Weak bases ionization constants

A strong base is a substance that reacts completely with water, so that the base ionization constant defined in Eq. (14) or (15) is effectively infinite. This situation can only be realized if the conjugate acid of the strong base is very weak. A weak base will be characterized by a base ionization constant that is considerably less than unity, so that the position of equilibrium in the reaction represented in Eq. (12) favors the existence of unreacted free base. 

Base ionization constants You won t be surprised to leam that like weak acids, weak bases also form equihbrium mixtures of molecules and ions in aqueous soluhon. Therefore, the equilibrium constant provides a measure of the extent of the base s ionization. The equilibrium constant for the ionization of methylamine in water is defined by this equilibrium constant expression. 

The constant is called the base ionizahon constant. The base ionization constant is the value of the equilibrium constant expression for the ionizahon of a base. The smaller the value of K, the weaker the base. values and ionization equahons for several weak bases are listed in Table 19-4. 

For weak acids and weak bases, the value of the acid or base ionization constant is a measure of the strength of the acid or base. 

TABLE 18-6 Ionization Constants and Values for Some Weak Bases Ionization Reaction iq, at 2S°C ... 

For weak acids, the magnitude of is very small, and as a result the resulting H3O+ and A ions will be produced in small amounts. Under those conditions, both Yjj+ and y will be approximately equal to one, and then one can approximate the thermodynamic equilibrium constant, K, by the concentration-based ionization constant, Ka. 

WEAK BASES AND BASE IONIZATION CONSTANTS Review Questions... 

Just as for weak acids, the equilibrium constant expression is written without including a concentration term for pure water. The resulting equilibrium constant is called a base ionization constant, Ki, ... 

Specific examples of important equilibrium systems include sparingly soluble salts, weak acids, and weak bases. For salts, the solubility product constant, K, provides a way to calculate equilibrium concentrations and molar solubilities. Weak acids and bases also have specifically named equilibrium constants the acid ionization constant, and the base ionization constant, Aj,. 

Base ionization constant (K, ) (12.7) Equilibrium constant for the reaction of a weak base with water, producing OH" and the conjugate acid of the weak base. 

Acid ionization constant Conjugate acid-base ( a). P-671 pair, p. 660 Base ionization constant Ion-product constant, p. 662 (K ), p. 678 Lewis acid, p. 697 Lewis base, p. 697 Strong acid, p. 666 Percent ionization, p. 677 Strong base, p. 667 pH, p. 663 Weak acid, p. 667 Salt hydrolysis, p. 689 Weak base, p. 668... 

Amines are weak bases, and aqueous solutions of amines are basic. The base ionization constant for an amine in water is given the symbol Kt,. 

Base Ionization Constants (Kb) for Select Weak Bases at 298 K ... 

Acid and Base Ionization Constants Strong acids and strong bases are assumed to ionize completely. Most weak acids and bases ionize to a small extent. The concentrations of the acid, conjugate base, and ion at equilibrium can be calculated from... 

Suppose you wish to calculate the pH of 0.10 M NaCN. As you have seen, the solution is basic because of the hydrolysis of the CN ion. The calculation is essentially the same as that for any weak base (Example 17.5). You would require the base-ionization constant of the cyanide ion. However, frequently the ionization constants for ions are not listed directly in tables, because the values are easily related to those for the conjugate molecular species. Thus, the Kb for CN is related to the Ka for HCN. 

Like ammonia, amines are weak bases. Table 16.2 gives base-ionization constants of some amines. 

SECTION 16.6 Weak Bases and Base Ionization Constants... 

A weak base ionizes only partially. The base ionization constant, K, is the equilibrium constant that indicates to what extent a weak base ionizes. 

Base ionization constant (Kb), 652 Lewis acid, 669 Strong conjugate acid, 655 Weak conjugate base, 654... 

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