Classifications of Acids and Bases

Pearson has also attacked the problem of the classification of acids and bases in a much more qualitative way. Realising that the pattern of rate... 

Rather than introduce new words into an already confusing terminology, Bell (64) is of the opinion that the words acid and base should be confined to proton acids, and the Sidgwick classification of molecules as electron donor and electron acceptor (which is essentially equivalent to the Lewis classification of acids and bases) be employed, together with the categories of nucleophilic and electrophilic reagents as defined by Ingold (65). 

In dilute solutions, strong acids and strong bases are completely ionized or dissociated. In the following sections we consider dilute aqueous solutions of salts. Based on our classification of acids and bases, we can identify four different kinds of salts. 

Donor atoms Metals removed HSAB classification of acid and base... 

A listing of the classification of acids and bases is given in Table 1.3. From this table it is apparent, for example, that Fe " (a hard acid) will tend to bond strongly with hard bases such as phosphate and carboxylate, but not with softer bases such as sulfide or aromatic amines. Fe ", a softer acid, bonds more strongly with these softer bases. 

Fig. 7 Classification of acids and bases according to their strength in water...

Concepts of acidity and basicity are, in practice, defined and evaluated by their utility. Since overly formd definitions can be restrictive the concepts of acidity evolve towaids more comprehensive definitions. For example the Lewis definition includes the Broensted definition simply regarding the proton as an electron acceptor. Because the interaction of Broensted acids and bases in solutions involves a common process, protic transfer, scales of acidity can be established, for example the Hammett [1] acidity function. For Lewis acid-base interaction there is no common process to provide a unique basis for comparisons of acid strength. Experimentally, the strength of a Lewis acid depends upon the particular Lewis base. The classification of acids and bases as hard or soft in the principle of hard and soft acids and bases (HSAB principle) clarifies the interactions of Lewis acids and bases [2a]. Strong interactions occur between hard acid and hard base, or between soft acid and soft base, hi the hard-hard interaction there is a considerable electrostatic contribution to bonding and in the soft-soft interaction there is a major covalent contribution to bonding. The use of density functional analysis has clarified the concepts of hardness and softness and an empirical ranking of Lewis acids, based on local hardness is, proposed [2c]. 

Besides the classification of acids and bases, the Pearson principle introduced the idea of binding preference, namely hard acids prefer to bind to hard bases, and soft acids prefer to bind to soft bases. 

Figure 2. Classification of Acids and Bases by Hardness and Softness. (Reproduced with permission from ref. 15. Copyright 1963 American Chemical Society.)...

Additionally, the soft-to-hard classification of acids and bases can in any case be established through identifying on the Y(h) diagrams the soft, borderline, and hard detached islands of Y values respecting r range. All these aspects should leave with quantum elucidation of the chemical bond and bonding nature via maximum hardness index Y. 

According with Pearson, classification of acids and bases as hard and soft needs the recourse to the concept of strength although litde insight this... 

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