Acid-base behavior, correlation with

The pattern of oxidation states correlates with the pattern of acid-base behavior of d-metal oxides. Although most d-metal oxides are basic, the oxides of a given element show a shift toward acidic character with increasing oxidation number, just as the oxoacids do (recall Section 10.10). The family of chromium oxides is a good... 

The "equilibrium boxes" for the solvents (Fig. 10-1) indicate the range over which differentiation occurs outside the range of a particular solvent, all species are leveled. For example, water can differentiate species (i.e., they are weak adds and bases) with pKa s from about 0 to 14 (such as acetic acid). Ammonia, on the other hand, behaves the same toward acetic acid and sulfuric acid because both lie below the differentiating limit of —12. The extent of these ranges is determined by the autoionization constant of the solvent (e.g, —14 units for water). The acid-base behavior of several species discussed previously may be seen to correlate with Fig. 10.1.11... 

It has been shown (Schindler and Stumm, 1987) that equilibrium constants characterizing the amphoteric behavior of metal oxides (the surface acidity) are linearly correlated with the equilibrium constants for the amphoteric behavior of the same metal ions in solution (acidity in solution). This finding supported the model in which the metal oxide surface is described with the same acid-base equations as the metal ions in solution, and allows the equilibrium constants for the oxide surfaces to be estimated from those determined in solution. 

Surface characteristics such as acid-base behavior, specifically at the temperatures of catalyst operation (Relevant characteristics include the number and concentration of acidic sites, their types [Bronsted or Lewis], and the correlation of these properties with structure and composition of the bulk catalyst.)... 

For nucleophilic displacement reactions, some relationship might be expected between the ability of an anion A to act as nucleophile and the dissociation constant of the acid HA the best nucleophiles should be the anions of the weaker acids. For an uncharged nucleophile B there should be a relationship with the dissociation constant of the conjugate acid BH. Nucleophilicity is often treated in terms of Bronsted-type relationships, but deviations and sometimes almost complete breakdown are found when wide ranges of substrates and nucleophiles are considered. Nucleophilicity is a more complex function of structure than acid-base behavior. This situation has led to the development for such reactions of a special system of correlation analysis, which involves scales of nucleophilicity constants. The first of these, the n scale, was proposed by Swain and Scott in 1953 and was based on methyl bromide as a standard substrate for an LFER. Other scales have subsequently been developed and applied in both simple and multiple regression. 

Plausibly there is a correlation of the solubility of the stable oxide, hydroxide, or oxyhydroxide of a cation with the stability of the first hydrolysis product (Figure 6. 7). Many multivalent hydrous oxides are amphoteric because of the acid-base equilibria involved in the hydrolysis reactions of aquo metal ions. Alkalimetric or acidimetric titration curves for hydrous metal oxides provide a quantitative explanation for the manner in which the chaige of the hydrous oxide depends on the pH of the medium. The amphoteric behavior of solid metal hydroxides becomes evident from such titration curves. From an operational point of view, such hydrous oxides can be compared with amphoteric polyelectrolytes and can be considered hydrated solid electrolytes, fn -... 

Results from the correlation of stability constants in conjunction with redox data have led to insights regarding the coordination chemistry of thia macrocycles. For example, the electrochemical behavior of a number of copper(II)/(I) redox couples has been investigated, and redox potentials as well as protonation and stability constants of Cu species were determined for a number of tetradentate and pentadentate thia-derived macrocycles with thia- and mixed thia-aza rings with the basic backbones (51) and (52). Results of the examination of the stability constants in conjunction with the Cu redox potentials indicate that the stability constants for the Cu oxidation state are relatively constant regardless of the mixing in of nitrogen donor atoms. Hence, the dramatic increase in the Cu / redox potential which is observed in the presence of the sulfur macrocycles can be attributed to a destabilization of the Cu state rather than stabilization of the Cu state, contrary to popular belief from the hard-soft acid-base system. 

Modification of the Lewis acid-base character of a. surface can also be achieved by the presence of inorganic ions in the scdution. In this case, however, a net correlation between the (more representative) value of Ys the entropy of hydration of the ions. 45i, has been found (48). In Fig. 9 a linear correlation is observed between those quantities for a series of cations and anions, with the only exception of K and OH which show a special behavior. Although AS deals with ions in solution and describes the surface behavi[Pg.148]

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