Qualitative Aspects of Acid-Base Reactions

So far, we have considered only reactions in which an acid or a base reacts with water. In this section, we will focus on the general case of a reaction involving an acid HA and a base B. 

Of course, the reaction between HA and B is reversible. In this section, we are interested in obtaining an answer to the question. Does the equilibrium favor the reactants, HA and B, or the products, A and BH  

For an acid-base reaction, equilibrium favors the formation of the weaker acid and the weaker base. 

To justify this statement, we may proceed as follows. Reaction (16.21) is the sum of the following reactions. 

Suppose that HA is a stronger acid than BH . Then fCa(HA) Kb(BH ) and fC 1. Because K is greater than 1, equilibrium favors the formation of BH+, 

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Ions as Acids and Bases 16-9 Qualitative Aspects of Acid-Base Reactions... 

Qualitative Aspects of Acid-Base Reactions—For an acid-base reaction, equilibrium favors the formation of the weaker acid and the weaker base. If the acid or base in an acid-base reaction is strong, then the reaction goes essentially to completion. 

In the practice of potentiometric titration there are two aspects to be dealt with first the shape of the titration curve, i.e., its qualitative aspect, and second the titration end-point, i.e., its quantitative aspect. In relation to these aspects, an answer should also be given to the questions of analogy and/or mutual differences between the potentiometric curves of the acid-base, precipitation, complex-formation and redox reactions during titration. Excellent guidance is given by the Nernst equation, while the acid-base titration may serve as a basic model. Further, for convenience we start from the following fairly approximate assumptions (1) as titrations usually take place in dilute (0.1 M) solutions we use ion concentrations in the Nernst equation, etc., instead of ion activities and (2) during titration the volume of the reaction solution is considered to remain constant. 

Which catalyst should be chosen for a given reaction will depend upon chemical, steric, and mechanistic factors. The application of Pearson s soft and hard acid-base (SHAB) principle has often proved a valuable qualitative guide as to suitable surface sites for a particular reactant. In fact, certain solids actually owe their catalytic power to attached Bronsted or Lewis acid and base groups as exemplified by weak acid ion exchange resins (Sect. 2.3), alumina (Sect. 3.2), and sometimes charcoals. Steric aspects can be con-... 

Popular qualitative chemical concepts such as electronegativity [1] and hardness [2] have been widely used in understanding various aspects of chemical reactivity. A rigorous theoretical basis for these concepts has been provided by density functional theory (DFT). These reactivity indices are better appreciated in terms of the associated electronic structure principles such as electronegativity equalization principle (EEP), hard-soft acid-base principle, maximum hardness principle, minimum polarizability principle (MPP), etc. Local reactivity descriptors such as density, Fukui function, local softness, etc., have been used successfully in the studies of site selectivity in a molecule. Local variants of the structure principles have also been proposed. The importance of these structure principles in the study of different facets of medicinal chemistry has been highlighted. Because chemical reactions are actually dynamic processes, time-dependent profiles of these reactivity descriptors and the dynamic counterparts of the structure principles have been made use of in order to follow a chemical reaction from start to finish. 

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