The Brpnsted-Lowry Theory

In 1923, two chemists working independently of each other, proposed a new theory of acids and bases. (See Figure 10.4.) Johannes Bronsted in Copenhagen, Denmark, and Thomas Lowry in London, England, proposed what is called the Brpnsted-Lowry theory of acids and bases. This theory overcame the problems related to the Arrhenius theory. 

Use the Arrhenius theory and then the Brpnsted-Lowry theory to describe the following concepts. If one of the theories does not apply, state that this is the case. 

In 1923, Johannes N. Brpnsted (1879-1947) and Thomas M. Lowry (1874-1936) independently defined acids and bases in a different way from the Arrhenins definitions. The resulting theory is sometimes called the Brpnsted-Lowry theory, bnt more often is referred to as just the Brpnsted theory. The Brpnsted theory extends the definitions of acid and base in a way that explains more than the Arrhenins definitions can explain. According to this theory, a Brpnsted acid is a proton donor, and a Brpnsted base is a proton acceptor. 

The Br0nsted-Lowry theory expands the definition of acids and bases to allow us to explain n ch more ol solution chemistry. For example, the Brpnsted-Lowry theory allows us to explain why a solution af ammonium nitrate tests acidic and a solution of potassium acetate tests basic. Most of the substances that we cofcider acids in the Arrhenius theory are also acids in the Brpnsted-Lowry theory, and the same is true of bases. Injboth theories, strong acids are those that react completely with water to form ions. Weak acids ionize only slightly. We can now explain this partial ionization as an equilibrium reaction of the weak acid, the ions, and the w ater. A similar statement can be made about weak bases ... 

The next theory of acids and bases is called the Brpnsted-Lowry Theory, proposed by Danish chemist Johannes Brpnsted and the English chemist Thomas Lowry, independently, in 1923. The definition of an acid, in this theory, sounds essentially the same as the Arrhenius acid. ABr0nsted-Lowry acid is defined as a substance that donates a proton to another species. Now, you might say, A proton is a hydrogen ion (H+), so what is the difference between the two definitions for acids Most notably, this definition doesn t require that the acid be in an aqueous solution. So a substance that donates protons, even when in a solid or vapor phase, is still acting as a Brpnsted-Lowry acid. 

In 1923, two chemists, J. N. Brpnsted in Denmark and J. M. Lowry in England, proposed independently a theory of acid/base behavior that is particularly useful in analytical chemistiy According to the Brpnsted-Lowry theory, an acid is a proton donor and a base is a proton acceptor. For a molecule to behave as an acid, it must encounter a proton acceptor (or base). Likewise, a molecule that can accept a proton behaves as a base if it encounters an acid. 

It should be noted that the terms acid and base in the Brpnsted-Lowry theory refer only to the function of the given substance in a protolytic reaction. The same substance under certain conditions may react as an acid, and in other cases it may demonstrate basic properties. Also, the relative strength of acids and bases is considerably affected by solvation of the reagents and reaction products. 

In the course of the reaction, HCl donates, or gives up, a proton to form a Cl ion, and H2O accepts a proton to form the ion. Thus, HCl is an acid and H2O is a base, according to the Brpnsted-Lowry theory. 

In this ionization reaction, water serves not only as a solvent but also as a base according to the Brpnsted-Lowry theory. 

Bases are much the same in the Lewis theory and in the Brpnsted-Lowry theory, because in the Br0nsted-Lowry theory a base must donate a pair of electrons in order to accept a proton. 

According to the Brpnsted-Lowry theory, a conjugate acid-base pair consists of molecules or ions related by the loss of one H by an acid, and the gain of one H by a base. 

Reaction (4.9) is not equilibrated. This fact must not be interpreted as being a consequence of a shortcoming in the Brpnsted-Lowry theory. Actually, and simply, the chloride ion is too weak a base in water. Additionally, it is recognized as being a base in some other solvents. 

The Brpnsted-Lowry theory states that acid-base reactions are a competition for a proton. For exeunple, take a look at the reaction of ammonia with water ... 

According to the Brpnsted-Lowry theory, a conjugate acid-base pair consists of molecules or ions related by the loss of one by an acid, and the gain of one by a base. Every acid-base reaction contains two conjugate acid-base pairs because an is transferred in both the forward and the reverse directions. When an acid such as HE loses an H, the conjugate base F is formed. When the base H2O gains an H, its conjugate acid, H30, is formed. 

Acids and bases are an important class of compounds dissolved in aqueous solution and often found in common household products as well as in many drugs. When you eat a grapefruit or a lemon, it tastes sour due to the presence of acids. Coffee and tonic water, on the other hand, taste bitter, due to the presence of bases. The definition of an acid is based on the Brpnsted-Lowry theory, which specifies that acids produce protons (H+) in aqueous solution, that is, they are proton donors. 

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