Aug., 1941
2267
THERMODYNAMICS OF DIMETHYL ETHER
ment of this surface equilibrium, the fact that the crystals are not of uniform composition when formed, as was mentioned above, introduces no difficulty. Indeed, even if the crystals were originally uniform, they would lose this uniformity during transition. At low sulfate concentrations, transition may take place to either the hexahydrate or the tetrahydrate. I t is of interest to consider which of these transitions will actually take place under given conditions. The hexahydrate is more stable than the tetrahydrate a t any temperature below that a t which the hexahydrate undergoes transition into the tetrahydrate. Consequently, if any hexahydrate is present, the transition is from decahydrate into hexahydrate, provided the concentration of sodium sulfate is not too high. In the absence of hexahydrate, the transition may be into the tetrahydrate, although this cannot readily be controlled. If any hexahydrate crystals have been in the room recently, it is almost impossible to prevent the hexahydrate from forming, as was mentioned in a different connection by Richards and Kelley. Sometimes tetrahydrate forms first, and then changes spontaneously to the hexahydrate. To obtain transition to the tetrahydrate, it is necessary to exclude completely any hexahydrate nuclei from the
[CONTRIBUTIONFROM
THE
atmosphere and to warm the system fairly rapidly past the temperature of transition into the hexahydrate. Under these conditions tetrahydrate may be obtained.
Summary 1. The transition of mixed crystals of sodium sulfate and sodium chromate decahydrate into hexahydrate, tetrahydrate or anhydrous crystals and solution has been investigated. The relation between temperature and the composition of the solution in equilibrium with decahydrate and another solid phase has been determined. 2. The existence of sodium sulfate hexahydrate in mixed crystals with sodium chromate hexahydrate has been established. Addition of sodium sulfate to sodium chromate hexahydrate lowers the temperature a t which hexahydrate, tetrahydrate and solution are in equilibrium. 3. When the concentration of sodium sulfate is low, the solid formed from the decahydrate by transition is usually mixed crystals of sodium chromate and sodium sulfate hexahydrate. Under some conditions the transition product is mixed tetrahydrate crystals. When the sodium sulfate concentration is high, the transition product is anhydrous sodium sulfate. HAVERFORD, PENNSYLVANIARECEIVED MARCH4, 1941 PHILADELPHIA, PENNSYLVANIA
SCHOOL O F CHEMISTRY AND PHYSICS O F
THE PENNSYLVANIA
STATE COLLEGE]
The Heat Capacity and Entropy, Heats of Fusion and Vaporization, and the Vapor Pressure of Dimethyl Ether. The Density of Gaseous Dimethyl Ether BY R. M. KENNEDY, MALCOLM SAGENKAHN AND J. G. ASTON The comparison of the gaseous heat capacity of dimethyl ether with that calculated from spectroscopic and molecular data has indicated a potential of 2500 cal. hindering the internal rotation of the methyl groups.' This paper presents the results of an investigation of the thermal properties of dimethyl ether from 14.13'K. to its normal boiling point which yields a value of 3100 j= 150 cal. for the hindering potential. This latter value is probably more nearly correct for reasons which will be presented presently. Preparation and Purification of Dimethyl Ether.Somewhat to our surprise, a preliminary and rather hasty preparation of dimethyl ether by dehydration of methyl alcohol over alumina gave a product which, for some reason (1) Kistiakowsky and Rice, J . Chem. Phys., 8, 618 (1940).
unknown, could not be purified readily by fractional distillation. On the other hand, the reaction of methyl iodide with sodium methylate gave a product which was purified readily to yield the purest organic compound yet prepared in this Laboratory. Four moles of methyl iodide was added to a solution of four moles of sodium in one liter of aldehyde-free alcohol, which had been distilled from magnesium, then from an equal atomic mixture of aluminum and zinc. The solution was contained in a 3-necked flask fitted with a mercury seal stirrer, a dropping funnel and a reflux condenser. The condenser outlet led to a carbon dioxide-snow trap through drying towers of calcium chloride and potassium hydroxide. The solution of sodium methylate was surrounded by a water-bath a t 25" and the methyl iodide was added over a period of three hours. The bath was heated to 75-80' to obtain a reasonable rate of gas evolution. When the rate of gas evolution slowed down, the reaction was discontinued. The yield was approximately 180-190 g.
The dimethyl ether was finally dried by passing the gas over activated alumina on glass wool in a tube 25 cm. long a t the rate of one mole per hour. This operation was repeated after reactivating the alumina and the product finally distilled through the laboratory low temperature column (twelve theoretical plates). Two fractions of 30 and 40 g. were arbitrarily taken from the middle of a 100g. portion boiling over a negligible range. The first of these samples together with about one-third of the second contained less than one part in 1,000,000 of solid-insoluble, liquid-soluble impurity as estimated from the freezing point curve and the premelting heat capacity.
The Heat Capacity Measurements.-The apparatus, method and temperature scale were as already d e s ~ r i b e d . ~The ? ~ heat capacity measurements are listed in Table I and plotted in Fig. 1. The temperature rises can be estimated TABLE I THEHEATCAPACITY OF DIMETHYL ETHER Mol. wt. 46.069: 0.68487 moles in calorimeter: O'C. = 273.16 OK. Temp., O K .
VOl. 63
R. M. KENNEDY, MALCOLM SAGENKAHN AND J. G. ASTON
2268
CP
cal./deg(/mole
13.70 15.99 18.69 21.15 24.27 28.20 32.30 36.18 39.49 43 . 02 47.98 53.88 60.00
Series I11 0.777 1.164 1.714 2.257 2.903 3.679 4.563 5.302 5.929 6.496 7.311 8.159 8.967
56.48 61.87 68.34 74.14 79.68 85.34 91.29 96.93 102.70 108.71 116.45 122.12 127.75
Series I 8.531 9,279 10.102 10.790 11.432 12.009 12.62 13.18 13.'79 14.39 15.11 15.60 16.12
137.18 142.70 148.75 155.04
Series TI 23.45 23.48 23.50 23.56
CP,
Temp., OK.
cal./deg./mole
173.98 180.03 185.89 191.62 202.89 212.79 217.96 223.29 228.73 231.33 240.28 245.48
23.52 23.51 23.51 23.52 23.65 23.83 23.93 24.01 21.12 21.15 24.54 24.65
148.57 167.86 196.99 207.58
Series IV 23.40 23.53 23.63 23.69
129.31
Series V 17.40
162.92 164.92 169.44
Series V I 23.51 23.53 23.51
Series VI1 186.18 23.68 190.87 23.70 196.30 23.73 200.63 23.77 205.59 23.83 172.95 23.56 178.97 23.61 210.51 23.88
(2) Aston and Messerly, THTSJ O ~ J R N A L .68, 2834 (19%) ( 3 ) htrsscrly and Anton, ;bid,, 62, 8%; (1940).
from the intervals between points in a series. In Table I1 is listed the heat capacity a t round values of the temperature. One defined calorie was taken equal to 4.1833 international joules. Corrections to the heat capacities for vaporization into the filling line were made using the density of the liquid given by Maass and Boomera4 TABLEI1 THE MOLAL HEATCAPACITY OF DIMETHYL ETHER C p , cal./'K.
T , OK.
I3 15 17 20 25 30 35 40 50 60 70 80 90 100 110 120 130
C p , cal./'K.
I', OK.
Solid
Liquid
0.5
