Aqueous ions, heat capacities

Pitzer (1973) developed a semi-empirical equation (ion-interaction model) to reproduce accurately the volumetric properties of aqueous electrolyte solutions. This model has been used to calculate accurately other thermodynamic properties such as expansivity, compressibility, free energy, enthalpy, and heat capacity. The ion-interaction model... 

Kakihana, H. and Maeda, M. (1970) The hydrolysis of the beryllium ion in heavy water. Bull. Chem. Soc. Jpn., 43, 109—113. Kakihana, H. and Sillen, L.G. (1956) Studies on the hydrolysis of metal ions. XVI. The hydrolysis of the beryllium ion, Be ". Acta Chem. Scand., 10, 985 — 1005. Khodakovskiy, I.L., Ryzhenko, B.N., and Naumov, G.B. (1968) Thermodynamics of aqueous electrolyte solutions at elevated temperatures (temperature dependence of the heat capacities of ions in aqueous solution). Geokhimiya, 12, 1200—1219 (English translation). 

Table 9.2 Standard heat capacities, entropies, enthalpies, and Gibbs free energies of formation of some common ions in aqueous solution at T= 298.15 K...

Allred G. C. and Woolley E. M. (1981). Heat capacities of aqueous acetic acid, sodium acetate, ammonia, and ammonium chloride at 283.15 K, 298.15 K, and 313.15 K ACfp for ionization of acetic acid and for dissociation of ammonium ion. J. Soln. Chem., 14 549-560. 

Heat capacity data for ions in aqueous solution over the temperature range 25-200°C. Such data for ionic species of uranium, plutonium, other actinides and various fission products such as cesium, strontium, iodine, technetium, and others are of foremost interest. 

Other properties of aqueous solutions where discontinuities were observed were molar volumes, heat capacities and viscosities.210-212 From all these pieces of evidence, the general opinion was that n in [M(H20) ]3+(aq) was nine for the lighter lanthanide ions, but eight for the heavier,213 though some workers were of the opinion that n did not change along the series.214 Solution X-ray studies have provided evidence for an average coordination number of 8.9 for Nd3+(aq),215 while neutron diffraction supports a value of 8.5 for the same ion.216... 

The bare proton has an exceedingly small diameter compared with other cations, and hence has a high polarising ability, and readily forms a bond with an atom possessing a lone pair of electrons. In aqueous solution the proton exists as the H30+ ion. The existence of the H30+ ion in the gas phase has been shown by mass spectrometry [4], and its existence in crystalline nitric acid has been shown by NMR [5], Its existence in aqueous acid solution may be inferred from a comparison of the thermodynamic properties of HC1 and LiCl [6]. The heat of hydration of HC1 is 136 kcal mole"1 greater than that of LiCl, showing that a strong chemical bond is formed between the proton and the solvent, whereas the molar heat capacity, molar volume and activity coefficients are similar,... 

The thermodynamic properties of aqueous species are for a one-molal concentration of the species (1 mol/kg of solvent water), which, for relatively insoluble species, may be strictly hypothetical. The thermodynamic properties of dissolved ionic species are based on the assumption that the heat capacity, entropy, A/// and AGf of the hydrogen ion [H+(aq)], all equal zero at all temperatures and pressures in other words, it is assumed that AG/ = AH/ = S° 0 for the hydrogen ion and that AG ° = AH/ = AS/ = 0 for the reaction... 

Several studies of the physical properties of sulphates have been carried out these will not be treated in detail but are listed as follows a determination of the dissociation constants of some univalent sulphate ion-pairs,the electrostriction of ammonium sulphate, dielectric and n.m.r. investigations of phase transitions in lithium ammonium sulphate, the surface structure of barium sulphate crystals in aqueous solution, optical activity and the electro-optical effect in crystals of Cd2(NH4)2(S04)3, apparent molal volumes and heat capacities of Na2S04, K2SO4, and MgS04 in water, and densities, heats of fusion, and refractive indices of double sulphates of univalent metals. ... 

The heat capacity change of "iso-Coulombic" reactions (reactions which are symmetrical with respect to the number of ions of each charge type) is nearly independent of temperature (13,14). Similarly, the molar volume change of "iso-Coulombic reactions will be expected to display a relatively minor temperature dependence because most of the temperature-dependent changes in the Coulombic and non-Coulombic contributions to the volumes of individual ions will cancel out in the AV term. Thus, the 25°C value of AV can be used in Equation 9 if the reaction is "iso-Coulombic", or is made so by the addition or subtraction of an appropriate number of water dissociation reactions. For example, the dissociation reaction of the aqueous complex HjCO can be written in the "iso-Coulombic" form (Reaction 12) by combining Reactions 10 and 11 ... 

This table contains standard state thermodynamic properties of positive and negative ions in aqueous solution. It includes en-thcdpy and Gibbs energy of formation, entropy, and heat capacity, and thus serves as a companion to the preceding table, Standard Thermodynamic Properties of Chemical Substances . The standard state is the hypothetical ideal solution with molality m = 1 mol/kg (mean ionic molality in the case of a species which is assumed to dissociate at infinite dilution). Further details on conventions may be found in Reference 1. 

The analysis of the results of [1997HOV] in Appendix A is essentially that from [2003GU1/FAN], as these results served as basis for the adoption of new values for the heat capacity of the and aqueous ions. 

The standard partial molal heat capacity of electrolytes, is probably the best thermodynamic property for providing information on ion-solvent interactions, but until very recently only two publications reported this quantity for electrolytes in non-aqueous systems. The reasons for the scarcity of data become obvious when one considers the formidable experimental and theoretical problems in obtaining this function. 

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