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 nadechworld.comist 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. TABLE 12.1 Common hydroxides and acids Common hydroxides Common acids sodium hydroxide NaOH hydrochloric acid HCl potassium hydroxide KOH acetic acid HC2H3O2 calcium hydroxide Ca(OH)2 nitric acid HNO3 aluminum hydroxide Al(OH)3 sulfuric acid H2SO4 ammonium hydroxide NH4OH carbonic acid H2CO3 phosphoric acid H3PO4 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.
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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.
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.