Thermodynamic aspects oxidation states in aqueous solution

12 Thermodynamic aspects oxidation states in aqueous solution 

In the preceding sections, we have, with some degree of success, attempted to rationalize irregular trends in some thermodynamic properties of the first row J-block metals. Now we consider the variation in values for equilibrium 

Consider now the variations in °(M +/M +) across the row. The enthalpy of atomization is no longer relevant and we are concerned only with trends in the third ionization energy (Table 21.15) and the hydration energies of M and Experimental values for °(M +/M ) 

These oxidations correspond to changes in electronic configuration of for V and d — d for Cr. The 

Cr and Cr hexaaqua ions are high-spin oxidation of is accompanied by a loss of LFSE (Table 21.3), while there is a gain in LFSE (i.e. more negative) upon oxidation of Cr (minor consequences of the Jahn-TeUer effect are ignored). Using values of from Table 21.2, these changes in LFSE are expressed as follows  

This turns out to be a difficult problem. Water is relatively easily oxidized or reduced, and the range of oxidation states on which measurements can be made under aqueous conditions is therefore restricted, e.g. Sc(II) and Ti(II) would liberate H2. Values of (M /M) are related (see 

In crossing the first row of the f-block, the general trend is for Ahyj/f to become more negative (Fig. 20.36). There is also a successive increase in the sum of the first two ionization energies albeit with discontinuities at Cr and Cu (Fig. 20.39). Values of A N vary erratically and over a wide range with a particularly low value for zinc (Table 6.2). The net effect of aU these factors is an irregular variation in values of (M /M) across the row, and it is clearly not worth discussing the relatively small variations in LFSEs. 

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Chapter 20 Thermodynamic aspects oxidation states in aqueous solution 589... 

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. 

Oxidation-reduction potentials for complexes in solution are determined by the relative stabilities of the complexes of the metal ion in the lower and higher oxidation states. The thermodynamic cycle connecting redox potentials and stabifity constants is shown in Fig. 7. This cycle can be useful both in rationalizing aspects of aqueous solution chemistry of complexes and in predicting or estimating values for stabifity constants or redox potentials for systems which are difficult or impossible to access experimentally. Thus knowledge of stabifity... 

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