THE! ENTROPY OF SOLUTION OF HEXANE WITH HEXADECANE J. H. HILDEBRAND AND J. W. SWENY Department of Chemistry, University of California, Berkeley, Calijornia Received December 81, 1938
I n a section of the contribution' by the senior author to the Symposium on Intermolecular Action some experimental figures were presented indicating that normal hexane and normal hexadecane form ideal solutions in accordance with an earlier prediction. At the same time mention was made of further experimental verifications then under way. This work has now been completed and is reported here. Since hexane and hexadecane must be synthesized h order to guarantee adequate purity and are rather expensive, it was desirable to use a method requiring but small quantities. We had small samples of these hydrocarbons, which had been obtained from the Eastman Kodalr Company. The vapor pressure of hexadecane is so minute at room temperatures that it is unnecessary to determine the composition of the vapor phase over the solution, the vapor pressure being due solely to hexane. Measurements were carried out in the tensimeter illustrated in figure 1. The calibrated tube, A, contains pure hexane. The amount present in the tube at any one time is determined by a cathetometer reading of the distance of the meniscus above the mark etched in the glass while the tube A is immersed in a bath of known temperature. The bulb, B, contains a known amount of hexadecane introduced from a weight pipet prior to sealing. Both components are frozen by the aid of liquid air, the mercury serving as a manometer is poured into the bulb, C, and the apparatus is evacuated through the stopcock. The mercury is then poured down into the triple manometer stems, and both substances are melted. A small afhount of air is of course present in both hydrocarbons, but it is largely expelled on freezing and escapes as small bubbles during the subsequent melting. This freezing and melting is repeated several times, after which the hydrocarbons are finally frozen and the apparatus reevacuated. A certain amount of hexane is then distilled from A into B, giving a solution whose composition is determined by freezing both components, pouring the mercury into the manometer, melting the hexane, 'J. Phys. Chem. 43, 109 (1939). 297
298
J. H. HILDEBRAND AND J. W. SWENY
and determining the new height of its meniscus. Portions A and B of the apparatus are then placed in a large thermostat,, the manometer portion being outside in a glass-enclosed box maintained ~t somewhat higher temperature in order to prevent distillation of tht? hexane onto the mercury. Readings of the three mercury columns permit the simultaneous determination of the vapor pressures of hexane over the solution and over the pure liquid. Since these two pressures will normally have about the same temperature coefficients in the case of regular solutions and, in the present system exactly the same coefficient, extreme accuracy in temperature control is unnecessary. Equilibrium is attained so quickly that the
FIG.1. The tensimeter
well-insulated thermostat remains constant in temperature within 0.002°C. over periods of time far in excess of the period of a single measurement. A number of different compositions can be investigated with a single filling of the apparatus simply by repeating the distillation in either direction as desired. The results are given in table 1 and plotted in figure 2. Series 1 represents the results previously reported, obtained by another method. It will be seen, first, that the ratio of the vapor pressure of hexane from the solution to that from the pure liquid, p / p o , agrees with its mole fraction, N , within what is doubtless the experimental error. This agreement is closest for the second series of measurements, where the manipulation
TABLE 1 N
= 1.025 0.989
YOLB PRAC'ION BEXANI
-___ 1
2
3
P
'C.
mm.
mm.
25.0 25.0 25.0
106.2 99.6 92.7
149.7 149.7 149.7
0.708 0.665 0.619
0.708 0.648 0.626
25.00 25.00 25.00 25.00 25.00
111.3 111.7 111.7 66.8 66.9
150.4 150.7 150.7 150.9 150.9
0.740 0.742 0.742 0.443 0.443
0.738 0.738 0.738 0.430 0.430
1.002 1.004
25.01 25.01 25.01 25.01 25.00 25.00 24.8 25.02 25.01 25.01 14 .O 17.8 19.8 20.0 23.2 23.4 25.00 25.00 25.00
102.5 102.6 102.5 102.8 96.5 96.5 72.9 74.3 73.7 73.6 44.7 24.0 27.7 28.0 31.9 31.9 33.3 33.6 33.7
155.3 154.9 154.7 154.7 153.0 153.0 153.3 155.6 155.8 155.8 91.6 110.4 123.9 125.2 143.0 144.2 154.7 156.0 157.0
0.660 0.662 0.663 0.664 0.631 0.631 0.476 0.478 0.473 0.473 0.489 0.217 0.223 0.224 0.223 0.221 0.215 0.215 0.215
0.663 0.663 0.663 0.663 0.635 0.635 0.478 0.478 0.478 0.478 0.478 0.222
0.996 0.998
0.222
0.222 0.222 0.222 0.222 0.222 0.222
1.000
1.004
1.029 1.029
.ooo
1
1.002 0.994 0.994 0.996 1 .ooo 0.989 0.989 1.023
0.978 1.003 1.008 1.003 0.996 0.969 0.969 0.969
Mole FractioQ N
FIQ.2. Relative vapor pressures of hexane from solutions with hexadecane 299
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J. E. EILDEBRAND AND J. W. SWENY
was carried out with the greatest care. The small discrepancies between the values of p o at 25.00"C. may be attributed mainly to the use of different samples of hexane and should have but little effect upon the ratio, p / p o . It will be seen, also, that the ratio p / p o is practically independent of temperature, a further and most direct evidence that the entropy of solution follows Raoult's law. This system, therefore, corresponds excellently with the model used in analyzing the possible configurations in a system of linear molecules of different lengths in parallel arrangement.
