The Bronsted-Lowry Concept

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  

The Bronsted-Lowry concept describes many more processes as acid-base reactions than does the Arrhenius concept. For example, the ionization of hydrogen chloride gas as it dissolves in water, HC1 (g) — HC1 (aq), can be described as an acid-base process as the proton from HC1 is transferred to water. 

In water, HC1 (aq), acts as a Bronsted-Lowiy acid donating a proton to water, HzO(I), which accepts the proton and acts as a Bronsted-Lowiy base. The solvent, water, is more than just a solvent, it is part of a neutralization. 

Proton donation can also occur in the gas phase as HC1 (g) reacts with NH3(g) to form solid ammonium chloride. 

Hydrogen chloride, the acid, donates a proton to ammonia, the base, which becomes the ammonium ion, NH4+. 

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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. 

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 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). 

The Bronsted-Lowry concept looks at the equilibrium reaction and ties the acid on the left to a base on the right, called a conjugate acid-base pair or, more simply, a conjugate pair. Suppose we were to consider an acid reacting with a compound in equilibrium with the acid s anion and the products, as... 

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. 

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

In terms of the Bronsted-Lowry concept, define an acid, a base and neutralization. 

Nitric acid ionizes in water to produce hydrogen ion, which exists in water as the hydronium ion. This makes it an Arrhenius acid. The equation showing the formation of the hydronium ion, H30+, shows nitric acid as a proton donor, making it an acid in the Bronsted-Lowry concept. 

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. 

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

Because the emphasis in the Bronsted-Lowry concept is on proton transfer, the concept also applies to reactions that do not occur in aqueous solution. In the reaction... 

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. ... 

In the year 1923, two more theories defining acid-base character were proposed. The first theory, Bronsted and Lowry theory, is very satisfactory for understanding physiological processes and will therefore form the basis of all further discussions. The second theory, proposed by G. N. Lewis is much more general than the Bronsted - Lowry concept. A brief discussion of this theory is given in Box 1.1. 

The old definition of neutralization states that an acid and a base react with each other to form salt and water. The Bronsted-Lowry concept offers a much broader view of the process of neutralization. According to this concept, neutralization is a process of proton transfer from an acid to a base. Neutralization need not result in the formation of a recognizable salt and may not Involve water. 

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