General concepts of acids and bases

In 1923, the same year in which Bronsted and Lowry defined acids and bases in terms of their proton donor/acceptor properties, the American chemist G. N. Lewis proposed an even more general concept of acids and bases. Lewis noticed... 

A still more general conception of acids and bases is possible (Lewis), in which bases are compounds or groups such as O2-, OH, H20, NH3, NH2- and F, with a free electron pair which can take part in a reaction (electron donors or nucleophilic compounds), for example, with a proton in bases regarded according to Bronsted s concept. 

In recent years several more general concepts of acids and bases have been introduced. They are useful for some purposes, such as the discussion of non-aqueous solutions. One of these concepts, due to the Danish chemist J, N. Bronsted, is that an acid is any molecular or ionic species which can give up a proton (which is a proton donor), and a base is any one which can take up a proton (which is a proton ac ceptqr). Thus NH + is called an acid, since it can give up a proton ... 

Hydrolysis of Salts. Anion hydrolysis cation hydrolysis hydrolysis in general. Buffered solutions. Coordination number of elements formulas of the oxygen acids. Strengths of the oxygen acids. Simple rules. General concepts of acids and bases. Proton donors and acceptors. Acid strength and tendency to undergo condensation. 

A more general concept of acids and bases was introduced by Gilbert N. Lewis. The Lewis theory deals with the way in which a substance with an unshared pair of electrons reacts in an acid-base type of reaction. According to this theory, a base is any substance that has an unshared pair of electrons (electron pair donor), and an acid is any substance that will attach itself to or accept a pair of electrons. 

Equations (7) and (8) are special cases for aqueous solutions of the equation for generalized acid-base catalysis. As shown by Lowry 54a), the muta-rotations of sugars are reactions involving simultaneous catalysis by both acids and bases, in the generalized concept of acids and bases proposed by Lowry and by Bronsted. Water functions as a complete catalyst because of its amphoteric dissociation into ions H20<- H+ + OH. Acids or bases alone are not effective catalysts but in mixture are complete catalysts. 

There are various definitions of acids and bases, and in discussing them it should be emphasized that the question is not one of validity but one of utility. Indeed, the problem of validity does not arise because of the fundamental nature of a definition. The problem is entirely one of choosing a definition which is of greatest use in the study of a particular field of acid-base chemistry. One point that needs to be borne in mind is that a concept of acids and bases is required that is neither too general nor too restrictive for the particular field of study. 

Any text on acids and bases would not be deemed complete if mention were not made of the extended definition of acids and bases that is embodied in the Lowry-Bronsted theory. The theory basically proposed a more general definition of acids and bases to overpower the limitations of the theory arising from the Arrhenius concept. 

Usanovich (1934) modified the Lewis concept of acid and base by removing the restriction of either donation or acceptance of the electron pair in a more generalized fashion. According to him ... 

Things are a little more complicated than implied by this statement. Molecules that donate protons and molecules that accept them are formally known as Bronsted acids and bases. There is a more general definition of acid and base— Lewis acids and bases—in which a Lewis acid is any molecule having the ability to accept a pair of electrons and a Lewis base is any molecule having the ability to donate a pair of electrons to a Lewis acid. The Bronsted concept will suffice for all our purposes. 

The generalized Lewis concept of acids and bases also includes common proton-transfer reactions.1 Thus water acts as a base because one of the... 

Even more general is the Lewis concept of acids and bases a Lewis base has a lone pair available for formation of a coordinate bond, and a Lewis acid has a vacant acceptor orbital handy. This concept is applicable to reactions in the gas phase or in inert solvents (as discussed in the previous section) as well as to complex formation in solution and the acid/ base phenomena studied by Arrhenius, Br0nsted and Lowry. 

General definitions of acids and bases were given by G. N. Lewis as an extension of the concept of the electron-pair covalent bond an acid is an electron-pair acceptor, a base an electron-pair donor. All Lewis bases are Bronsted bases—an unshared electron pair is required to accept a proton. H+ is an acid in the Lewis sense since it can accept an electron pair from a base such as NH3, but LLO and NH4... 

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. 

Earlier in this chapter we considered Arrhenius s concept of acids and bases An acid is a substance that produces H+ ions when dissolved in water, and a base is a substance that produces OH- ions. Although these ideas are fundamentally correct, it is convenient to have a more general definition of a base, which covers substances that do not produce OH- ions. Such a definition was provided by Bransted and Lowry, who defined acids and bases as follows ... 

The first person to recognize the essential nature of acids and bases was Svante Arrhenius. Based on his experiments with electrolytes, Arrhenius postulated that acids produce hydrogen ions in aqueous solution, and bases produce hydroxide ions. At the time of its discovery the Arrhenius concept of acids and bases was a major step forward in quantifying acid—base chemistry, but this concept is limited because it applies only to aqueous solutions and allows for only one kind of base—the hydroxide ion. A more general definition of acids and bases was suggested independently by the Danish chemist Johannes N. Bronsted (1879-1947) and the English chemist Thomas M. Lowry (1874-1936) in 1923. In terms of the Bronsted—Lowry definition, an acid is a proton (H+) donor, and a base is a proton acceptor. For example, when gaseous HCl dissolves in water, each HCl molecule donates a proton to a water molecule, and so HCl qualifies as a Bronsted-Lowry acid. The molecule that accepts the proton—water in this case—is a Bronsted-Lowry base. 

Definition of Acids and Bases. —The old definitions of an acid as a substance which yields hydrogen ions, of a base as one giving hydroxyl ions, and of neutralization as the formation of a salt and water from an acid and a base, are reasonably satisfactory for aqueous solutions, but there are serious limitations when non-aqueous media, such as ethers, nitro-compounds, ketones, etc., are involved. As a result of various studies, particularly those on the catalytic influence of un-ionized molecules of acids and bases and of certain ions, a new concept of acids and bases, generally associated with the names of Br nsted and of Lowry, has been developed in recent years. According to this point of view an acid is defined as a substance with a tendency to lose a proton, while a base is any substance with a tendency to gain a proton the relationship between an acid and a base may then be written in the form... 

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 ... 

Acids are defined classically as substances which liberate hydrogen ions in aqueous solutions, while bases are substances which dissociate in water with the formation of hydroxyl ions. This formulation of the concept of acids and bases had developed chiefly on the basis of acidic and basic properties displayed in aqueous solutions. When an attempt was made to establish a more general theory of acids and bases, the old formulation appeared in many ways to be one-sided and incomplete. 

The Arrhenius concept of acids and bases was a tremendous advance in the understanding of these compounds, but is it limited to aqueous solutions, and a lot of chemistry takes place out of water. In 1923 a Danish chemist, Johannes Bronsted (1879-1947), and an English chemist, Thomas Lowry (1874-1936), proposed a more general way to describe acids and bases centered on the ability of a species to donate or accept a proton, IT. It was not limited to aqueous solutions. Here is how they defined acids, bases and neutralization ... 

Although the Arrhenius concept of acids and bases was a major step forward in understanding acid-base chemistry, this concept is limited because it allows for only one kind of base—the hydroxide ion. A more general definition of acids and bases was suggested by the Danish chemist Johannes Bronsted and the English chemist Thomas Lowry. 

The most general view of acids and bases was advanced by G. N. Lewis. In this model, acids are substances which have an affinity for lone electron pairs, and bases are substances which possess lone electron pairs. Water and ammonia are the most common substances which possess lone electron pairs, and therefore behave as bases in the Lewis scheme. The reaction of silver ion, Ag with cyanide ion, CN , and boron trifluoride, BF3 (an electron-deficient compound), with ammonia, NH3, are two examples of Lewis acid-base reactions. The Lewis acid-base concept is most useful in chemical reactions in nonaqueous solvents. We will not find it useful in our study of ionic equilibria in water. 

SECTION 16.2 The Bronsted- Lowry concept of acids and bases is more general than the Arrhenius concept and emphasizes the transfer of a proton (H ) fiom an acid to a base. The H ion, which is merely a proton with no surrounding valence electrons, is strongly bound to water. For this reason, the hydronium ion, H30" (flq), is often used to represent the predominant form of H in water instead of the simpler H"( q). 

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 Lewis concept of acids and bases includes proton-transfer reactions all Bronsted-Lowry bases (proton acceptors) are also Lewis bases, and all Bronsted-Lowry acids (proton donors) are also Lewis acids. The Lewis model, however, is more general in that it is not restricted to proton-transfer reactions. 

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