Ionic entropies solvation

Spectroscopic methods, molten salts, 702 Spectroscopy detection of stmctnral nnits in liquid silicates, 747 and structure near an ion, 72 Standard partial gram ionic entropies, absolute, II Thermodynamics, applied to heats of solvation, 51 of ions in solution, 55 Time average positions of water near ions. 163 Tools, for investigating solvation, 50 Transformation, chemical, involving electrons, 8 Transition metals... 

As in the case of heats and heat capacities, ionic entropies have been examined more extensively in methanol and methanol-water mixtures than in other solvents. The earliest study was that of Latimer and Slansky who report entropies for NaCl, KCI, KBr, and HCl in water-methanol mixture from 0 to 100% methanol. Jakuszewski and Taniewska-Osinska " have determined the entropies of numerous halide salts and HCl at 25 C, and more recently Franks and Reid, using data from the literature, have calculated standard ionic entropies for several species in water-methanol mixtures covering the whole concentration range. It should be observed that these authors chose the mol fraction standard state for the solute in solution and ideal ionic gas as the standard state for the pure solute instead of the conventional hypothetical one molal solution and pure solid at OK. The former standard states are convenient when comparing entropies of solvation for the various species. Ionic entropies in methanol are considerably more negative than in water. 

Partial molal entropy data in ethanol are nearly as sparse as the heat capacity data. The only comprehensive entropy data in this solvent are those of Jakuszewski and Taniewska-Osinska, who report 5 for HCl and several alkali metal halides in ethanol. Ionic entropies have been calculated for the alkali metals from free energies and enthalpies of solvation, but since extra-thermodynamic assumptions were necessary, the meaning of the values is questionable. Ionic entropies in ethanol are somewhat more negative than in methanol and considerably more negative than in water. 

Born Treatment. Several attempts have been made to evaluate absolute ionic entropies by means of the Born equation (eqn. 2.11.18) or the modified Born equation. The method consists mainly of employing the temperature coefficient of the ionic free energies of solvation as discussed in sect. 2.11.2 to obtain the entropy of solvation. The entropy of the individual gaseous ions can be calculated by the methods also discussed in sect. 2.11.2 and consequently ionic entropies evaluated. 

In an extension of these ideas, Franks and Reid have examined the ionic entropies in mixed water-methanol solutions (see Appendix 2.4.42) and in 20 % aqueous dioxan, and have observed that the entropies of the ions for each of these systems can be expressed by an equation having the same form as eqn. 2.11.36. Similar to the pure solvent systems, the entropy of a given ion has no correlation with the solvent dielectric constant, nor is there a linear correlation of the entropy with solvent composition. Instead, the entropies reach a maximum in the vicinity of 40 mol per cent methanol. The authors explain this in terms of the solvent having the highest degree of structure near this composition. As in the pure non-aqueous solvents, the relative magnitude of the effect of ions on the solvent structure is the same for all ions, both negative and positive. This observation led the authors to conclude that there is no evidence for preferential solvation in these mixed solvent systems. 

A further method to obtain solvation numbers depends on the immobilization of the solvent molecules near the ions as derived from the molar ionic entropies of solvation, (Section 4.3.2.3). The method proposed by Marcus [23] specifies... 

Similar observations hold for solubility. Predominandy ionic halides tend to dissolve in polar, coordinating solvents of high dielectric constant, the precise solubility being dictated by the balance between lattice energies and solvation energies of the ions, on the one hand, and on entropy changes involved in dissolution of the crystal lattice, solvation of the ions and modification of the solvent structure, on the other [AG(cryst->-saturated soln) = 0 = A/7 -TA5]. For a given cation (e.g. K, Ca +) solubility in water typically follows the sequence... 

In cases where the solvation energies are large, as for example when ionic compounds dissolve in water, these hydrophobic effects, based on adverse changes in entropy, are swamped. Dissolving such compounds can be readily accomplished due to the very large energies released when the ions become hydrated. 

One more difference between ionic liqnids and conventional organic solvents of the C-Cl cleavage should be mentioned. Normally, the intrinsic solvation of the developing halide ion disfavors the cleavage via the entropy term. Such a term cannot be significant in the ionic liquids composed of very bulky cations and anions. 

There are other close-range forces related to entropy changes, including various interactions between solution species and a solid surface, such as solvation (in water, hydration) forces. Hydration forces can occur when hydrated cations are adsorbed at interacting surfaces. As these surfaces approach each other closely, loss of water of hydration is necessary in order to allow closer approach. While these forces can be repulsive, attractive or oscillating, they are most likely to be repulsive under the conditions of CD. Such forces may be very important for CD, which is almost always carried out in the presence of a high ionic concentration. For example they could be a cause of poor adhesion of some CD films. Solvation forces are treated in detail in Israelachvili s book—see Further Reading at the end of this chapter, Forces subsection. 

What values might we expect for AHsoln and ASsoin Let s take the entropy change first. Entropies of solution are usually positive because molecular randomness usually increases during dissolution +43.4 J/(K mol) for NaCl in water, for example. When a solid dissolves in a liquid, randomness increases on going from a well-ordered crystal to a less-ordered state in which solvated ions or molecules are able to move freely in solution. When one liquid dissolves in another, randomness increases as the different molecules intermingle (Figure 11.2). Table 11.2 lists values of ASsoin for some common ionic substances. 

Partial molar entropies of ions can, for example, be calculated assuming S (H+) = 0. Alternatively, because K+ and Cl ions are isoelectronic and have similar radii, the ionic properties of these ions in solution can be equated, e.g. analysis of B-viscosity coefficients (Gurney, 1953). In other cases, a particular theoretical treatment which relates solvation parameters to ionic radii indicates how the subdivision could be made. For example, the Bom equation requires that AGf (ion) be proportional to the reciprocal of the ionic radius (Friedman and Krishnan, 1973b). However, this approach involves new problems associated with the definition of ionic radius (Stem and Amis, 1959). In another approach to this problem, the properties of a series of salts in solution are plotted in such a way that the value for a common ion is obtained as the intercept. For example, when the partial molar volumes of some alkylammonium iodides, V (R4N+I ) in water (Millero, 1971) are plotted against the relative molecular mass of the cation, M+, the intercept at M + = 0 is equated to Ve (I-) (Conway et al., 1966). This procedure has been used to... 

Several methods involve a study of the properties of solutions in equilibrium and are hence reasonably described as thermodynamic. These methods usually involve thermal measurements, as with the heat and entropy of solvation. Partial molar volume, compressibility, ionic activity, and dielectric measurements can make contributions to solvation studies and are in this group. 

Solvation entropy changes are large if the reactions involve ionic charges. If opposite electric charges are created the contribution to the... 

The present theories of the effects of solvents on the rates of polar (ionic) reactions do not permit a quantitative analysis of the above cited results. Fractions of AHp and ASp, due to solvation, apparently compensate each other, because the increase in the energy needed for desolvation is just compensated by equal contributions of the entropy (a more firmly bound or larger number of molecules are desol-vated). This compensation phenomenon is well known in organic chemistry. For instance, the difference between the activation enthalpies (AAH ) of the reaction of benzyl chloride with pyridine in DMF and CH3OH is equal to -5.3 kcal mole". ... 

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