958
D. L. HILDENBRAND, W. R. KRAMER AND D. R. STULL
Vol. 62
THE HEAT CAPACITIES OF HEXACHLOROBENZENE AND PENTACHLOROPHENOL FROM 15 TO 300'K. BY D. L. HILDENBRAND, W. R. KRAMER AND D. R. STULL Thermal Laboratory, The Dow Chemical Company, Midland, Michigan Received April 9,1068
The heat capacities of crystalline hexachlorobenzene and pentachlorophenol have been measured from 13 to 300°K. in an automatically operated adiabatic calorimeter. The entropies of the solids a t 298.15"K. were found t o be 62.20 and 60.21 cal. deg.-l mole-1 for hexachlorobenzene and pentachloro henol, respectively. Thermodynamic functions, based on the experimental data, have been tabulated from 15 t o 3 0 0 ' 2 The free energies of formation of both compounds a t 298.15"K. have been derived from the measured entropies and previously published heats of formation.
Reliable thermodynamic data are relatively scarce for most of the chlorinated aromatic compounds. The measurements reported herein were undertaken to extend our knowledge of the thermodynamic properties of this group of industrially important substances. Andrews and Haworth' previously have reported the heat capacity of hexachlorobenzene over the range 100 to 330'K. as measured in a heat conduction calorimeter, but the entropy value derived from their data appears to be somewhat higher than might be expected for such a compound. No heat capacity data have been reported for pentachlorophenol. Materials.-The purified calorimetric samples of crystalline hexachlorobenzene and pentachlorophenol were supplied by research groups within The Dow Chemical Company. Purities were determined by the dynamic freezing curve method. The C&le sample was found to have a purity of 99.89 mole % ' and a melting point of 228.49 f 0.05 , while the C&OH had a purity of 99.73 mole % ' and a melting point of 189.65 i 0.05'. The hexachlorobenzene Sam le used in the heat capacity measurements had a weight oP58.092 g., while the pentachlorophenol sample weighed 65.480 g. Weights were corrected to vacuum. Calorimetric Appaiatus.-The adiabatic calorimeter and the details of its automatic operation have been fully described e l s e ~ h e r e . ~ , ~ The sample container used for the solid samples of hexachlorobenzene and pentachlorophenol was constructed of copper joined with soft solder. It contained ten re-entrant heater wells clustered around a central resistance thermometer well and had a volume of about 65 cm.s. A system of removable vertical heat transfer vanes was provided by a sheet of copper interwoven among the heater wells and sprung into place. The top of the sample container wa9 also removable, and i t fitted into a circular groove filled with low melting (90") alloy. This vacuum tight arrangement proved very convenient for filling, emptying and flushing the sample container. A l / 8 inch diameter monel tube inch in length was soldered to the container top for use in adding helium exchange gas and sealing. All parts of the sample container were nickel plated for corrosion resistance. The rocedure in filling and sealing the sample container waa as gllowa: with top removed, the crystalline sample was packed evenly into the container. The top was then soldered into place with the low melting alloy, heat being provided by a disc heater attached directly to the top. After evacuating and filling the container with helium at atmospheric plressure, a small copper plug was soldered into the end of t e monel tube for closure. Corrections were applied to each run for small differences in the weights of solder used. The platinum resistance thermometer ( Ra = 96 ohms) was one of a group of several recently calibrated by comparison with a Bureau of Standards calibratedplatinum thermometer. (1) D. H.Andrews and E. Haworth, J . A m . Chem. Soc., 80, 3000 (1928). (2) D.R. Stull, Anal. Chin. Acta, 17, 133 (1957). (3) D. L. Hildenbrand, W. R. Kramer, R. A. McDonald and D. R. Stull, J . A m . Chem. Soc., 80, 4129 (1958).
The ice point was taken as 273.15"K. and one defined calorie was taken as 4.1840 absolute joules.
Heat Capacity Data.-The reliability of the automatically operated calorimeter previously has been establisheda by comparing measurements of a pure sample of n-heptane with values reported by Ginnings and Furukawa4 in their paper on heat capacity standards. From this comparison, the heat capacity measurements are considered to be accurate to within 0.3% above 50'K., with the uncertainty increasing to 1% a t 25'K. Several inter-laboratory comparisons have shown these accuracy estimates to be correct. Two complete low temperature runs, covering the range 13 t o 300'K., were made on both hexachlorobenzene and pentachlorophenol. Since a single run, requiring little human attention, could be made in about 54 hours, the small extra effort involved in making a second run was considered well spent in view of the additional reduction in uncertainty. Temperature rises were approximately 10% of the absolute temperature below 50'K. and 5 to 6' at higher temperatures. For space economy, only the smoothed heat capacities at even temperatures are reported here. These data are given in Tables I and 11. The great majority of the experimental points deviate from the smooth curve by less than 0.2%. The heat capacities of both compounds followed the normal S-shaped curve. The additional thermodynamic properties So,(Ho- Hoo)/T and - ( F O - HoP)/T are also given in Tables I and 11. Molecular weights of CeCle and C6C160Hwere taken as 284.81 and 266.36, respectively. The heat capacity data of Andrews and Haworth' on C6Cle deviate considerably from the values in Table I. The results of Andrews and Haworth are 16% lower at lOO'K., 7% higher at 200'K. and 28% higher at 300'K. They have used a heat conduction calorimeter heated at a rate of about one degree per minute, but it appears that this rate may have been too high and the thermal conductivity of C&l6 too low to allow proper distribution of heat throughout the sample. The Entropies of Hexachlorobenzene and Pentachlorophenol.-The entropies, obtained by graphical integration of the heat capacity data, are given in Tables I and 11. Heat capacities were extrapolated to 0°K. by fitting the first 12 to 14 points between 13 and 30°K. to a combination of Debye and Einstein functions. For CeCl6 a Debye func(4) D. C. Ginnings and G. T. Furukawa, J . A m . Chem. Soe., 76, 522 (1953).
HEATCAPACITIES OF HEXACHLOROBENZENE
August, 1958
959
TABLE I TABLE I1 THERMODYNAMIC PROPERTIES OF HEXACHLOROBENZENE, THERMODYNAMIC PROPERTIES OF PENTACHLOROPHENOL, CAL. MOLE-^ D E G . - ~ CAL.MOLE-1D E G . - ~ T,O K .
cRo
SQ
15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 298.15 300
2.63 4.53 6.34 8.03 9.56 11.00 12.36 13.65 14.90 16.12 17.30 18.46 19.57 20.65 21.69 22.70 23.68 24.62 26.41 28.10 29.69 31.19 32.61 33.97 35.27 36.50 37.69 38.84 39.93 40.99 42.02 43.01 43.96 44.89 45.78 46.65 47.48 48.11 48.29
0.98 2.00 3.21 4.52 5.87 7.24 8.62 9.99 11.35 12.70 14.04 15.36 16.67 17.97 19.25 20.52 21.77 23.01 25.44 27.81 30.13 32.38 34.58 36.73 38.83 40.88 42,89 44.85 46.77 48.65 50.50 52.31 54.08 55.82 57.54 59.22 60.87 62.20 62.49
0.73 1.44 2.24 3.07 3.89 4.69 5.47 6.22 6.95 7.67 8.36 9.04 9.71 10.36 10.99 11.61 12.22 12.82 13.98 15.08 16.15 17.17 18.15 19.10 20.01 20.89 21.74 22.57 23.37 24.15 24.90 25.64 26.35 27.05 27.72 28.38 29.03 29.54 29.66
0.25 .56 .97 1.45 1.98 2.55 3.15 3.77 4.40 5.03 5.68 6.32 6.96 7.61 8.26 8.91 9.55 10.19 11.46 12.73 13.98 15.21 16.43 17.63 18.02 19.99 21.15 22.28 23.40 24.50 25.60 26.67 27.73 28.77 29.82 30.84 31.84 32.66 32.83
tion (3 degrees of freedom) with e = 80' together with three Einstein functions each with 9 = 85' was used for the extrapolation. In the CeC160H extrapolation, the same combination of functions was used, with OD = 82.5' and BE = 92'. At 298.15'K. the entropies of C6Cl6 and CsC160H are 62.20 h 0.10 and 60.21 f 0.10 cal. mole-' deg.-l, respectively. Parks and Huffmans have calculated the entropy of C6Cl6 as 71.2 at 298.15'K., based on the data of Andrews and Haworth' but this value should be discarded for reasons mentioned above. Free Energy Data.-The entropy data given above may be combined with the heats of forma( 5 ) G. 8. Parks and €€.M. Huffman, "The Free Energies of Some Organic Compounds," Reinhold Publ. Corp., New York, N. Y.,1932.
T,OK.
CPO
80
15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 298,15 300
2.45 4.11 5.83 7.40 8.86 10.25 11.60 12.92 14.17 15.37 16.53 17.65 18.73 19.78 20.80 21.79 22.75 23.68 25.47 27.17 28.79 30.33 31.81 33.23 34.59 35.90 37.12 38.33 39.50 40.63 41.72 42.77 43.79 44.78 45.73 46.66 47.56 48.27 48.43
0.87 1.79 2.90 4.10 5.35 6.63 7.91 9.20 10.49 11.78 13.05 14.32 15.57 16.82 18.05 19.27 20.47 21.66 24.00 26.29 28.53 30.72 32.86 34.96 37.02 39.03 41.01 42.94 44.84 46.70 48.53 50.33 52.10 53.84 55.54 57.22 58.88 60.21 60.51
0.65 1.30 2.04 2.81 3.56 4.31 5.04 5.76 6.47 7.16 7.84 8.50 9.15 9.78 10.40 11.00 11.60 12.18 13.30 14.39 15.44 16.44 17.42 18.36 19.28 20.17 21.03 21.86 22.67 23.46 24.24 24.98 25.72 26.43 27.13 27.81 28.48 29.01 29.13
0.22 .49 -86 1.29 1.79 2.32 2.87 3.44 4.02 4.62 5.21 5.82 6.42 7.04 7.65 8.27 8.87 9.48 10.70 11.90 13.09 14.28 15.44 16.60 17.74 18.86 19.98 21.08 22.17 23.24 24.29 25.35 26.38 27.41 28.41 29.41 30.40 31.20 31.38
tion given by Sinke and StulP and the entropies of the elements7 in order to evaluate the free energies of formation of c6cl6 and CeClsOH. These values are given in Table 111. TABLE I11 HEATS-4ND FREEENERGIES OF FORMATIOX AT 298.15'K., KCAL.MOLE-1 A H f O
CaC16 (S)
CsClSOH
(8)
-31.3 zk 1 . 0 -70.6 f 0 . 7
A F f O
+ 0.3 f1.0 -34.4 f 0 . 7
THISJOURNAL, 62, 397 (1958). (7) D. R. Stull and G. C. Sinke, "Thermodynamic Properties of the Elements. Number 18 of the Advances in Chemistry Series Edited by the Staff Industrial and Engineering Chemistry," American Chemical Society, 1155 Sixteenth Street, N. W., Washington 6, D. C. (6) G. C. Sinke and D. R. Stull,
