ENTHALPIES OF ALUMINUM OXIDE AKD SODIUM CHLORIDE
August, 1963
The evidence from the entropies of vaporization, surface tension, thermal conductivity, coefficient of thermal expansion, isothermal compressibility, sound velocity, and internal pressure indicates that ethylene glycol contains more hydrogen bonds per unit volume than does methanol. The n.m.r. hydroxyl proton shifts would also probably imply that the hydrogen bonds are not only more numerous in glycol but that they might perhaps also be somewhat stronger. Due to difference in size, shape, strength of hydrogen bond, and number of hydroxyl groups per molecule the average statistical configuration of methanol molecules in the pure liquid state will be different from the average statistical configuration of glycol molecules in the pure liquid state. When a small number of methanol molecules are added to pure glycol the methanol molecules must be incorporated into the glycol configuration. This accommodation could be accomplished by the methanol molecules hydrogen bonding to the oxygen atoms in glycol which are already involved in hydrogen bonds in glycol chains or rings, or by hydrogen bonding in one of two ways with the free hydroxyl groups a t the end of glycol chains. I n either case the average statistical configuration would be that of the glycol structure. The same argument could be applied to methanol solutions containing a small amount of glycol, except the average statistical configuration would be that characteristic of methanol. As the number of solute niolecules is increased in the solution the average statistical configuration of the molecules in the solution must change if the two molecular species differ in size, shape, hydrogen bond strengths, and number of hydroxyl groups per molecule. This new statistical configuration could be one in which the molecules of the two
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molecular species were more or less randomly distributed in the solution, or there could be a preference for each of the molecular species to prefer their own configuration environment thus leading to partial “clustering” of the two molecular species into two somewhat different average statistical configurations which coexist in the same solution. These configurations are to be viewed as an assemblage of molecules whose average configuration lifetime is longer than the time required for them to disperse and regroup with new members into a new but similiar configuration. The times required for the above processes could very well be of the order of a microsecond or less. When the “clustering” effect become very large it can lead to phase separation. Measurements of many different types indicate that there is partial “clustering” in solutions which contain alcohols dissolved in nonpolar solvents. The n.m.r. spectra of glycol-methanol solutions would seem to indicate that perhaps there is a tendency for partial “clustering,” especially in the concentration range of methanol between mole fractions of 0.5 to approximately 0.72. The excess thermodynamic properties imply that the binary system glycol-methanol is less stable than would be an ideal solution formed from the same two components, and that this decrease in stability is due both to an energy and entropy factor which make about equal contributions. Acknowledgment.-The partial financial support of this research by the Chemstrand Corporation is gratefully acknowledged. Appreciation is extended to Mr, W. D. Henderson for his construction of the calorimeter.
A HIGH TEnlPERATURE CALORIMETER ; THE ENTHALPIES OF a-ALUMINUM OXIDE AND SODIUM CHLORIDE BY RAPIERDAWSON, ELIZABETH B. BRACICETT,* AR’D THOMAS E. BRACKETT” Department of Chemastry, TVdliam Marsh Rice Unaversity, Houston, Texas Received February 4, 1963 The construction and operation of a high temperature calorimeter is described. The enthalpy of a sample of a-aluminum oxide furnished by the National Bureau of Standards has been measured from 750 to 1400°K. and found t o he about 0.3% higher than the enthalpy reported by the N.B.S. in the region in which it can be = compared. The enthalpy of sodium rhloride has been measured and found to fit the equations ( H T - H288.15) 10.452’ 0.002442Tz - 3334 cal./mole [600”K. < 2’ 6 1073.8”K.; i 0 . 2 % ] and ( H T - H298.16) = 25.242’ 0.00376T2 - 5307 cal./mole [1073.8”K. T < 1300’K.; &O.l%].
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