Creator:D.A. Shirley and W.F. Giauque
Date Created:1959
Place Created:
Keywords:iodine entropy,sublimation
Context:article reprinted from Journal of American Chemical Society
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[Reprinted from the Journal of the American Chemical Society, 81, 4778 (1959).)
The Entropy of Iodine. Heat Capacity from 13 to 327 K. Heat of Sublimation
By D. A. Shirley and W. F. Giauque
Sept. 20, 1959
Entropy, Heat Capacity and Heat of Sublimation of Iodine
4779
T, °K. 10 15 20 25 30 35 40 45 50 55 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290
298.15
300
310
320
325
330
Table II
Thermodynamic Properties of Solid Iodine
--0°c. = 273.15°k.-
•Cal. deg.-1 mole-C„
0.963
2.450
3.866
5.140
6.156
6.909
7.557
8.110
8.571
8.972
9.343
9.955
10.353
10.693
10.911
11.199
11.407
11.587
11.716
11.853
11.966
12.068
12.158
12.243
12.325
12.441
12.539
12.617
12.678
12.726
12.767
12.813
12.873
12.941
13.011
13.027
13.145
13.313
13.404
(13.488)
S« 0.350 1.010 1.912 2.916 3.948 4.955 5.921 6.844 7.724 8.560 9.356 10.846 12.202 13.441 14.581 15.636 16.620 17.540 18.404 19.216 19.985 20.713 21.406 22.066 22.696 23.300 23.879 24.438 24.977 25.495 25.995 26.478 26.945 27.397 27.758 27.838 28.266 28.686 28.892 (29.098)
— (F> - B0/
T
0.090 0.277 0.570 0.938 1.354 1.797 2.252 2.711 3.169 3.621 4.066 4.930 5.755 6.541 7.289 8.001 8.679 9.326 9.943 10.535 11.102 11.646 12.170 12.673 13.159 13.627 14.080 14.518 14.943 15.355 15.754 16.143 16.520 16.887 17.180 17.245 17.594 17.934 18.100 (18.266)
(IP - Ho0)/T 0.260 0.733 1.342 1.978 2.594 3.158 3.699 4.133 4.555 4.939 5.290 5.916 6.447 6.900 7.292 7.635 7.941 8.214 8.461 8.681 8.883 9.067 9.236 9.393 9.537 9.673 9.799 9.920 10.034 10.140 10.241 10.335 10.425 10.510 10.579 10.593 10.672 10.752 10.792 (10.832)
relationship
AF"/T = -R In P = A- H0')/T + AH0°/T
The results are given in Table III. It is evident that there is no significant drift in the values of AH00 calculated from the data over the range 273.15 to 368.15°K.
Table III
Heat of Sublimation and Vapor Pressure op Iodine
rp ©	K.	--Inter, mm.-» Pox pt- .PoBlod.		.—Cal. mole"'— AH from AHt' Pav t. calcd.		Ref.
273.	.15	0.030	0.03051	15667	15025	7
288	.15	.131	.1286	15647	14964	7
298	.15	.305	.3080	15664	14922	7
303.	.15	.469	.4669	15655	14901	7
308.15		.699	.6971	15656	14880	7
313.	.15	1.025	1.028	15659	14858	7
318.	15	1.498	1.494	15657	14836	7
323.	.15	2.154	2.147	15656	14814	7
323.	15	2.154	2.147	15656	14814	8
328.	,15	3.084	3.052	15651	14791	7
328.	.15	3.069	3.052	15654	14791	8
333.	15	4.285	4.293	15659	14768	8
338.	.15	5.962	5.969	15658	14744	8
343.	15	8.196	8.216	15659	14720	8
348.	15	11.21	11.22	15656	14696	8
353.	15	15.09	15.15	15659	14672	8
358.	15	20.21	20.28	15660	14647	8
363.	15	26.78	26.95	15663	14619	8
368.	15	35.24	35.52	15663	14575	8
			Av.	15658	±3	
ing to a rigid rotator and harmonic oscillator. Convenient functions for the latter have been given by Johnston, Savedoff and Belzer.6 We find at 250, 273.15, 298.15, 300, 400 and 500°K. the values 52.759, 53.469, 54.176, 54.227, 56.588 and 58.455, respectively, for - (F°-H0°)/T of I2(g). All intermediate values needed were interpolated as mentioned above. At 298.15°K„ for I2(g) = 54.176 cal. deg.-1 mole-1.
The heat of sublimation of iodine at the absolute zero may now be calculated from the accurate vapor pressure measurements of Baxter, Hickey and Holmes7 and Baxter and Grose8 by use of the
(6)	H. L. Johnston, Lydia Savedoff and J. Belzer, Navexos P646, Office of Naval Research, U. S. Navy, Washington, D. C.
(7)	G. P. Baxter, C. H. Hickey and W. C. Holmes. This Journal, 29, 127 (1907).
(8)	G. P. Baxter and M. R. Grose, ibid., 37, 1061 (1915).
The equivalent calculation made earlier by Giauque2 with less accurate data was used to show that the entropy of the solid approaches the entropy due to the nuclear spin multiplicity and unlike hydrogen it does not deviate in a specific manner due to the ortho and para states which are well-known in the gas. The present more reliable data confirm the earlier result. While an additional decrease in the entropy due to spin states will occur at extremely low temperatures, it is of course convenient to delete this effect from the entropy values used in ordinary thermodynamic calculations. The entropy value 27.76 cal. deg.-1 mole-1 given here is the absolute value less the nuclear spin effect.
The heat of sublimation at temperature T may be calculated from AHt = A (7/° — Ha°)r + Ai/0°. In columns 3 and 5 of Table III calculated values of the vapor pressure and heat of sublimation of iodine are given at the several temperatures based on the average value of AHo" = 15658 cal. mole-1.
Acknowledgments.—We thank L. E. Murch and J. B. Ott for assistance during the experiments and the National Science Foundation for a Fellowship (to D.A.S.).
Berkeley, California
[Reprinted from the Journal of the American Chemical-Society, 81, 4778 (1959).] Copyright 1959 by the American Chemical Society and reprinted by permission of the copyright owner.
[Contribution from the Low Temperature Laboratory, Departments of Chemistry and Chemical Engineering,
University of California, Berkeley]
The Entropy of Iodine. Heat Capacity from 13 to 327 K. Heat of Sublimation1
By D. A. Shirley and W. F. Giauque Received February 24, 1959
The heat capacity of iodine has been measured from 13 to 327°K. The thermodynamic properties, Cp, S", —(F° — H0°)/ T and (H° - H0<>)/T have been tabulated to 330°K. The entropy of the solid at 298.15°K. = 27.76 cal. deg."1 mole""1, the heat of sublimation at 0°K., AH0° = 15658 cal. mole-1 and the heat of sublimation at 298.15°K. was found to be 14922 cal. mole-1.
The generally accepted value 27.9 cal. (leg. mole-1 for the entropy of solid iodine at 298.15°K. rests less on the rather inadequate heat capacity measurements from which it was calculated, than on its successful use in a calculation2 of the heat of sublimation of iodine from the vapor pressure data over a temperature range of nearly one hundred degrees. A value based on sufficiently complete heat capacity data will have a greater reliability and the present work was done to provide these data.
Experimental Procedure and Iodine Sample.—Undoubtedly the reason why more complete calorimetric data on the important substance iodine have not become available long ago is that it will react with typical low temperature calorimeters. To avoid this difficulty a special calorimeter was constructed from a 90-cc. Pyrex bottle, blown to fit approximately into a copper cylindrical shell with a diameter of 4.4 cm. and length 9.5 cm. The bottle had a small glass stoppered neck which extended slightly above a small monel neck and collar at the top of the copper shell. The final closure was by means of a copper cap which could be soldered to the monel collar. Monel was used to prevent excessive heat conduction to the calorimeter during the soldering process. As usual, the interior of the calorimeter was filled with helium to improve heat conduction. As would be expected the glass bottle increased the time required for thermal equilibrium and thus caused some increase in the magnitude of the corrections for heat transfer between the calorimeter and its surroundings; however, this involved only a minor decrease in accuracy. In general the calorimetric equipment was similar to that described previously.3 A gold resistance thermometer-heater was used for high precision temperature measurements and Laboratory Standard Thermocouple No. 105 was used as a temperature reference. The thermocouple was checked for reliability during this work with the following results: 0.02° high at both the triple point (13.94°K.) and the boiling point (20.36°K.) of hydrogen; 0.01° high at the triple point (63.15°K.) and 0.06° high at the boiling point (77.34°K.) of nitrogen.
The sample of iodine was taken from Baker and Adamson Lot No. L098 (resublimed) and the maximum limits of impurities given were: non-volatile, 0.010% and CI and Br, 0.005%.
An amount of 253.271 g. in vacuo was used for the measurements. 0°C. was taken as 273.15°K. and 1 defined calorie was taken equal to 4.1840 absolute joules.
The Heat Capacity and Thermodynamic Properties of Iodine.—The experimental data are given in Table I and the derived thermodynamic functions
(1)	This work was supported in part by the National Science Foundation.
(2)	W. F. Giauque, This Journal, 53, 507 (1931).
(3)	W. F. Giauque and C. J. Egan, J. Chem. Phys., 5, 45 (1937).
are given for CP, S°, (F°-H00)/T and (H°-H0<>)/T at even temperature intervals in Table II.
Table I
Heat Capacity of Iodine, Cal. Deg.-1 Mole-1
,-Cal. deg. -» mole-"--,	Mol. wt. = 126.91'-.
T, °k.	c„	T, °k.	cp	T, °k.	cp
13.62	2.143	78.18	10.30	192.96	12.27
15.71	2.644	84.61	10.51	202.06	12.34
18.04	3.322	90.99	10.73	209.66	12.44
20.55	4.018	97.27	10.88	216.84	12.47"
23.33	4.754	104.01	11.05	223.32	12.64
26.16	5.359	110.99	11.21	231.05	12.67
29.24	6.051	118.43	11.37	239.68	12.67
32.30	6.510	126.10	11.60	248.58	12.73
36.13	7.051	133.48	11.64	256.85	12.75
39.93	7.549	140.90	11.72	264.96	12.83
44.24	8.043	148.36	11.83	273.32	12.73
49.26	8.520	155.65	11.99	282.02	12.86
54.52	8.891	162.71	12.00	291 14	12.88
60.05	9.361	170.04	12.07	301.22	13.12
65.79	9.757	177.56	12.15	311.19	13.15
71.87	10.044	185.21	12.20	321.93	13.33
" This result is uncertain by ±1% due to water vapor in the apparatus and is thus given no weight.
The data of Lange4 cover the range 9 to 52° K. His measurements deviate from the present work by ± 1% near and somewhat above the boiling point of hydrogen and above 30 °K. they are 2 to 4% high.
A Debye extrapolation below 10°K. gives 0.350 cal. deg.-1 mole-1 as the entropy less the nuclear spin effect at that temperature.
The thermodynamic functions for iodine gas have been given in the Bureau of Standards Tables.6 We have applied a small correction to convert these values to 0°C. = 273.15°K. instead of 273.16° K. and have also altered the values given for 0 and 25° slightly in addition since these values appear to have been interpolated in the main series without complete accuracy. The only satisfactory method of interpolating such values to 0.001 appears to be a difference plot with a function correspond-
(4)	E. Lange, Z. physik. Chem., 110, 343 (1924).
(5)	"National Bureau of Standards," Series III, June 30, 1948; March 1, 1954. '