Other conceptions of acids and bases

The remarks made at the end of the section on the Lowry-Bronsted definition apply with even greater foree to two other definitions that have been proposed for acids and bases, namely  

These take us a long way from the original eoneeptions of acid and base, and might be better divorced from these ideas eompletely. At the very least, they should only be used in a qualified way, as in the expressions Lewis aeid and Lewis base . 

For further details about these two definitions, see textbooks. 

Today s ]ob market values ambitious, innovative, perceptive team players. Swedish universities foster these qualities through a forward-thinking culture where you re close to the latest ideas and global trends. 

Whatever your career goals may be, studying in Sweden wall give you valuable 

---

ACIDS AND BASES. OXIDATION AND REDUCTION OTHER CONCEPTS OF ACIDS AND BASES... 

These other concepts of acids and bases are not so easily applied quantitatively as the Lowry-Bronsted concept. Nevertheless they have proved to be very useful as ways of classifying chemical substances and—more importantly— these ideas have been a stimulus to many advances in inorganic chemistry. 

The first substantial constitutive concept of acid and bases came only in 1887 when Arrhenius applied the theory of electrolytic dissociation to acids and bases. An acid was defined as a substance that dissociated to hydrogen ions and anions in water (Day Selbin, 1969). For the first time, a base was defined in terms other than that of an antiacid and was regarded as a substance that dissociated in water into hydroxyl ions and cations. The reaction between an acid and a base was simply the combination of hydrogen and hydroxyl ions to form water. 

The protonic concept of acid and bases is applicable to many of these high lemperatures solvent systems such as the fused ammonium salts which possess the oniunt ion or solvated proton, and the fused anionic acids which arc salts possessing a metallic ion and a hydrogen containing anion. One of the most useful of the anionic acids is KHF which is used to dissolve ore minerals containing silica, tilania and other refractory oxides. 

This general idea has led to the very broad concept of acids and bases proposed by Br0nsted. (The same concept was suggested by Lowry.) Accordingly, an acid is simply defined as any substance that can donate a proton to any other substance, and a base is defined as any substance that accepts a proton from another substance that is, an acid is a proton donor and a base is a proton acceptor. Thus a proton transfer can occur only if an acid reacts with a base ... 

In the simple theory based on Lewis concepts exemplified above, the key aspects are an empty orbital on one atom and a filled orbital (with a pair of electrons present, the lone pair) on the other. Many of the ligand species providing the lone pair are considered bases in the classical Brpnsted-Lowry concept of acids and bases (which has as its focus the transfer... 

SECTION 16.11 The Lewis concept of acids and bases emphasizes the shared electron pair rather than the proton. A Lewis acid is an electron-pair acceptor, and a Lewis base is an electron-pair donor. The Lewis concept is more general than the Bronsted Lowry concept because it can apply to cases in which the add is some substance other than H. ... 

The electrolytic conception of acids and bases can be extended to other ionizing solvents by means of the following definitions ... 

Although the Arrhenius concept of acids and bases is useful, it is somewhat limited. For example, it tends to single out the OH ion as the source of base character, when other ions or molecules can play a similar role. In 1923, Johannes N. Brpnsted and Thomas M. Lowry independently noted that many reactions involve nothing more than the transfer of a proton (H ) between reactants, and they reaUzed that they could use this idea to expand the definitions of acids and bases to describe a large class of chemical reactions. In this view, acid—base reactions are proton-transfer reactions. 

The Swedish chemist Svante Arrhenius framed the first successful concept of acids and bases. He defined acids and bases in terms of the effect these substances have on water. According to Arrhenius, acids are substances that increase the concentration of H ion in aqueous solution, and bases increase the concentration of OH ion in aqueous solution. But many reactions that have characteristics of acid-base reactions in aqueous solution occur in other solvents or without a solvent. For example, hydrochloric acid reacts with aqueous ammonia, which in the Arrhenius view is a base because it increases the concentration of OH ion in aqueous solution. The reaction can be written... 

In the first part of this chapter, we will discuss the Arrhenius, the Bronsted-Lowry, and the Lewis concepts of acids and bases.The Bronsted-Lowry and Lewis concepts apply to nonaqueous as well as aqueous solutions and also enlarge on the Arrhenius concept in other ways.This chapter expands on what you learned in Chapter 4 about acids and bases. 

The solvent-system definition is also particularly useful for extending the concepts of acid and base to non-protonic non-aqueous systems. Many such systems are considered to undergo a self-ioniza-tion reaction analogous to autoprotolysis but involving the transfer of an anion such as F , Cl , O ", etc., rather than the transfer of a proton. In a few cases conductivity and other measurements provide substantial support for the existence of the postulated equilibrium, while in other cases there is no evidence at all for such a self-dissociation equilibrium. Nevertheless, the assumption that the equilibrium exists in a given solvent has proved useful for systematizing the chemistry in that solvent. 

Although the Bronsted concept of acids and bases focuses on the transfer of a proton, electron pairs are more fundamental to the process. Covalent bonds are formed or broken when a proton is transferred from one atom to another. To account for this possibihty, Gilbert N. Lewis proposed a definition of acids that focuses on electron pairs. A Lewis acid is an electron pair acceptor a Lewis base is an electron pair donor. This is a general definition of an acid and a base. For example, hydrochloric acid, a Bronsted acid, is also a Lewis acid because it contains a proton that accepts an electron pair. Ammonia is a Lewis base because it can act as an electron pair donor. However, many other species can serve as electron pair acceptors or donors. Consider the following general reaction between a Lewis acid and a Lewis base. 

Scepticism about the quantitative usefulness of the Lewis concept of acids and bases is still frequently encountered in chemistry textbooks. We expect that this book will demonstrate that quantitative data exist for acid/base systems other than those involving proton donors and acceptors, and will encourage textbook authors to go beyond a mere qualitative presentation of the Lewis acid/base concepts. 

According to the Lewis concept of acids and bases the catalyst in this reaction behaves as an acid and the halide reacts as a base. Many other metallic halides catalyze this reaction in a similar manner. All of them can be shown to be acids by titrating with indicators in the proper solvent. 

Hydrogen was recognized as the essential element in acids by H. Davy after his work on the hydrohalic acids, and theories of acids and bases have played an important role ever since. The electrolytic dissociation theory of S. A. Arrhenius and W. Ostwald in the 1880s, the introduction of the pH scale for hydrogen-ion concentrations by S. P. L. Sprensen in 1909, the theory of acid-base titrations and indicators, and J. N. Brdnsted s fruitful concept of acids and conjugate bases as proton donors and acceptors (1923) are other land marks (see p. 48). The di.scovery of ortho- and para-hydrogen in 1924, closely followed by the discovery of heavy hydrogen (deuterium) and... 

It was G. N. Lewis who extended the definitions of acids and bases still further, the underlying concept being derived from the electronic theory of valence. It provided a much broader definition of acids and bases than that provided by the Lowry-Bronsted concept, as it furnished explanations not in terms of ionic reactions but in terms of bond formation. According to this theory, an acid is any species that is capable of accepting a pair of electrons to establish a coordinate bond, whilst a base is any species capable of donating a pair of electrons to form such a coordinate bond. A Lewis acid is an electron pair acceptor, while a Lewis base is an electron pair donor. These definitions of acids and bases fit the Lowry-Bronsted and Arrhenius theories, and cover many other substances which could not be classified as acids or bases in terms of proton transfer. 

According to the Arrhenius theory of acids and bases, the acidic species in water is the solvated proton (which we write as H30+). This shows that the acidic species is the cation characteristic of the solvent. In water, the basic species is the anion characteristic of the solvent, OH-. By extending the Arrhenius definitions of acid and base to liquid ammonia, it becomes apparent from Eq. (10.3) that the acidic species is NH4+ and the basic species is Nl I,. It is apparent that any substance that leads to an increase in the concentration of NH4+ is an acid in liquid ammonia. A substance that leads to an increase in concentration of NH2- is a base in liquid ammonia. For other solvents, autoionization (if it occurs) leads to different ions, but in each case presumed ionization leads to a cation and an anion. Generalization of the nature of the acidic and basic species leads to the idea that in a solvent, the cation characteristic of the solvent is the acidic species and the anion characteristic of the solvent is the basic species. This is known as the solvent concept. Neutralization can be considered as the reaction of the cation and anion from the solvent. For example, the cation and anion react to produce unionized solvent ... 

Usanovic acid-base theory — A general theory of acids and bases taking into consideration electron exchange processes (redox reactions) was proposed in 1939 by Usanovic. His definition is a symmetrical one and includes all concepts discussed above, i.e., an acid is defined as a substance which is able to liberate protons or other cations (cation donator) or to take up anions or electrons (anion acceptor, electron acceptor). A base is defined as a substance which is able to release anions or electrons (anion donator, electron donator) or to take up protons or other cations (cation acceptor). According to that theory, all chemical reactions (excluding reactions between radicals resulting in covalent bindings) can be considered as acid-base reactions. 

The concepts and equations of acid-base dissociation have referred chiefly to aqueous solutions. Recently, interest in the behavior of acids and bases in solvents other than water has increased considerably. The classical definition of an acid and a base, which is satisfactory for water solutions, is too limited for other solvents. Because of the great importance of the general question of the acid-base equilibrium, the clear and fruitful views of Bronsted are exhaustively considered in a special (fourth) chapter. ... 

The concepts of acidity and basicity are used by scientists from all branches of theoretical, experimental, and engineering chemistry. They are fundamental in nature and are of unquestionable utility as a powerful classification tool. However, it is interesting to observe that the meaning of these two simple words may vary depending on the compounds or materials discussed, experimental conditions or methods of study, particular applications, state of matter or size of particles, and, in some instances, even the context in which the terms are used. Indeed, any discussion about acid-base properties is an assertion in comparative terms a debate about relativity. By definition, acidity and basicity are opposite properties (or principles) that manifest themselves only in relationship to each other. Acids are revealed and measured by bases the presence of bases is revealed and quantified by acids. 

Despite its successes, the Arrhenius concept is limited. In addition to looking at acid-base reactions only in aqueous solutions, it singles out the OH ion as the source of base character, when other species can play a similar role. Broader definitions of acids and bases are described in the next sections. 

---