2538
P. GOURSOT, H. L. GIRDHAR, AND E. F. WESTRUM, JR.
Acknowledgment. The authors wish to express their sincere appreciation to Carolyn Barber and James Huntsicker for their a'ssistance in the experimental ~ 0.200 composition of measurements of the N C C I= mixture; to Dr. H. G. Carlson, Dr. S.-S. Chang, Dr. J. C. Trowbridge, R. Radebaugh, and M. L. Hougen
for assistance in the experimental measurements of the other four compositions of mixture; and to Dr. L. 0. Case for the helpful discussion on the phase diagram of this system. The partial financial support of the United States Atomic Energy Commission is gratefully acknowledged.
Thermodynamics of Polynuclear Aromatic Molecules.
111. Heat
Capacities and Enthalpies of Fusion of Anthracene'
by P. Goursot, H. L. Girdhar, and Edgar F. Westrum, Jr.2 Department of Chemistry, University of Michigan, Ann Arbor, Michigan
4810.4
(Received November 26, 1969)
The heat capacity of anthracene was determined from 5 to 520°K by adiabatic calorimetry and the thermodynamic functions calculated at selected temperatures from these data. No thermal anomaly has been found in the crystalline phase. The values of C,, Sa, (H" - Hoo)/T and (Go - H"o)/T for the crystal at 298.15"K are 50.31, 49.51, 25.04, and -24.46 cal/mol O K , respectively. The enthalpy and entropy values of melting are 7020 i 12 cal/mol and 14.35 A 0.02 cal/mol OK at the melting temperature of 488.97"K. A plot of Szss us. Kitaigorodskii's coefficient shows that a linear relationship exists for condensed polynuclear aromatic molecules. Introduction During a series of investigations of the thermal and thermochemical aspects of the condensed phases of polynuclear aromatic hydrocarbons, 3-6 it was noted that previous measurements of the heat capacity of anthracene' over the rather limited range from 90 to 305°K deviated considerably from the average of data on naphthalenes and naphthacene.* This study covers the above region and extends the range of temperature over which measurements mere made to 5°K and into the liquid phase (to 520OK). The resultant data not only provide definitive thermodynamic functions but also enhance the fund of data available for correlations of the thermal properties of the solid state and the melting transition with other physicochemical properties of these molecules. Experimental Section Cryostat and Experimental Calorimeter f a y Measurements from 6 to 350°K. lleasurements were made in the 1Iark I11 vacuum cryostatg by the quasiadiabatic technique described previously.'" The gold-plated calorimeter (laboratory designation W-42) used has a capacity of 93 ema, horizontal vanes, a gold-gasketed closure, and is generally similar to one described preThe Journal of Physical Chemistry, Vo1. 74, N o . 12, 1970
viously. A capsule-type strain-free platinum resistance thermometer (laboratory designation A-5) located within the entrant well of the calorimeter was used to determine temperatures which above the oxygen point are believed to accord with the thermodynamic scale to within 0.03"K. The heat capacity of the calorimeter-heater-thermometer assembly was deter11i12
(1) This research was supported in part by the U . s. Atomic Energy Commission. (2) To whom correspondence concerning this work should be addressed. (3) W.-K. Wong and E. F. Westrum, Jr., J . Phys. Chem., 74, 1303 (1970). (4) W.-K. Wong and E. F. Westrum, Jr., unpublished data. (5) S.-w. Wong and E. F. Westrum, Jr., J . Amer. Chem. SOC.,in press. (6) D. Rodgers and E. F. Westrum, Jr., unpublished data. (7) H. M . Huffman, G. S. Parks, and J. M. Barmore, J . Amer. Chem. Soc., 53, 3876 (1931). (8) J. P. McCulIough, H. L. Finke, J. F. Messerly, S. S. Todd, T. C. Kincheloe, and G. Waddington, J . Phys. Chem., 61, 1105 (1957). (9) E. F. Westrum, Jr., J . Chem. Educ., 34, 443 (1962). (10) E. F. Westrum, Jr., J. B. Hatcher, and D. W.Osborne, J . Chem. Phys., 21, 419 (1953). (11) D. W.Osborne and E. F. Westrum, Jr., ibid., 21, 1884 (1953). (12) E. F. Westrum, Jr., and E. Chang, Colloq. Int. Cent. Nat. Rech. Sci., 156, 163 (1967).
HEATCAPACITIES AND ENTHALPIES OF FUSIQN OF ANTHRACENE
Series X I
mined in a separate series of measurements. Minor adjustments were applied for the difference (between these determinations and those on the loaded calorimeter) in the amount of 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 mas 68.125 g in vacuo and its heat capacity ranges from 68 to 81% of the total. Buoyancy corrections were made using the reported13density of 1.25 g/cm3. A helium pressure of 117 Torr at 300°K was used to facilitate thermal equilibration in the sample space. All measurements of mass, temperature, resistance, voltage, and time are referred to calibrations or standardizations made by the U. S. National Bureau of Standards. Thermostat and Calorimeter for Measurements from SO0 to 620°K. The previously described Atark IV intermediate range t h e r m o ~ t a t , 'a~ silver calorimeter (laboratory designation W-22), and a capsule-type platinum thermometer (laboratory designation A-8) were used. The mass of the calorimetric sample was 57.197 g in vacuo, and its heat capacity range from 70 to 78Oj, of the total. A helium pressure of 198 Torr a t 300°K was used to facilitate thermal equilibration in the sample space. Anthracene Sample. Dr. James Hinton of Newport News, Va. purified the anthracene sample by zone melting and independently established the purity as better than 99.99%. Fractional melting studies subsequently described in this paper further attest to the purity.
AH Detn D A H m Detn E
Results and Discussion Heat Capacity and Thermodynamic Functions. The
Table I : Experimental Heat Capacity of Anthracene" T
CB
-
C,
T
Mark I11 Cryostat Series I 204.43 208.61 212.74 216.79 220.75
229.15 37.74 238.38 39.35 247.30 40.94 257.64 42.76 267.87 44.65 277.29 46.34 286.39 48.04 296.53 49.81 306.36 51.65 315.91 53.38 327.59 55.49 336.60 57.14 345.37 58.76 350.05 59.55 Series IV 6.31 0.129 7.86 0.287 8.89 0.443 9.89 0.614 10.81 0.792 11.88 1.131 12.99 1.293 13.39 1.534 15.01 1.803 16.22 2.140 17.64 2.548 19.39 3.026 21.38 3.579
33.45 33.21 34.93 35.64 36.37
Series I1 88.80 16.06 100.01 17.54 109.76 18.86 119.33 20.21 128.52 21.56 138.03 22.96 147.67 24.41 154.44 25.41 162.98 26.74 165.80 27.20 174.15 28.46 182.64 29.83 192.14 31.46 201.85 33.12 210.88 34.16 219. ,59 36.15 Series I11 219.13
35.96
23.76 26.68 29.98 33.25 36.92 41.32 45.99
4.242 5,029 5.870 6.670 7.519 8.448 9.361
Series V 47.52 52.05 56.92 62.16
9.650 10.469 11.302 12.153
Series VI 58.40 63.64 69.34 75.50 81.98 89.31
11.151 12.374 13.226 14.130 15.084 16.122
Series VI1 80.71 88.40 96.39 104.73
14.90 16.02 17.06 18.17
Mark IV Thermostat Series VI11 304.80 313.63 323.42 333.59 343.51 353.58 363.80 373.78 384.04 394.58 396.78 406.76 417.22 427.47 437.51 T
51.77 53.34 55.07 56.69 58.72 60.51 62.38 64.12 65.99 67.84 68.26 70.05 71.99 73.77 75.63
447.34 457.51 468.00 477.32
Series IX
Series XI1
465.75 81.54 475.97 84.21 484.94 125.9 Melting Detns A 497.01 88.96
AH Detn F Melting Detns G 496.74 87.50 501.63 88.02 506.50 88.56 511.34 89.13
Series X
Series XI11
AH Detn B A H m Detn C
AH Detn H Melting Detns J
AT
CB
Melting Detns A 488.89 488.92 488.93 488.93 488.94 491.80
0.055 0.018 0.014 0.013 0.019 5.722
21400 65000 82000 89000 70400 204.4
Melting Detns G 488.90 Units:
0.023 cal, mol,
77.59 79.74 82.06 84.62
44200 OK.
T
2539
AT
C,
488.92 0.012 81600 488.93 0.008 129000 488.93 0.002 430000 224 491.62 5.365 Melting Detns J 488.91 0.019 488.92 0.007 488.93 0.009 500.30 22.737
58000 150000 126000 107
saturation pressure heat capacity values, C,, a t the mean temperature of each determination are listed in chronological order in Table I and are depicted in Figure 1 together with literature data.' These data have been adjusted for curvature and are given in terms bf the defined thermochemical calorie of 4.1840 J, an ice point of 273.15"K1and a gram formula mass of 178.236. The smoothed heat capacity and the thermodynamic functions derived from these data are given in Table I1 a t selected temperatures. The probable error of the heat capacity is 0.1% above 25"K, 1% at 12"I
