Base-ionization Constant Kb

At this point, the calculation becomes a little different. Because the benzoate ion is a base, the equilibrium constant expression must be the base ionization constant, Kb. You haven t been given the value of Kb, however, so you will have to generate it using Equation 13.8 ... 

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

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

A handbook that lists only pfCg values for weak electrolytes has the following entry for 1,2-ethanediamine, NH2CH2CH2NH2 p/aminopropanoic acid, NH2CH(CH3)C00H pKi = 2.34(-l-1) p/<2 = 9.87(0). Interpret these handbook entries by writing equations for the ionization reactions to which these pfC values apply. What are the corresponding values of the base ionization constants Kb, and Kbj ... 

Kb is also widely called the base ionization constant. 

Ka and Kb are particular types of equilibrium constants that give us an idea of the relative strengths of acids and bases, respectively. The acid-dissociation constant, Ka, is the equilibrium constant for the ionization of a weak acid to a hydrogen ion and its conjugate base. Likewise, the base-dissociation constant, Kb, is the equilibrium constant for the addition of a proton to a weak base by water to form its conjugate acid and an OH ion. 

Dissociation (Ionization) Constants (Kb) of Selected Amine Bases... 

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. 

I Acid- and base-ionization constants are often iisted as I pKflS and pKbS pKb = -log Kb). 

The equilibrium constant for the reaction of an acid with water is called the acid ionization constant (K ), equation (16.10), and that for the reaction of a base with water is called the base ionization constant (K, ), equation (16.13). Strong acids and strong bases have large ionization constants (Xa or Kb >i> 1) and are essentially completely ionized in water. Weak acids and weak bases have small ionization constants K or Xt, 1) and ionize to a limited extent in water. The extent to which acids or bases ionize in water is described in terms of either the degree of ionization (a) (equation 16.15) or the percent ionization (equation 16.16). For a weak acid or weak base, the degree of ionization increases with increasing dilution. 

The mathematical relationship between ATa, the ionization constant for a weak acid, and Kb, the base hydrolysis constant for the anion of the weak acid is = ATa x Kt,. 

Since NH4+ is a stronger acid than N02 is a base, the solution will be acidic. The ionization constants were computed from data in Table 17-3 and with the relationship Kw = Ktx Kb. 

Kv is the self-ionization constant for water (Table 3.2) and equation (3.18) reflects the not surprising inverse relation between Ka and Kh. It is only when Ka and Kv for a compound are of different magnitudes that it may be classified as an acid or a base. An example which is difficult to classify is hypoiodous acid (HOI) where K = 2.5 x lO11 mol dm 3 andKh = 3.2 x 10 10 mol dm3. Although Kb has been widely used in the past, it is a quantity which is largely redundant, for Ka (or pKa) may be used to express the strength of bases as well as acids, see Table 3.3. 

The answer is in the expression for the ionization constant, K l or Kb, where the ratio of the conjugate acid and base concentrations is found. In the case of a weak monoprotic acid, HA, we have the following ... 

Relative quantitative strengths of acids and bases are given either by their ionization constants, Ka and Kb, or by their pKa and pKh values as defined by ... 

Bq = becquerel (nuclear Kb = ionization constant (base) V = volume... 

Relative basicities are expressed using the basicity constant, Kb, which is defined as the concentrations of the ionized amine products in water (ammonium salt and hydroxide) divided by the concentration of the un-ionized amine. Larger Kb s mean greater basicity. pKb is the negative logarithm of Kb the smaller the pKb the stronger the base. 

The ionization constants of acids (Ka) and bases (Kb) are a measure of relative tendencies to lose or gain protons in solutions. Hence, they enable us to arrange a series of compounds in order of ascending acidic strength. 

In practice one prefers to use the pKa value instead of ionization constant. Kg (la). In 1923 Bronsted extended the use of acidic ionization constants to bases defining Kb as well as pKb (lb). Consequently, we may infact be able to use pKg values to measure basicities of various nitrogen atoms in complex heterocyclic molecules. 

A simple relationship between the ionization constant of a weak acid (K ) and the ionization constant of its conjugate base (Kb) can be derived as follows, using acetic acid as an example ... 

HA with an ionization constant. Ka and of a base BOH whose constant is Kb, in separate solutions. Ionization proceeds as follows ... 

Notice that neutrality, that is, xa — xb = 0, occurs at pH 7 only if [A ]/Ka — [B ]/Kb- This is why a solution of sodium acetate, for example, a neutral salt of a strong base (large Kp) and a weak acid (small Ka), exhibits a high pH. The slope of a pH curve is influenced strongly by the ionization constants of both acid and base. The ionization of weak acids and bases is severely limited by the concentration of their companion ions a solution whose pH is thus limited is said to be buffered. ... 

X 10 /4.2 X 10 = 2.4 X 10 1 Thus, Na2C03(aq) is basic. 16-8. Since equilibrium favors the formation of the weaker base, we need to identify bases that have an ionization constant, /Tb significantly larger than that of CHsCOO" (say, 100 times larger). For CHsCOO , Kb = K /Ka(CH3COOH) =... 

Equilibrium constants of weak bases can be measured in the laboratory by procedures very much like those used for weak acids. In practice, though, it is simpler to take advantage of a simple mathematical relationship between Kb for a weak base and Ka for its conjugate acid. This relationship can be derived by adding together the equations for the ionization of the weak acid HB and the reaction of the weak base B- with water ... 

Kb is an equilibrium constant governing the ionization of weak bases. 

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