Acid-base chemistry solvent theory

Almost all of the reactions that the practicing inotganic chemist observes in the laboratory take place in solution. Although water is the best-known solvent, it is not the only one of importance to the chemist. The organic chemist often uses nonpolar solvents sud) as carbon tetrachloride and benzene to dissolve nonpolar compounds. These are also of interest to Ihe inoiganic chemist and, in addition, polar solvents such as liquid ammonia, sulfuric acid, glacial acetic acid, sulfur dioxide, and various nonmctal halides have been studied extensively. The study of solution chemistry is intimately connected with acid-base theory, and the separation of this material into a separate chapter is merely a matter of convenience. For example, nonaqueous solvents are often interpreted in terms of the solvent system concept, the formation of solvates involve acid-base interactions, and even redox reactions may be included within the (Jsanovich definition of acid-base reactions. 

When one considers the incredible number of chemical reactions that are possible, it becomes apparent why a scheme that systemizes a large number of reactions is so important and useful. Indeed, classification of reaction types is important in all areas of chemistry, and a great deal of inorganic chemistry can be systematized or classified by the broad types of compounds known as acids and bases. Many properties and reactions of substances are understandable, and predictions can often be made about their reactions in terms of acid-base theories. In this chapter, we will describe the most useful acid-base theories and show their applications to inorganic chemistry. However, water is not the only solvent that is important in inorganic chemistry, and a great deal of chemistry has been carried out in other solvents. In fact, the chemistry of nonaqueous solvents is currently a field of a substantial amount of research in inorganic chemistry, so some of the fundamental nonaqueous solvent chemistry will be described in this chapter. 

Kolthoff, I. M. and M. K. Chantooni, General introdnction to acid-base equilibria in nonaqueons organic solvents, in Treatise on Analytical Chemistry, Part I, Theory and Practice (I. M. Kolthoff and P. J. Elving, eds.), John Wiley Sons, New York, 1979, pp. 239-301. 

The Bronsted-Lowry theory contributed several fundamental ideas that broadened our understanding of solution chemistry. First of all, an acid-base reaction is a charge-transfer process. Second, the transfer process usually involves the solvent. Water may, in fact, accept or donate a proton. Last, and perhaps most important, the acid-base reaction is seen as a reversible process. This leads to the possibility of a reversible, d)mamic equilibrium (see Section 8.4). 

In all proton containing solvents acid-base phenomena can be described in terms of the Bronsted-Lowby theory. All of these solvents have the solvated proton in common as the solvent cation, and this determines to a considerable extent the chemistry in their solutions. Bronsted acids are usually characterized by their acidic strength in water, e.g. by the acidity constant in this solvent. Thus acetic acid and hydrofluoric acid both behave as moderately weak acids in water with at room temperature. When acetic acid is dissolved in liquid hydrogen fluoride, the former is successfully competing for the protons, so that acetic acid acts as a base ( acetic-base ) in this medium just as it does in nitric acid ... 

Solvent extraction of metals embodies all aspects of coordination chemistry rates, equilibria, stereochemistry, crystal field theory, covalent bonding, hard-soft acid-base theory, hydrogen bonding, steric hindrance, enthalpy and entropy. All of these basic principles can link together to produce pure metals on an industrial scale from dilute aqueous solutions — a remarkable achievement of elegant coordination chemistry. To achieve this result it is only necessary to form within the aqueous medium a neutral species containing the metal to be extracted. 

Acid-base equilibria have played a predominant role in structural theories of chemistry [1-8]. Yet, with the exception of limited studies in the gas phase with Lewis acids, such as B(CH3)3 [7, 8], the interpretation of the observations has been clouded by the uncertain role of the solvent. Recent advances in high pressure mass spectrometry [9,10] and pulsed ion cyclotron resonance spectroscopy [11-16] have made possible the study of proton-transfer equilibria in the gas phase with a precision which allows determination of even small effects of changes in molecular structure. 

The addition of acid to the solution leads to shifts in the equilibria that favor the aqua-form of the complex. Thus, Pfeiffer, in 1906, established the reversible transformation of aqua-complexes into hydroxo-complexes. Werner later used this observation in the development of add-base concepts in coordination chemistry.32 Werner s work in this field is now largely of historical interest but two points, emphasized by Werner, remain espedally noteworthy, i.e. the importance of solvent in add-base equilibria and that bases may be considered as proton acceptors. These statements were incorporated into the acid-base theory developed by Bronsted and Lowry in 1923. 

We have seen that Walden s fears that Lewis would deliberately eliminate the important part played by the solvent in acid-base properties were groundless. Lewises acids and bases, dissolved in suitable amphoteric solvents, have the typicaF properties of acids and bases. These typicaF properties are the properties with which we are familiar from our study of water chemistry. Now that we are beginning to branch out into other fields, we may expect to find increasingly that the electronic theory of acids and bases is the only one so far proposed that is at all adequate. 

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