Conjugate Pairs According to the Brønsted-Lowry theory, an acid is any substance (molecule or ion) that can transfer a proton ( ion) to another substance, and a base is any substance that can accept a proton. Acid-base reactions are proton-transfer reactions, as follows: Chemical species whose formulas differ only by one proton are said to be conjugate acid-base pairs. Thus, is the conjugate base of the acid , and is the conjugate acid of the base . Similarly, is the conjugate base of the acid , and is the conjugate acid of the base . Strong acids have very weak conjugate bases, and very weak acids have strong conjugate bases. pKa, pKb, and Acid and Base Strength Learning Goal: To understand the relation between the strength of an acid or a base and its and values. The degree to which a weak acid dissociates in solution is given by its acid-ionization constant, . For the generic weak acid, , and the acid-ionization constant is given by Similarly, the degree to which a weak base reacts with in solution is given by its base-ionization constant, . For the generic weak base, , and the base-ionization constant is given by Another way to express acid strength is by using : Another way to express base strength is by using : Conjugate pairs For any conjugate acid-base pair Acid-Base Properties of Salt Solutions Salt solutions can be neutral, acidic, or basic, depending on the acid-base properties of the constituent cations and anions. Salts formed by reaction of a strong acid with a strong base are neutral, salts formed by reaction of a strong acid with a weak base are acidic, and salts formed by reaction of a weak acid with a strong base are basic. If a salt is derived from a weak acid and a weak base, the of the solution depends on the relative acid strength () of the cation and base strength () of the anion. If , the solution of the salt is acidic. If , it is basic. If , it is neutral. Keep these values in mind for the remaining questions: for is . for is . Dissociation Constant For the dissociation reaction of a weak acid in water, the equilibrium constant is the acid-dissociation constant, , and takes the form Weak bases accept a proton from water to give the conjugate acid and ions: The equilibrium constant is called the base-dissociation constant and can be found by the formula Percent Dissociation In addition to the acid-dissociation constant, , another measure of the strength of an acid is percent dissociation, determined by the following formula: Percent dissociation increases with increasing . Strong acids, for which is very large, dissociate completely (100). For weak acids, the percent dissociation changes with concentration. The more diluted the acid is, the greater percent dissociation. A certain weak acid, , has a value of 4.7×10?7. pH and Percent Ionization of a Weak Base Ammonia, , is a weak base with a value of . Exercise 15.48: Problems by Topic - Autoionization of Water and pH Exercise 15.76: Problems by Topic - Acid Solutions Find the of each of the following solutions of mixtures of acids. Exercise 15.86: Problems by Topic - Base Solutions Amphetamine is a weak base with a of 4.2. Exercise 15.105: Problems by Topic - Polyprotic Acids Exercise 15.126: Cumulative Problems Lakes that have been acidified by acid rain can be neutralized by liming, the addition of limestone . Exercise 15.140: Challenge Problems When 2.55 of an unknown weak acid () with a molar mass of 85.0 is dissolved in 250.0 of water, the freezing point of the resulting solution is -0.255.
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