The Bronsted-Lowry concept of acids and bases

The Bronsted-Lowry concept of acids and bases will be discussed in detail in Chapter 7. 

In terms of the Bronsted-Lowry concept of acids and bases, identify the acid, base, conjugate acid, and conjugate base in the following equation ... 

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). 

The Bronsted-Lowry concept of acids and bases4 makes it unnecessary to distinguish between acid and base indicators emphasis is placed upon the charge types of the acid and alkaline forms of the indicator. The equilibrium between the acidic form InA and the basic form InB may be expressed as ... 

The problem with the Arrhenius definitions is that they are specific to one particular solvent, water. When chemists studied nonaqueous solvents, such as liquid ammonia, they found that a number of substances showed the same pattern of acid-base behavior, but plainly the Arrhenius definitions could not be used. A major advance in our understanding of what it means to be an acid or a base came in 1923 when two chemists working independently, Thomas Lowry in England and Johannes Bronsted in Denmark, came up with the same idea. Their insight was to realize that the key concept underlying the properties of acids and bases was the transfer of a proton (a hydrogen ion) from one substance to another. The Bronsted-Lowry definition of acids and bases is as follows ... 

Why is ammonia, NH3, considered to be a base in both in the Arrhenius and Bronsted-Lowry concepts of acids and bases ... 

Apart from this modification, the Arrhenius definitions of acid and base are still valid and useful today, as long as we are talking about aqueous solutions. However, the Arrhenius concept of acids and bases is so intimately tied to reactions that take place in water that it has no good way to deal with acid-base reactions in nonaqueous solutions. For this reason, we concentrate in this chapter on the Bronsted-Lowry definitions of acids and bases, which are more useful to us in our discussion of reactions of organic compounds. 

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 Bronsted-Lowry theory focuses on the transfer of a proton from one species to another. However, the concepts of acids and bases have a much wider significance than the transfer of protons. Even more substances can be classified as acids or bases under the definitions developed by G. N. Lewis ... 

Each of the three definitions expands our concept of acids and bases. Arrhenius basic definition is adequate for understanding many of the properties of acids and bases. It is important to recognize, though, that acids and bases are not fixed labels that can be applied to a substance. Bronsted-Lowry and Lewis showed that acid-base characteristics are dependent on the reactions that take place between substances. A... 

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... 

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. 

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 ... 

In the Bronsted-Lowry concept, an acid is a proton donor, a base a proton acceptor and neutralization is the donation of one proton by an acid to a base. 

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. 

Skill 25.1 Compare the Arrhenius, Bronsted-Lowry, and Lewis concepts of acids and bases. 

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. 

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. 

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. 

On the above basis it is, in principle, unnecessary to treat the strength of bases separately from acids, since any protolytic reaction involving an acid must also involve its conjugate base. The basic properties of ammonia and various amines in water are readily understood on the Bronsted-Lowry concept. 

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. 

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. 

The base in the Bronsted-Lowry concept is any substance that can accept the proton it can even be the solvent. A Bronsted-Lowry base has an electron pair (a lone pair) that will accept the proton. The point that needs to be stressed here is that the proton is involved in both the definition of an acid (donates a proton) and a base (1accepts a proton). 

Note that HA and B are not necessarily neutral. They could be ions that are capable of acting as an acid or a base. This is one of the features of the Bronsted-Lowry concept that broadens the definitions of acids and bases over the Arrhenius concept—there are many more substances that can behave as acids or bases. Further, we can write the reaction including the solvent, water in this case, and the associated Ka. 

---