The thermodynamic equation (aCp/BP), = -T(aZV/aT2), is used as a basis for relating thermal expansion to structure (at several temperatures and pressures) of water and "normal" liquids. Similar considerations lead to a relation between the sign of (aZTzo/aTZ)p for solutions and classification of solutes as structure-making or structure-breaking. Canadian Journal of Chemistry, 47, 4613 (1969)A variety of evidence has led to the view that water is a partly "structured" liquid that can be loosely described as a sort of partly broken down ice. Possibly the simplest useful description of liquid water, is that in which the water is pictured as being a mixture of two species; one a bulky icelike species with relatively low density and the other a denser species that is formed by breaking (or bending) the hydrogen bonds that maintain the ice-likeness of the bulky species. Since both increasing temperature and increasing pressure would have the effect of increasing the fraction of the dense species at the expense of the bulky species, this simple model can account qualitatively for the maximum in density, the unusually high heat capacity, and several other properties of water. Some of the properties of aqueous solutions have also been interpreted in terms of the "structure-making" or "structure-breaking" characteristics of various solutes. This paper is concerned with the relation between these structural ideas and the thermal expansion of water and of aqueous solutions.Partial molal heat capacities of aqueous electrolytes are commonly negative. An early explanation (for example, Lewis and Randall (1)) that is consistent with other properties of aqueous electrolytes is that these solutes break up the bulky aggregates and thereby decrease the effective heat capacity of the water. Since increasing pressure would also break up the bulky aggregates, the same reasoning suggests that the heat capacity of pure water should decrease with increasing pressure. We therefore expect (aCp/ aP), to be negative and use the thermodynamic equation in predicting that (a2 V/aT2), for water is positive. Because increasing temperature also breaks up the bulky aggregates, we expect that the magnitude of (aC,/BP), should decrease with increasing temperature, which requires that T(a2 V/aT2), should also decrease with increasing temperature. Similarly, we expect that T(a2 v/aT2), should decrease with increasing pressure. Since a variety of evidence suggests that D 2 0 is a more structured liquid than is H 2 0 , we also expect that the magnitude of (aC,/aP), and thence T(a2 v/aT2), would be larger for D 2 0 than for H 2 0 .Kell (2) has tabulated values of density (p) and coefficient of thermal expansion (a = (I/V)(aV/aT),) for H20(liquid) at 1 atm. over a range of temperatures. These values have been used with the equation in which M represents the molecular weight, to obtain values of (a2 V/aT2), and thence thevalues of T(a2 V/aT2), that are shown in Fig. 1. Densities of D20(liquid) tabulated by Kell (2) The curves in Fig. 1 show the predicted d...

No abstract

. Can. J. Chem. 53, 3841 (1975).We have made atomic absorption measurements leading to the solubility of zinc hydroxide at 12.5,25.0,50.0, and 75.0 "C from p H = 6.7 to p H = 13.8. Results have been used for evaluation of the solubility product and stepwise constants for association of ZnZ+(aq) with OH-(aq) to form solute species of type Zn(OH),Z-'(aq) with i = 1 4 . Evidence is presented to support the reliability of the various equilibrium constants. Uncertainties in derived enthalpies are assessed. RANDY A. REICME, KEITH G. MCCURDY et LOREN G. HEPLER. Can. J. Chem. 53,3841 (1975).On a fait des mesures d'absorption atomique conduisant A la solubilitk de I'hydroxyde de zinc A 12.5, 25.0, 50.0 et 75.0 "C, B des p H = 6.7 B 13.8. On a utilisk les rksultats pour evaluer le produit de solubilitk et les constantes successives pour I'association de ZnZ+(aq) avec OH-(aq) pour former des espkces solutks du type Zn(OH),2-'(aq) avec i = 1 4 . On prksente des donnkes pour supporter la fiabilitk de diverses constantes d'kquilibre. On ktablit les incertitudes dans les enthalpies qui en dbivent.[Traduit par le journal] Introduction It is well established that zinc hydroxide is slightly soluble in water, becoming more soluble as the p H is either lowered or raised. There are, however, substantial differences between solubilities at 25 "C reported by various investigators and we know of no solubility measurements that have covered a substantial range of temperature. As a result of these limitations on the available solubility data there are related uncertainties in the ' equilibrium constants for association of zn2+(aq) with OH-(aq) and in the solubility product of zinc hydroxide. We have therefore measured (by atomic absorption spectrophotometry) the solubility of zinc hydroxide at 12.5,25.0, 50.0, and 75.0 "C from p H = 6.7 to p H = 13.8. Results have been used for evaluation of the solubility product and stepwise equilibrium constants for association of Zn2+(aq) with OH-(aq).

Densities of completely miscible (water + methyldiethanolamine) and (water + ethyldiethanolamine) systems have been measured over the full range of compositions at temperatures from 25 to 80°C. Results of these measurements have been used in calculating excess molar volumes and partial molar volumes of each component. We have also identified different measures of the thermal expansion of these systems and have calculated some of these derivative quantities. The partial molar volumes and their derivatives with respect to temperature provide a basis for extending our understanding of molecular interactions in these (water + organic) systems.

No abstract