SOME HEAT-CAPACITY DATA ON ORGANIC COMPOUNDS, OBTAINED WITH A RADIATION CALORIMETER BY MONROE E. SPAGIIT, S. B E S S O S THOMAS,‘ A S D GEORGE S . P A R K S
In a recent publication from this laboratory by Thomas and Parks,* in which specific heat data on boron trioxide glass were presented, a so-called “radiation” calorimeter was described. In order to obtain thermal data a t higher temperatures on some organic compounds which have already been investigated at low temperatures by Parks, Huffman and their co-worker~,~ this apparatus has now been used for the measurement of the heat capacities and heats of fusion of the following nine substances: pentacosane, tritriacontane, hexamethylbenzene, diphenyl, triphenylmethane, naphthalene, dibenzoylethane, erythritol and mannitol. In addition some data have been obtained on liquid ethylbenzene and on ethyl azoxybenzoate, the latter being a substance that exists as a liquid-crystal within the temperature range I 13.7°-122.50c. Method The apparatus used consists of a calorimeter suspended in air within a heavy copper jacket. This jacket is maintained at a given temperature difference (constant to i. 0.01’)with respect to the calorimeter by means of a differential thermocouple, used in conjunction with an appropriate potentiometer, galvanometer and photoelectric relay-system. The rate of heat exchange between the calorimeter and its surrounding jacket a t any instant is a function of the two temperatures involved. Thus, q = KT (TJ- Tc),
(1)
where q is the number of calories flowing from the jacket to the calorimeter per minute, T j and T c are the respective temperatures of the jacket and calorimeter, and KT is the constant for Newton’s law of cooling a t the particular temperatures involved. We may also write the equation,
where C, is the heat capacity of the calorimeter and contents and dTc/dt is the rate of change of the temperature of the calorimeter per minute. Combining Equations I and z we then obtain,
Holder of the Shell Research Fellowship at Stanford University for the scholastic year 1930-31. Thomas and Parks: J. Phys. Chem., 35, 2031 (1931). ZParks, Huffman and Thomas: J. Am. Chem. SOC., 52, 1032; Huffman, Parks and Daniels: 1547 (1930); Parks and Anderson: 48, 150611926).
HEAT-CAPACITY DATA ON ORGANIC COMPOUNDS
883
If the rate dTc/dt is first determined when the calorimeter is filled with a material of known heat capacity (in this case various weights of metallic copper and water), the constant KTmay be evaluated for a range of calorimeter temperatures. Then, in turn, the heat capacities of the calorimeter and a second substance may be determined by measuring the corresponding values of dTc/dt when the calorimeter is filled with this substance. As the heat capacity of the calorimeter itself is known, the specific heat of the second substance can be readily calculated. Heats of fusion and transition may be determined by summing up the total heat input to the calorimeter as its temperature is raised from a point TI, just below the region of premelting, to a point TB,slightly above the melting point, and then subtracting the heat necessary to raise the temperature of the calorimeter and contents from TI to the melting point and from the melting point to TP.These latter quantities are calculated from the extrapolated specific heat curves.
where AHfuslon is the heat of fusion of the given sample and C, is the total heat capacity of the calorimeter and contents.
Materials Pentacosane and Tritriacontane.-These were fractions of paraffin wax, prepared in the Research Laboratory of the Standard Oil Company of Indiana.' Each sample was recrystallized three times from ethylene dichloride in our own laboratory. The melting points were: pentacosane, j3.4'; and tritriacontane, 71.o'C. Ethylbenzene.-This compound was synthesized by the Friedel-Crafts reaction in the Chemical Laboratory of Johns Hopkins University. I t was purified by three fractional distillations. The final product boiled at 135.6' and melted sharply at -95.1'C. Hexamethylbenzene, Diphenyl and Triphenylmethane.-These were relatively pure compounds obtained from the Eastman Kodak Company. I n all cases they were subjected to two or more fractional crystallizations from ethyl alcohol. The melting points of the final products were found to be: hexamethylbenzene, 165.jo; diphenyl, 68.3'; and triphenylmethane, 92.1'C. Naphthalene.-Kahlbaum's naphthalene was subjected to four fractional distillations. The unusually sharp melting curve shown by this material indicated that it was very pure. Dibenzoy1ethane.-This sample (melting point 145.4OC) was supplied to us in pure form by Professor Conant* of Harvard University. Buchler and Graves: Ind. Eng. Chem., 19, 718 (192;). SOC.,45, 1303 ( 1 9 2 3 ) .
* Conant and Lutz: J. Am. Chem.
884
MONROE E. SPAGHT,
s. BENSON THOMAS AND GEORGE 8. PARKS
Erythritol and Mannitol.-C. P. Pfanstiehl products with melting points and 166.ooC, respectively, were used without further purification. Ethyl azoxybenzoate.-A very pure sample of this substance was kindly loaned to us by Professor J. W. McBain. In our experiments we found the following melting points: crystalline solid to liquid-crystal I 13.7'; and liquidcrystal to liquid, 122.5'C. of
I 18.4'
Experimental Results Table I presents specific heat data for the solid and liquid stateg of the several compounds investigated (save ethylbenzene). I n each case a large number of individual determinations (fifty to one hundred) of the specific heats were made. From a plot of these results a smooth curve was then constructed, and from this the values given in the table were taken. In no case were the actual experimental points more than two per cent off this curve. The absolute accuracy of the tabulated mean values is believed to be within three per cent.
TABLEI Specific Heat Data (in calories per gram) Hexarnethylbenzene Crystals
Erythritol Crystals
Mannitol Crystals
Dibenzoylethane Crystals
,315
,380
,334
,321
,303
60
,332 ,350 ,367
'393 ,407 ,420
,345 '357 ,370
,331 ,341 '352
70 80
,385
M.P. '79.9'C
,383 ,396
90
,424
,434 ,448 .463
IO0
'432
.4i8
. 4 2I
'363 ,373 ,384 '394
,313 ,323 ,333 ,344
Temp. "C Na hthalene Zrystals
30 40
50
,408
,354 '344 ,374
Transition IIO
'
I20
.447
110.6" ,466
130 140
,455
,480
,462
'493
,683 ,688
150 160 170
,470 .47 j
507
,693
M . P . 166.5
,456
'485
'555
M.P. 166.0'
I80 190
,493
,566 .576
.720
.587
,723
200
440
,500
'
,434
M . P . 128.4'
,404 ,415
,384 ,395
'425
'405
,435 ,446
M . P . 145.4
,721
,506 '
509
,512
'51: ,518
HEAT-CAPACITY DATA ON ORGANIC COMPOUNDS
885
TABLE I (Continued) Temp. "C
EthylAzoxybenzoate Crystals
Triphenylmethane Crystals
Diphenyl Crystals
Pentacosane Crystals
Tritriacontane Crystals
30 40
,315 ,326
,302 ,314
,307 ,320
,464 ,483
50 60
'337
,328
,333
,453 ,468 M . P . 5S.4"
,348 ,358
'342 ,355
,369 ,380
~ 1 f . P92.1' .
'39'
,442
70 80 90 IO0 I10
116.2 I20
M.P. 71.0'
,520
,422
'
569
'572
,430 '438
.578 ,586
. 586
,401 '449 1st M.P. llS.7'
,579 '592
,471
2nd M.P.122.5' ,456
130
,472
140
,475 ,478
150
,368
,345
N . P . 68.3'
,501
'553 ,561
As an example of the results obtained, we have plotted our specific heat curves for solid and liquid naphthalene (Fig. I ) together with the values reported by Battelli,' Schlamp,* and A n d r e ~ s .Being ~ stable and easily purified, naphthalene serves as an excellent reference substance with which the results of different calorimetric methods may be compared. The specific heat data given for the solid states of pentacosane and tritriacontane were not determined in this investigation (because of the premelting which occurred within the temperature range studied), but were obtained by extrapolation of the low temperature results published by Parks, Huffman and tho ma^.^ The values given for the liquid states of these compounds are those found in this investigation. Using a special adiabatic calorimeter Williams and Daniels5 found an irregular curve for the heat capacity of liquid ethylbenzene in the temperature Measurements made for the purpose of investigating this range 20'-4o°C. reported irregularity show a perfectly smooth curve from 5' to 60°C. The actual values obtained check closely with those of Huffman, Parks, and Daniels,G who previously worked on the same material.
' Battelli: Atti. del reale instituto Veneto di scienze, lettere ed arti, 3, 1781 (1884).
* Schlamp: Ber. Oberhess. Ges. f. 3
Naturw. u. Heilk., 31, 1 0 0 (1895). Andrew, Lynn and Johnston: J. Am. Chem. SOC.,48, 1274 (1926). Parks, Huffman and Thomas: J. Am. Chem. SOC.,52, 1032 (1930). Williams and Daniels: J. Am. Chem. SOC.,46, 1569 (1924). Hutfman, Parks and Daniels: J. Am. Chem. SOC.,52, 1547 (1930).
886
MONROE E. SPAGHT, S. BENSON THOMAS AND GEORGE S. PARKS
FIG.I Specific heat curves for crystalline and liquid naphthalene
The heats of fusion determined for the several compounds are presented in Table IT. TABLEI1 Fusion and Transition Data Nature of change
Substance
Pentacosane Tritriacontane Hexamethylbenzene 1,
Diphenyl Triphenylmethane Naphthalene Dibenzoylethane Erythritol Mannitol Ethyl azoxybenzoate 1,
Fusion IJ
Transition Fusion J> 1J
,, ,f
3)
JJ
1st fusion 2nd ”
Temperature Heat effect ‘C (cal. per gram)
53.4 71.0 110.6 165 ’ 5 68.3 92. I 79,9 145.4 118.4 166.0 113.7 122.5
53.8 54.0 2.6 30.4 28.9 21.5
35.8 39.1 82.9 70.3 14.3 3.8
The results for pentacosane and tritriacontane, 53.8 and 54.0 calories per gram, respectively, are in very good agreement with the corresponding values of 53.5 and 54.0 calories per gram obtained by Parks and Todd,’ who used a method of mixtures. The value of 28.9 calories per gram found for diphenyl is in close agreement with that of Eykmann2 (28.5 calories). The heat of fusion of triphenylmethane was found to be 21.5 calories per gram. This Parks and Todd: Ind. Eng. Chem., 21, 1235 (1929).
* Eykmann: 2. physik.
Chem., 4, 518 (1889).
HEAT-CAPACITY DATA ON ORGANIC COMPOCNDS
887
FIG.2 Time-temperature curve of ethyl azoxybenzoate
is considerably higher than the value (17.8 calories) obtained by Hildebrand and his co-workers.' Their value, however, is probably low, owing to an inadequate allowance for the premelting of the sample. Hexamethylbenzene is of particular interest in that it undergoes a sharp crystalline transition a t 110.6OC. The heat of this transition was found to be 2.6 cal. per gram. I t appears that hexamethylbenzene is unusually prone to undergo crystalline changes, as Huffman, Parks, and Daniels previously found a similar transition at -165'C, which involved a heat effect of 1.5 cal. per gram.
FIG.3 The specific heat curves for the three states of ethyl azoxyhenzoate Hildebrand, Duschak, Foster and Beebe:
J. Am. Chem. SOC.,39,2293 (1917).
888
MONROE E. SPAGHT,
s.
BENSON THOMAS AND GEORGE
s.
PARKS
The heat of fusion of naphthalene is generally accepted as being close to 35.6 cal. per gram. The value of 35.8 obtained in this investigation thus agrees as well as can be expected, considering the limitations of the method employed. The two heats of fusion of ethyl azoxybenzoate represent the heat effect accompanying the transition from the solid state to an anisotropic liquid or liquid-crystal, 113.7’c, and that from the liquid-crystal state to the true isotropic liquid, 1 2 2 . 5 O C . Very little accurate thermal data have been obtained for liquid crystals, but our results are in general agreement with other direct measurements made on such compounds as well as with the values calculated from cryoscopic data.’ The thermal properties of substances that undergo complicated transitions are frequently more clearly shown by time-temperature curves than by the corresponding heat capacities and heats of fusion. Accordingly, in Fig. z we have shown the time-temperature curve for ethyl azoxybenzoate as it is slowly heated from the solid to the liquid-crystalline and true liquid states. Fig. 3, which is in effect derived from Fig. 2, presents the heat capacity curves for the three states. The single value calculated for the specific heat of the liquid-crystal is probably somewhat high, due to the close proximity of the two transition stages.
summary By use of a “radiation” calorimeter, the heats of fusion of nine organic compounds have been determined and the heat capacities of these substances have been measured in both the solid and liquid states. A short series of measurements on liquid ethylbenzene shows no 2. evidence of previously reported irregularities in the heat capacity curve. 3. Ethyl azoxybenzoate, a substance showing the liquid-crystal phenomenon, has been investigated over the temperature range 30’-I j0”C. I.
Department o j Chemzstry, Stanjord Universzty, Calzjornza. Oc!ober 27, 1,931. 1 For other data re arding h,-t effects with liquid crystals see: Dr. Rudolf Schenck: “Kristallinische Fliissigfceiten und 7ussige Kristalle,” 84 (1905); Amerio: Kuovo Cimento Vol. 2, Nov. Dec. 1901; Hulett: 2. physik. Chem., 28,645 (1899); Schenck: 2. physik. hem., 28, 285 (1899).
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