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Acids and Bases

In the tradtional (Arrhenius) system, an acid is a substance whose aqueous solution contains more hydrogen ions than hydroxide ions, and a base is a substance whose aqueous solution contains more hydroxide ions than hydrogen ions. In the Brønsted-Lowry system, an acid is a proton donor, and a base is a proton acceptor. There is a system for the nomenclature of acids. Some names must be memorized.

The most common reactions of acids are: (1) neutralization, the reaction of an acid with a base, and (2) displacement of hydrogen by a metal.

Acids can be characterized as strong (completely ionized in solution) or weak (partially ionized in solution). The anion or conjugate base of a strong acid is a weak base; that of a weak acid is a strong base. Molecules of a weak acid exist in aqueous solution in equilibrium with ions. The concentration of the weak acid molecules, hydrogen ion, and the anion of the acid are related by an equilibrium constant known as the acid dissociation constant, Ka, of the acid. This constant always has a value much less than1.

Changing the concentration of one of the components of an equilibrium results in a change in the concentration of the other components. In the equilibrium of a weak acid with its ions, the addition of a salt of the acid profoundly decreases the concentration of the hydrogen ion. this effect is known as the common-ion effect.

The pH of a solution is the negative log of its hydrogen ion concentration. The pKa of an acid is the negative log of its acid dissociation constant. Water is a weak acid with a dissociation constant Kw of 1x10-14; pKw =14. A neutral solution has pH 7. An acidic solution has pH < 7; a basic solution has pH 7.

Hydrolysis is the reaction of water with another substance. Salts whose anions are strong Brønsted-Lowry bases or whose cations are themselves weak acids hydrolyze, yielding either acidic or basic solutions.

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