I704
Vol. GS
DONALD R. DOUSLIN AND HUGHM. HUFFMAN
7. The reaction rate was retarded by increased viscosity. S. The reaction is composed of two separate. zero order reactions, the initial reaction being par tially chemically and diffusion controlled, and the later reaction being definitely diffusion controlled.
9. Thermodynamic considerations and influence of ionic strength, varied by chloride ion dddition, indicate that the rate controlling step for the initial part of the reaction is OH- 4OHCuo = X-.
+
RECEI\FU ~ I A K C2.3, I I 19Uj
~ ‘ r ~ r c 4 r , r11r . I I,\OIS
[CONTRIBUTION FROM 1 HC UURIXU OF ,\1ISES, PLCrROLLUhf ESPERIMENTAL STATIOS
Low-Temperature Thermal Data on the Five Isomeric Hexanes1 BY
DONALD R. DOUSLIN? AND HUGHhf.
The Bureau of Mines is carrying on a research program to obtain precise and accurate values of the thermodynamic constants of petroleum hydrocarbons and related compounds. In this paper are presented the results of low-tcmperature thermal studies on the five isomeric hexanes. All of these compounds have been studied by other workers. Parks, Huffman and Thomas4and Parks, Huffman and Barmorej studied n-hexane over the templerature range 90 to 300” K. Stul16 has also made a superficial investigation of all of the hexanes over the temperature range 90 to 320’ K. More recently Pitzer and Kilpatrick7 have made a careful study of 2,2-dimethylbutane over the temperature range 13 to 280” K. This duplication again affords an opportunity to compare results from (different) laboratories which claim high accuracy for thcir i i i e a ~ ~ r c i i ~ e n t s . Experimental The Materials.--The hydrocarbons used in this investigation were A. P. I.-N. B. S. “Best” samples purified by the A. P. I. Research Project 6 a t the National Bureau of Standardss and certified by them in regard to their purity. In the courw of the measurements the melting compounds have been studied ni conditions in the usual manner.9 (1) Published b y permishioil c i l t h e Uirrctur of the Bure;ru c t f Mines, C . S. Depar!mcnt 01 the Interior. S o t copyrighted. (2) Associate Physical Chemist, 13iireau of Mines, Petroleum Bxperiment Station, Bartlesville, Okla. (3) Principal Physical Chemist, Bureau of Mines, Petroleum Experiment Station, Bartlesville, Okla. (4) Parks, Huffman and Thomas, THISJOURWAL, 52, 1032 (1030). (5) Parks, Huffman and Barmore, ihid., 53, 3876 (1031). (6) Stull, ibid., 59, 2726 (1037). (7) Kilpatrick and Pitaer, ibid.,68, 1066 (1946). (8) These samples of A.P.I.-S.Ij.S. hydrocarbons have been made available by the American Petroleum Institute and the National Bureau of Standard!; through the A. P. I. Research Project 44 on the “Collection, analysis, and calculation of data on the properties of hydrocarbons.” The samples were purified a t the National Bureau of Standards by the A. P. I. Research Project 6 on the “Analysis, purification and properties of hydrocarbons,” under the supervision of Frederick D. Rcssini, from material supplied by the following laboratories: n-hexane, 2-methylpentane, 3-methylpentdne, and 2,%dimethylbutane, by the A . P. I. Research Project 6 on the “Analysis, lrurification a n d properties of hydrocarbons,” a t the National Rureau of Standards; 2,3-dimethylbutane, by the Standard Oil Compnuy (Indiana), D‘hiting, Indiana, nnd RZ. W. Kcllogg Cvmpariy, New York, N,Y. (‘J) Douslin mil Hulfman, ibid., 68, 173 (1‘346).
HUFFMAN3
Unfortunately i t was impossible to crystallize 3inethylpentane, so its purity could not be checked by the freezing-point method. The experimental and certain derived data for four of these compounds are surnniarized in Table I. ’I‘AULE I M I ~ L T I XPOINT G SUXMARY 0°C. = 273.16’ K.
-
7
% Xelted
T ,O R . A -
Obs.
Calcd.
T , OK. _-*__?
Melted
Obs.
Calcd.
n-Hexane, 2-Methylpentatie A\Tx = 0.0498 AT AVX = 0.0527 AT 9.3 177.82n2 177.8202 11.4 119.5075 119.504 26.1 ,8307 ,8307 2 4 . 2 ,5300 ,529 54.2 ,8336 ,5336 4 8 . 2 ,5405 ,541 77.4 ,8346 ,5345 72.2 ,5445 .544 8 8 . ii , 83-+ti ,8347 89.6 ,5465 ,546 ion. o ... ,8349 100.0 ... .546 ... ,8364 Pure ... ,552 Pure Triple p t . 177.84 * 0.05’K. Impurity 0.0075 nioIe7; 2,3-Ditnethylbutane iVx 0.00458 AT 22 145.058 145.038 ,118 ,117 47 58 ,129 ,130 90 ,150 .Id0 100 ... ,154 ... ,186 Pure Triple pt. 145.19 j=0.05”K. itiipurity 0.015 n ~ o l e ~ ~
Triplept. 119.55 * 0.05’K. Impurity 0.029 moley, 2,Z-Diinethvlbutane 1% 0.00229 A T 27.3 174.095 174.035 51.8 .159 ,150 77.4 .193 .193 91.1 ,206 ,206 100.0 ... .212 Pure ... ,279 Triplept. 174.28 * 0.05”K. Impurity 0.015 mole%
A study of these data indicate,s that 2,3-diinethylbutane and 2,2-dimethylbutane do not obey Kaoult’s law over the entire range of composition (liquid-solid) studied. The differences between T(obs) and T(calcd) are much greater than the probable uncertainty in the temperature nieasurements. Both compounds have extremely low heats of fusion and consequently suffer large depressions in the melting point for sniall amounts of impurity, thus snaking the nieasurements more sensitive to deviations from ideal behavior. It is also possible that these measurements may indicate that solid solutions are formed. The calculated triple point temperatures and impurity are
LOW-TEMPERATURE THERMAL DATAON FIVEISOMERIC HEXANES
Sept., 1946
therefore more or less arbitrary. It is quite unlikely that this uncertainty will have a very large effect on the thermal data. The materials were placed in a glass container connected to a high vacuum system and were alternately frozen and melted i n vacuo t o remove any dissolved gases. They were then distilled into the copper Calorimeter, which was joined to the glass system by means of a Housekeeper seal. l%rhenthe calorimeter was full, the small (1.0 mm. 0.d.) filling tube was pinched off and immediately closed with a drop of soft solder. The Apparatus.-The measurements were made in the a.pparatus described by Ruehrwein and HufYman,lowhich was loaned to the Bureau of Mines by the California Institute of Technology. Very briefly, the method is as follows: The material under investigation was contained in a sealed copper calorimeter, which was mounted in the adiabatic calorimetric system. A measured amount ‘of electrical energy was supplied to the calorimeter, and a t all times the temperature of the environment was maintained a t that of the calorimeter to prevent heat interchange. The initial and final temperatures of the calorimeter were measured by means of a platinum resistance thermometer. The electrical measurements required for the determination of the resistance of the thermometer and for the electrical energy were made on a “1Vi’hite’’double potentiometer in conjunction with :ihigh-sensitivity galvanometer and accurately calibrated resistances. The potential was in terms of a bank of-six saturated cadmiuni cells which had been certified by the National Bureau of Standards. Time nieasurenients were made with :ti1 electric stop clock, which was frequently coriipared against a stop watch. The measurements was, in general, , and above 30’ K . it is believed the accuracy uncertainty should not be greater than 0.2% in this temperature range. The energy measurements were made in terms of the international joule and were converted to calories by dividing by 4lS33. 13xperimental Results One of these compounds, 2-methylpentane, could be obtained either in the crystalline or glassy state. Measurements were made on both forms from h.ydrogen temperatures to slightly below the nie’lting point. I t was not possible to crystallize %methylpentane by any oi’ the methods used. Measurements were made on the glass and liquid from approximately 48’ E(.to room temperature. The results of the specific-heat measurements on these five compounds are listed in Table 11. In Table 111 are listed the values of the specific heats a t integral temperatures as selected from ljmooth curve:; drawn through all of the ‘data. Due to the relatively large uncertainties in the (10) Kue:irwc,iri
;!tirl
Huffniiiu, ‘rms
JOURNAI.,
66, 1620 (1943).
1705
TABLE I1 T H E hfOLAL HEAT CAPACITP
017
THE EIEXANES
0 “C. = 573.16” K., mol. n-t, = 84.17fi T,
OK.
AT
C,
T,
OK.
AT
+Hexane 13.0s
18.24 19,I)fi 23. 20 23.48 27.82 28.90 32.53 34.72 39.57 44.95 31 . 30
