A. The Arrhenius Definitions
In Chapter 5, we defined an acid as a substance that releases hydrogen ions in aqueous solutions and a base as a substance that releases hydroxide ions in aqueous solutions. Because this behavior depends on dissociation into ions, and because the theory of ionization was first proposed by the Swedish chemist Svante Arrhenius (1859-1927), these definitions are frequently referred to as the Arrhenius definitions. Table 12.1, a reproduction of Table 5.11, lists several familiar acids and bases.

B. The Brønsted-Lowry Definitions
The Arrhenius definitions of acids and bases describe the characteristics of aqueous solutions of acids and bases. In 1923, T. M. Lowry in England and J. M. Brønsted in Denmark proposed a system that defines acids and bases in terms of the mechanism by which they react. According to the Brønsted-Lowry definitions:

An acid is a proton (H⁺) donor.

A base is a proton (H⁺) acceptor.

Because a hydrogen ion consists of a nucleus containing a single proton, the terms hydrogen ion and proton are synonymous. These definitions somewhat broaden the category of substances that are acids or bases. The category of acids now includes those shown in Table 12.1 as well as ions such as ammonium ion, NH₄⁺, and bicarbonate ion, HCO₃^-. Among Brønsted-Lowry bases are the hydroxide ion, OH^-; the anion of any acid; and ammonia, NH₃. Many substances such as water, bicarbonate ion, and ammonia can act as either an acid or a base.

In the Brønsted-Lowry system, an acid reacts by donating a proton to a base. In doing so, the acid becomes its conjugate base. The formula of the conjugate base is the formula of the acid less one hydrogen. The reacting base becomes its conjugate acid. The formula of the conjugate acid is the formula of the base plus one hydrogen ion. Let us illustrate this system using the neutralization of hydrochloric acid with sodium hydroxide. When hydrochloric acid reacts with hydroxide ion, water and chloride ion are formed. In the equation for the reaction each acid-base pair has the same subscript. Acid₁ is HCl, its conjugate base is base₁; hydroxide ion is base ₂, and its conjugate acid (water) is acid₂.

Chloride ion is the conjugate base of hydrochloric acid. Water is the conjugate acid of the hydroxide ion. In this equation the sodium ion is a spectator ion.

The equation for the reaction of hydrochloric acid with ammonia is

When water reacts with ammonia, it is acting as an acid:

Hydroxide ion is the conjugate base of water. When water reacts with an acid, it is acting as a base:

The conjugate acid of water is the hydronium ion, H₃O⁺, an ion formed by the association of a hydrogen ion with a water molecule.

Example: In the following list, group A contains Brønsted-Lowry acids and group B contains Brønsted-Lowry bases. Whos by equation how each substance in group A acts as an acid using water as a base. Show by equation who each substance in broup B acts as a base using acetic acid as an acid. Group A: HSO4-, HNO3, H2S Group B: OH-, HS- CO32- Solution Group A 1. HSO4- +   H2O H3O+ +   SO42-   acid1 base2 acid2 base1 The conjugate base of the bisulfate ion is the sulfate ion. The conjugate acid of water is the hydronium ion. 2. HNO3 +   H2O NO3- +   H3O+   acid1 base2 base1 acid2 3. H2S +   H2O HS- +   H3O+   acid1 base2 base1 acid2 Notice that the formula of the conjugate base of the acid is the formula of the acid less one hydrogen ion. Group B 1. OH- + HC2H3O2 H2O +   C2H3O2-   base1 acid2 acid1 base2             2. HS- + HC2H3O2 H2S +   C2H3O2-   base1 acid2 acid1 base2             3. CO32- + HC2H3O2 HCO3- +   C2H3O2-   base1 acid2 acid1 base2 Notice that the formula of the conjugate acid of a base contains one more hydrogen ion than the formula of the base.

All of the reactions in Example 12.1 fit the general equation for a Brønsted-Lowry acid-base reaction:

The Brønsted-Lowry definitions greatly expand the classes of substances called acids and bases. Members of the acid group need not be compounds but can also be ions, and a base need not contain in its formula the hydroxide ion. The usefulness of these definitions is emphasized by considering the Lewis structures of typical members of the groups. Following are Lewis structures for some bases.

Each of these bases has at least one unshared pair of electrons on a very electronegative atom. When a proton or hydrogen ion is added to these structures, it forms a covalent bond with one of these unshared electron pairs, as shown below:

Example: Use the Lewis structure for the formate ion, HCO2-, to show how it acts as a Brønsted-Lowry base.