84
J. Chem. Eng. Data 2001, 46, 84-89
Low-Temperature Heat Capacities and Derived Thermodynamic Functions of Para-Substituted Halogen Benzenes. 2. p-Bromoiodobenzene and p-Diiodobenzene J. Cees van Miltenburg,* Harry A. J. Oonk, and Gerrit J. K. van den Berg Chemical Thermodynamics Group, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands
Heat capacity measurements were made on p-bromoiodobenzene and p-diiodobenzene from 5 K to 380 K and from 5 K to 420 K, respectively. The measurements were used to calculate the entropy and enthalpy values relative to 0 K. The enthalpy of melting of p-bromoiodobenzene was found to be (19375 ( 20) J‚mol-1, and the triple-point temperature, (363.52 ( 0.01) K. For p-diiodobenzene these values were respectively (22328 ( 50) J‚mol-1 and (402.40 ( 0.05) K. A small solid-solid transition occurs in p-diiodobenzene. The maximum of this transition was measured at 327 K for sublimated material and shifted to 322 K for crystallized material. The enthalpy of transition was found to be (223 ( 10) J‚mol-1.
Introduction This is the second article dealing with the thermodynamic properties of the para-substituted benzenes, with the substituting groups Cl-, Br-, and I-. The first article dealt with the thermodynamic properties of p-chlorobromobenzene and p-chloroiodobenzene between 10 K and 360 K.1 The family of the para-substituted halobenzenes has been used by our group as a model system for the study of the interactions in molecular mixed crystals.2,3 The compounds were used in a series of studies on the mixing behavior in the solid and liquid state. The object of this series of publications is to present the thermodynamic properties of the pure compounds. p-Bromoiodobenzene and p-diiodobenzene did receive very little interest in the literature. Molar heat capacities, measured over a small temperature range, were published.4,5 The crystal structure of p-diiodobenzene was studied by neutron diffraction by Alcobe´ et al.6 This compound exists in a low- and a high-temperature crystalline phase, both of which are orthorhombic, with space groups Pbca and Pccn, respectively. In this respect, p-diiodobenzene does form an exception, as it does not occur in the same crystal form (P21/a) as that for the other p-dihalobenzenes. Experimental Section The two compounds were purchased from Aldrich. pBromoiodobenzene had a stated purity of 98%, and pdiiodobenzene had one of 99%. Both compounds were vacuum sublimated once before use. In this procedure, the first and last part (about 10%) of the sublimated compound is rejected. For most compounds, this method does give an improvement in purity. The calorimeter vessel was filled to the maximum, using 13.389 g for p-bromoiodobenzene (molar mass 282.9062) and 15.73 g of p-diiodobenzene (molar mass 329.9067). The molar masses were calculated using the IUPAC tables of 1991.7 After filling, the vessel was evacuated and closed under a helium pressure of about 1000 Pa to improve heat conduction. The calorimeter used, * Corresponding author.
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laboratory-design indication CAL V, had been described in 1987.8 More recent improvements in design and datahandling were described in 1998.9 The temperature calibration (IPTS-68) of the rhodium-iron thermometer (Oxford Instruments 27 Ohms at 273 K) was corrected to the ITS-90 scale.10 The resistance of the thermometer was measured with an automatic alternating current bridge (Tinsley type 5840D). The standard deviation of the temperature-time curve in the second half of the stabilization periods was between 25 and 40 µK outside the transition regions. Energy was supplied in a step-shaped protocol; 30%-60%-100% of the maximum power was normally used. This improves the response of the shields to the changing temperature. A constant current supply (Keithley model 220), programmed by the measuring computer, was used. The reproducibility of the heat capacity measurements was estimated from fitting the measurements with polynomial functions and was found to deviate within (1% between 5 K and 30 K, 0.05-0.1% between 30 K and 100 K, and 0.03% or less above 100 K. The accuracy was checked by measuring standard substances (synthetic sapphire and n-heptane). No deviations larger than 0.2% of the recommended values were found. Measurements were made in the intermittent mode. Stabilization periods of about 600 to 1000 s were used between the heating periods. Below 30 K the periods were on the order of 150 s. We did use energy inputs in the heating periods leading to a temperature increment of about 2 K below 100 K and 3 K above this temperature. As the maximum heat input was limited to a value leading to this increment outside the transition region, a relatively large number of heat inputs were used in the fusion process. The total time spent in the melting region is for every melting experiment on the order of 9 h. To equilibrate the samples, they were first melted (series 1), followed by a slow cooling process. In this cooling process, the temperature of the first shield was set 10 K below the temperature of the vessel, resulting in a cooling speed of about 5 K‚h-1 outside the crystallization region. In the next measurement (series 2) the sample was heated to the beginning of the melting process and left under adiabatic conditions at that temperature overnight.
10.1021/je000068u CCC: $20.00 © 2001 American Chemical Society Published on Web 12/12/2000
Journal of Chemical and Engineering Data, Vol. 46, No. 1, 2001 85 Table 1. Experimental Data Series for p-Bromoiodobenzene T
Cp
K
J‚K-1‚mol-1
series 1 299.25 301.29 304.28 307.27 310.25 313.23 316.21 319.19 322.17 325.15 328.13 331.11 334.08 337.06 340.03 343.01 345.98 348.95 351.92 354.90 357.88 360.85 362.90 363.49 363.50 363.51 363.51 363.51 363.51 363.51 363.52 363.52 363.52 363.52 363.52 363.52 363.52 363.52 364.34 366.60 369.46 372.32 375.17 378.01 380.85 series 2
145.57 146.52 147.88 148.94 150.27 151.47 152.62 153.85 155.08 156.30 157.52 158.82 160.04 161.25 162.49 163.66 164.86 166.09 167.31 168.58 169.96 173.60 804 55969 173442 274477 464726 688238 850032 915057 >106 896651 >106 >106 >106 >106 >106 >106 510.82 196.28 196.70 197.21 197.78 198.40 198.89
T
Cp
T
Cp
T
Cp
T
Cp
K
J‚K-1‚mol-1
K
J‚K-1‚mol-1
K
J‚K-1‚mol-1
K
J‚K-1‚mol-1
296.21 297.27 299.31 302.30 305.29 308.27 311.25 314.22 317.20 320.17 323.14 326.11 329.08 332.06 335.02 337.99 340.96 343.92 346.88 349.84 352.80 355.75 358.70 361.57 363.23 series 3 6.97 8.10 9.19 10.57 12.06 13.67 15.44 17.34 19.35 21.42 23.56 25.75 28.01 30.35 series 4 6.03 7.58 8.68 9.96 11.43 13.01
148.42 145.33 146.50 147.57 148.81 149.84 151.10 152.22 153.21 154.24 155.26 156.27 157.31 158.31 159.16 160.09 160.98 162.14 163.40 164.74 166.33 168.03 170.51 195.24 1892
14.74 16.61 18.58 20.65 22.77 24.95 27.19 29.51 31.80 series 5 33.60 34.76 36.71 39.42 42.04 44.63 47.24 49.87 52.52 55.19 57.89 60.60 63.33 66.07 68.84 71.62 74.42 77.24 80.07 82.90 85.75 88.61 91.48 94.35 97.24 100.13 series 6 103.98 107.02 110.46 114.30 118.14 121.99 125.85 129.72 133.60 137.49
14.59 17.58 20.47 23.44 26.39 29.44 31.49 33.78 36.23
141.37 145.27 149.17 153.08 156.99 160.90 164.81 168.73 172.66 176.58 180.48 184.36 188.21 192.02 195.80 199.54 203.27 206.96 210.62 214.26 217.87 221.46 225.02 228.56 232.08 235.58 239.05 242.51 245.94 249.35 252.75 256.12 259.48 262.83 266.15 269.46 272.75 276.02 279.27 282.51 285.73 288.94 292.13 295.30 298.46 series 7 300.76
90.28 91.74 93.07 94.60 95.94 97.30 98.68 100.07 101.41 102.88 104.25 105.57 106.92 108.28 109.47 110.90 112.22 113.55 114.81 116.10 117.33 118.64 119.89 121.28 122.58 123.84 125.07 126.27 127.58 128.79 129.82 130.52 131.81 133.05 134.39 135.65 136.94 138.07 139.31 140.56 141.87 143.09 144.35 145.48 146.77
302.61 305.03 307.99 310.96 313.93 316.90 319.86 322.83 325.80 328.76 331.73 334.69 337.65 340.61 343.57 346.53 349.49 352.45 355.40 358.35 361.23 363.06 363.50 363.52 363.52 363.52 363.52 363.52 363.52 363.52 363.52 363.52 363.52 363.52 363.52 363.52 363.52 363.67 365.19 367.95 370.70 373.45 376.19 378.93
2.53 3.96 5.51 7.67 10.23 12.93 15.73 18.63 21.63 24.62 27.57 30.50 32.08 34.69 1.80 3.35 4.81 6.69 9.21 11.83
Results and Discussion p-Bromoiodobenzene. No phase transitions were found between 10 K and the melting point. The experimental data series are given in Table 1.The temperatures in the table are the mean temperatures of the temperature intervals used; the heat capacity values are mean values over the temperature interval. Each series forms a continuous set of measurements. Very high values for the heat capacity were measured in the melt. When negative values are reported, the equilibrium temperature (see further on) was found to be lower after a heat input. This phenomenon does not influence the calculation of the enthalpy of fusion, as this calculation is based on the enthalpy increment. Also, the effect is so small that the influence on the calculation of the purity was neglected. However, it is a phenomenon which is common to the p-substituted halobenzenes. It was also found, but more outspoken, in p-diiodobenzene, where it is discussed in more detail. Series 1 and 2 were made for annealing the sample and were not used for the calculation of the derived thermodynamic properties. To obtain a starting value for the calculation of the absolute entropy, the low-temperature data were fitted to Cp ) RT3
38.12 39.26 40.86 42.34 44.48 46.37 48.09 49.93 51.76 53.42 54.93 56.54 58.05 59.39 60.73 62.23 63.53 64.81 66.04 67.40 68.59 69.81 71.09 72.26 73.37 74.65 75.49 76.53 78.71 80.23 81.69 83.10 84.64 86.16 87.56 88.91
148.13 149.04 150.11 151.23 152.36 153.47 154.64 155.91 156.97 158.13 159.42 160.68 161.90 163.01 164.23 165.31 166.53 167.96 169.65 172.02 186.22 1152 41471 242726 774023 >106 106 >106 >106 >106 106 7950 230.85 207.77 208.18
332.40 333.36 334.32 335.27 336.22 337.17 338.12 339.07 340.01 series 4 9.15 10.06 11.67 13.29 14.71 15.94 17.04 18.04 18.96 19.83 20.65 21.42 22.15 22.85 23.51 24.16 24.77 25.36 25.94 26.49 27.03 27.55 series 5 5.73 7.26 8.32 9.37 10.74 12.23 13.90 15.71 17.64 19.70 21.84 24.02 26.25 28.52 series 6 33.37 35.03 37.45 39.98 42.51 45.04 47.60 50.19 52.80 55.45 58.11 60.79 63.50 66.23 68.98 71.74 74.52 77.32 80.13 82.95 85.78 88.62 91.48 94.34 97.21 100.09 series 7
158.69 158.97 159.36 159.85 159.99 160.53 160.83 161.06 161.35
102.47 103.83 106.47 110.36 114.20 118.04 121.90 125.76 129.64 133.50 137.32 141.09 144.81 148.48 152.10 159.32 162.83 166.30 169.75 173.16 176.54 179.90 183.23 186.53 189.81 193.07 196.30 199.52 202.71 205.89 209.04 212.17 215.29 218.38 221.46 224.52 227.57 230.60 233.61 236.61 239.59 242.55 245.51 248.44 251.37 254.28 257.18 260.07 262.95 265.81 268.66 271.50 274.33 277.14 279.94 282.73 285.51 288.28 291.03 293.78 296.52 299.24 series 8 301.01 301.71 302.57 303.58 304.58 305.58 306.59 307.59 308.60 309.60 310.61 311.61
76.91 77.51 78.44 79.90 81.33 82.80 84.23 85.76 87.31 88.65 90.05 91.35 92.69 93.98 95.84 97.88 99.13 100.33 101.50 102.70 103.96 105.15 106.28 107.47 108.52 109.73 110.76 111.98 113.09 114.25 115.29 116.44 117.54 118.61 119.74 120.78 121.95 123.04 124.08 125.18 126.27 127.30 128.42 129.44 130.49 131.00 131.83 132.85 133.96 135.06 136.15 137.22 138.30 139.30 140.34 141.42 142.53 143.64 144.72 145.60 146.68 147.59
312.61 313.62 314.62 315.62 316.62 317.62 318.62 319.60 320.55 321.49 322.40 323.32 324.28 325.26 326.26 327.25 328.25 329.24 330.24 331.24 332.24 333.24 334.24 335.24 336.23 337.23 338.23 339.23 340.23 series 9 341.19 342.09 344.03 347.01 349.98 352.96 355.93 358.90 361.87 364.84 367.80 370.74 373.68 376.61 379.53 382.44 385.34 388.23 391.11 393.97 396.81 399.52 401.37 402.07 402.24 402.31 402.34 402.36 402.37 402.38 402.38 402.38 402.38 402.38 402.38 402.38 402.38 402.38 402.39 402.39 402.45 403.53 405.89 408.59 411.29
152.69 153.27 153.74 154.30 154.48 154.95 155.37 155.88 156.28 156.49 156.89 157.20 157.51 158.02 158.25
6.75 8.34 11.43 14.50 17.01 18.96 20.69 22.09 23.44 24.51 25.53 26.67 27.51 28.51 29.35 30.09 30.93 31.50 32.13 32.84 33.51 33.94 2.11 3.72 4.91 7.18 9.60 12.58 15.51 18.56 21.61 24.29 27.18 29.90 32.55 35.51 38.32 40.12 41.53 43.14 45.04 46.84 48.58 50.45 52.25 53.87 55.44 57.09 58.63 60.03 61.42 62.94 64.28 65.58 66.88 68.24 69.49 70.68 72.03 73.20 74.36 75.61
148.49 148.07 148.57 149.00 149.26 149.62 149.90 150.22 150.67 150.93 151.38 151.78
152.13 152.51 152.95 153.31 153.96 155.54 159.46 166.35 178.79 191.75 201.04 191.27 177.69 168.58 163.63 161.05 159.54 158.78 158.46 158.43 158.40 158.71 158.83 159.16 159.38 159.82 160.05 160.46 160.91 160.58 161.03 162.53 163.69 164.74 165.83 166.75 168.04 168.96 170.11 171.20 172.34 173.45 174.64 175.82 176.94 178.11 179.34 180.84 183.11 188.38 229.06 819 4349 12479 30032 58679 72241 109794 218961 106 >106
