures of the determinations are presented in Table I in chronological order. These data have been adjusted for curvature and are given in terms of the defined thermochemical calorie of 4.1840 joules, an ice point of 273.15"K., and a gram formula mass of 316.99. The data in the region of the transition are presented in Figure 1. Table I: Heat Capacities of Uranium Monoselenide" T
Series I 104.89 10.69 114.25 11.43 123.93 12.21 133.93 13.06 143.77 14.01 153.45 15.28 162.99 15.71 183.59 12.93 194.17 12.75 204.64 12.72 214.99 12.75 225.20 12.83 229.15 12.84 237.67 12.93 246.11 12.95 254.48 12.95 263.06 12.96 271.82 13.00 280.51 13.03 289.44 13.05 298.58 13.10 307.88 13.13 317.31 13.20 326.66 13.24 336.25 13.24 345.78 13.26 Series I1 141.13 13.71 146.90 14.35 151.27 14.87 a
equilibration in the sample space. A capsule-type, strain-free plathum resistance thermometer (laboratory designation A-3) located within the entrant well of the calorimeter was used to determine temperatures which, above the oxygen point, are believed to accord with the therniodynamic temperature scale to within 0.03OK. All measurements of mass, temperature, resistance, voltage, and time are referred to calibrations or standardizations made by the U. S. National Bureau of Standards.
CP
c,
T
154.21 156.26 157.88 159.48 161.08 162.68 164.29 165.93 169.19 170.85 174.95 178.71 181.67
15.34 15.73 16.01 16.23 16.28 15.80 15.35 14.96 14.57 14.31 13.58 13.05 12.89
Series I11 5.85 0.144 6.43 0.154 7.22 0.179 8.20 0.201 9.49 0.271 11.11 0.344 12.63 0.438 13.80 0.556 14.71 0.639
T
CP
24.84 27.63 30.79 32.47 36.55 40.48 44.42 48.35 52.76 57.72 63.28 69.33 76.03 83.30 88.29 97.18 106.29
1.995 2.396 2.859 3.091 3.688 4.204 4.713 5.220 5.751 6.336 6.967 7.568 8.202 8.930 9.378 10.08 10.81
Series V 158.46 16.19 159.72 16.33 160.72 16.32 1 6 1 . i 2 16.09 163.45 15.55 165.89 15.00 168.36 14.60 170.85 14.24 173.36 13.73 176.33 13.26 179.76 12.98 184.86 12.84
Series IT13.95 0.565 15.17 0.695 16.68 0.869 18.40 1.082 20.32 1.346 22.44 1.651
Units: cal., g.f.m., "K.
The smoothed heat capacities and the thermodynamic functions derived from these data are given in Table I1 at selected temperatures. These values, obtained by means of a high-speed digital compilter using programs ~
~~
~
~~~~
(5) E. F. Westrum, Jr., J . Chem. Educ., 39, 443 (1962). (6) E.F. Westrum, Jr., J. B. Hatcher, and D. W, Osborne, J . Chem. Phys., 2 1 , 419 (1953). (7) D. TI-. Osborne and E. F. Westrum, Jr., ibid., 21, 1884 (1953).
Volume 69, Number 10 October 1966
3620
YOICHl
The lowest temperature heat capacity values may be represented by an expression
Table 11: Thermodynamic Properties of Uranium Monoselenide" -(Go
T
S O
CP
H a - Hoo
- H"o) T
5 10 15 20 25
0.112 0.277 0.674 1 ,301 2.017
0.107 0.232 0.409 0.685 1.052
0.27 1.22 3.47 8.34 16.62
0.0527 0.1102 0.1778 0.2681 0.3869
30 35 40 45 50
2 . 745 3.457 4.143 .&. 795 . j .417
1.484 1.961 2.468 2.993 3.531
28.53 44.04 63.05 85.41 110.95
0.5329 0.7024 0.8911 1.0954 1.3120
60 70 80 90 100
I;. 592 '7.656 S ,642 !J. 510 10.314
4.624 5.722 6.809 7.878 8.922
171.1 242.4 324.0 414.8 513.9
1.773 2.258 2.760 3.269 3.783
110 120 130 140 150
11.09 11.88 12.71 13.61 14.74
9.942 10.940 11.924 12.898 13.874
621.0 735.8 858.7 990.2 1131.7
4.296 4.809 5.318 5.825 6.329
160 170 180 190 200
IS. 40 14.28 12.99 12.74 12.73
14.874 15.803 16.578 17,271 17.925
1286.8 1440.0 1575 1704 1831
6.831 7.333 7.826 8.308 8.770
210 220 230 240 250
12.74 12.77 1:2.82 1'2.86 1'2.91
18.546 19.139 19.708 20.255 20.781
1958 2086 2214 2342 247 1
9.221 9.658 10.083 10.496 10.897
260 270 280 290 300
12.95 1:2.99 13.03 13.07 13.11
21.29 21.78 22.25 22.71 23.15
2600 2730 2860 2990 3121
11.29 11.67 12.04 12.40 12.75
325
13.21
24.20
3450
13.59
350
13.31
25.19
3782
14.38
273.15
13.00
21.93
2771
11.78
298.15
13.10
23.07
3097
12.68
a
Units: cal., g.f.m., OK,
previously described,*have been checked by comparison with large scale plots of the data. The thermodynamic functions are believed to have precision characterized by a probable error of less than 0.2% above 50°K. The entropy and Gibbs energy function have not been adjusted for nuclear spin or isotopic mixing contributions and are hence practical values for use in chemical thermodynamic calculations. The J o u r m l of Physical Chemistry
TAKAHASHI AND EDGAR F. WESTRUM, JR.
C,[cal./(g.f.m. OK.)] = 2.075 X 10-2T
+ 8.04 x
1 0 - 5 ~ 3
since the values of C, and C , will be practically the same at these temperatures. The T 3term represents the lattice contribution; the linear term is the electronic contribution. This equation was used to extrapolate the Use heat capacity below 5°K. The large electronic contribution is comparable to those of other isostructural uranium compounds (for example, UCg: 4.7 X 10-3, US1*: 4.9 X UNl': 9.6 X and corresponds to the observed high electrical conductivity4 ohm-cm.) of Use a t low temperatures. ( p 31 The Ferromagnetic Transition. The only reported measurements of the magnetic susceptibility of Use are those of Trzebiatowski and S ~ s k i who , ~ found that Use is ferromagnetic with a Curie point in the range 185 to 190"K., and a Curie-Weiss e of 188'K. Their calculated magnetic moment from the magnetization at 0°K. was 1.31 B.M. ( p ~ ) whereas , that calculated from the Curie-Weiss law for the paramagnetic region was 2.51 p ~ .Changes in the sense of the temperature dependency of the Hall coefficient and electrical resistivity were observed between 180 and 200°K. by Matson, et aL4 The experimental heat capacity of Use obtained by the present work is shown in Figure 1 to illustrate the features of the X-type transition observed at 160.5"K. As described above, this anomaly is believed to be associated with ferromagnetic ordering, though the observed transition temperature is 25 to 30" lower than that obtained by magnetic and electrical measurements. It should be noted that the heat capacity "tail" of the X-transition extends upward to about 200°K. If, as this suggests, the magnetization involves short range field dependence (instead of ordered, parallel alignment of atomic spins throughout macroscopic volumes), the transition temperatures would vary with the stoichiometry, homogenization, etc. Hence, more precise magnetic measurements on well-characterized samples are desiderata. (8) B. H. Justice, Ph.D. Dissertation, University of Michigan, 1961; USAEC Report TID-12722, 1961. (9) E. F. Westrum, Jr., E. Suits, and H. K. Lonsdale in "Advances in Thermophysical Properties at Extreme Temperatures and Pressures," S. Gratch, Ed., American Society of Mechanical Engineers, New York, N. Y., 1965, p. 156. (10) E. F. Westrum, Jr., and F. Grgnvold in "Thermodynamics of Nuclear Materials," IAEA, Vienna, 1962, p. 3. (11) E. F. Westrum, Jr., and C. M. Barber, unpublished data.
THERMODYNAMIC FUNCTIONS OF SbIa AND Bib
The estimation of the entropy and enthalpy associated with the magnetic ordering process was done by utilizing a Debye function to assist in drawing a smooth curve for the lattice heat capacity contribution. This yields 154 cal./g.f.m. for the enthalpy of transition and 1.05 cal./(g.f.ni. OK.) for the corresponding entropy increment. These results can be compared with the entropy increment, 1.17 cal./(g.f.m. OK.), observed in the similar magnetic transition of US’O a t 180°K.
3621
Acknowledgment. The authors appreciate the partial financial support of the U. s. Atomic Energy Commission and of the Selenium-Tellurium Development Association, Inc., and the cooperation of Wen-Kuei Wong and Mrs. Carolyn M. Barber in making the measurements and calculations. We thank Dr. H. L. Goering of the Chemistry and Chemical Engineering Department of Battelle Memorial Institute for his generosity in providing the calorimetric sample.
The Thermodynamic Functions above Room Temperature for Antimony and Bismuth Iodides and Their Absolute Entropies’
by Daniel Cubicciotti and Harold Eding Stanford Research Institute, M e d o Park, California 94085
(Received M a y 80, 1965)
The enthalpies of the condensed phases of Sb11 and Bi13 were measured from room temperature to the .boiling point. The fundamental vibration frequencies for gaseous Sb13 and BiIs were estimated. These, together with molecular structure data, were used to calculate their thermal functions (entropy, enthalpy, and free energy function). Literature information on vapor pressures was used to evaluate the absolute entropy for the condensed phases, and from this the thermal functions for the condensed phases were calculated.
Introduction The vapor pressures of SbI3 and %I3have been reported in the literature; however, without information on the heat capacities of the liquid and gas, a proper thermodynamic treatment of their vaporization has not been possible. We have measured the enthalpies of the condensed phases from room temperature to the boiling poin1,s and thus obtained the heat capacities and entropies above room temperature. I n the gas phase, these molecules are similar to the other trihalides of group V-A elements. By extrapolation of data from other trihalides a set of force constants was estimated which, together with structural information, was used to calculate the absolute entropies, as well as the other thermal functions, in the gas phase. The entropy of vaporization and thermal data were used
to calculate the absolute entropy of the condensed phase and, hence, the free energy functions.
Thermodynamic Functions for Gas Phases In order to calculate the thermal functions for the gas phase, one needs to know the structure of the gaseous molecules, the fundamental vibration frequencies, and information on low-lying electronic states.2 The structures of a number of the gaseous halides of arsenic, antimony, and bismuth have been dete~mined.~They are, in general, rather flat trigonal ~
~~
(1) This work was made possible by the support of the Research Division of the U. S. Atomic Energy Commission under Contract No.
AT(04-3)-106. (2) See, for example, K. S. Pitzer and L. Brewer, revision of “Thermodynamics,” by G. N. Lewis and M. Randall, McGraw-Hill Book Co., Inc., New York, N. Y., 1961, Chapter 27.
Volume 69, Number 10 October 1965
