Entropies of hydration

As is suggested frequently , this term might well result from the restriction of the hydrogen bonding possibilities experienced by the water molecules in the first hydration shell. For each individual water molecule this is probably a relatively small effect, but due to the small size of the water molecules, a large number of them are entangled in the first hydration shell, so that the overall effect is appreciable. This theory is in perfect agreement with the observation that the entropy of hydration of a nonpolar molecule depends linearly on the number of water molecules in the first hydration shell ". ... 

AH and AS to various notional subprocesses such as bond dissociation energies, ionization energies, electron affinities, heats and entropies of hydration, etc., which themselves have empirically observed values that are difficult to compute ab initio. 

The derivation of values of conventional and absolute molar enthalpies and molar entropies of hydration of ions... 

The models may be somewhat deficient, but estimates of the enthalpies and entropies of hydrated ions are accurate and are the subject of Section 2.4. 

The calculation of the values for the standard molar entropies of hydration of ions requires some groundwork using the data presented in the following sub-section. 

In this section the standard molar entropies of a small selection of cations and anions are tabulated and the manner of their derivation discussed. The values themselves are required in the calculation of entropies of hydration of ions, discussed in Section 2.7.2. 

Absolute Standard Molar Entropies of Hydration of Ions... 

Q Calculate (he value of ihc absolute standard entropy of hydration of the Mg cation. 

Inspection of the values for the entropies of hydration of the Group 2 cations in Table 2.17 shows that, with the exception of that for Be2 +, the values become less negative as the ionic radius increases. This effect is similar to that observed for the Group 1 cations. The exception of Be2+ to the general trend is possibly because of its tendency to have a tetrahedral coordination that causes it to affect fewer molecules of water in the hydration process. 

A comparison of the entropies of hydration of Na +, Mg2+ and Al3+ shows that they become much more negative as the positive charge on the cation increases. The increasing charge would be expected to be more and more effective in restricting the movement of water molecules, and there would be more water molecules participating in the restricted volume of the hydration spheres of the ions. The decreasing ionic radius in the series amplifies the trend. 

The Absolute Standard Molar Entropy of Hydration of the Proton... 

Table 2.17 Absolute standard molar entropies of hydration for some ions (in J K-1 mol-1)...

Q Calculate the value of the absolute entropy of hydration of the proton. 

The entropy of hydration of the proton has a value intermediate between those of Li+ and Na 1. This is consistent with the hydrated proton consisting of the hydrated hydroxonium ion, H30+(aq), rather than being the hydrated fundamental particle. The hydrated proton must have a radius intermediate between those of Li1 and Na +, consistent with its entropy of hydration. 

Absolute values lor the enthalpies and entropies of hydration of ions were discussed in terms of their sizes and chai ses. 

The entropies of solution are almost all positive, the positive values of the entropies of lattice vaporization predominating over the negative entropies of hydration of the pairs of ions. 

The nature of ions in solution is described in some detail and enthalpies and entropies of hydration of many ions are defined and recalculated from the best data available. These values are used to provide an understanding of the periodicities of standard reduction potentials. Standard reduction potential data for all of the elements, group-bygroup, covering the s-and p-, d- and/- blocks of the Periodic Table is also included. Major sections are devoted to the acid/base behaviour and the solubilities of inorganic compounds in water. 

This book offers no solutions to such severe problems. It consists of a review of the inorganic chemistry of the elements in all their oxidation states in an aqueous environment. Chapters 1 and 2 deal with the properties of liquid water and the hydration of ions. Acids and bases, hydrolysis and solubility are the main topics of Chapter 3. Chapters 4 and 5 deal with aspects of ionic form and stability in aqueous conditions. Chapters 6 (s- and p-block). 7 (d-block) and 8 (f-block) represent a survey of the aqueous chemistry of the elements of the Periodic Table. The chapters from 4 to 8 could form a separate course in the study of the periodicity of the chemistry of the elements in aqueous solution, chapters 4 and 5 giving the necessary thermodynamic background. A more extensive course, or possibly a second course, would include the very detailed treatment of enthalpies and entropies of hydration of ions, acids and bases, hydrolysis and solubility. 

It can be anticipated that the computation of A//soi and AAsoi is more delicate than the prediction of AGsoi, which benefits from the enthalpy-entropy compensation. Accordingly, the suitability of the QM-SCRF models to predict the enthalpic and entropic components of the free energy of solvation is a challenging issue, which could serve to refine current solvation continuum models. This contribution reports the results obtained in the framework of the MST solvation model [15] to estimate the enthalpy (and entropy) of hydration for a set of neutral compounds. To this end, we will first describe the formalism used to determine the MST solvation free energy and its enthalpic component. Then, solvation free energies and enthalpies for a series of typical neutral solutes will be presented and analyzed in light of the available experimental data. Finally, collected data will be used to discuss the differential trends of the solvation in water. 

Table 4-1. Experimental free energy, enthalpy and entropy of hydration (kcal/mol) for the series of neutral molecules considered in this study...

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