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16.8 Relationship Between Ka and Kb 639 16.8 Relationship Between Ka and Kt b VVe've seen in a qualitative way that the stronger acids have the weaker conjugate bases. To see if we can find a corresponding quantitative relationship, let's consider the NH4+ and NH3 conjugate acid-base pair. Each of these species reacts with water: '(aq) ^=± NH3(flt|) + H+(aq) [16.38] H2O(0 ^=^ NH4+(ag) + OH~(aq) [16.39] Each of these equilibria is expressed by a characteristic dissociation constant: [NH3][H+] [NH4+] [NH4+][OH1 [NH3] When Equations 16.38 and 16.39 are added together, the NH4+ and NH3 species cancel and we are left with just the autoionization of water. NH3(fl
640 Chapter 16 Acid-Base Equilibria Chemistry at Work Amines and Amine Hydrochlorides Many amines with low molecular weights have unpleasant "fishy" odors. Amines and NH3 are produced by the anaerobic (absence of O2) decomposition of dead animal or plant matter. Two such amines with very disagreeable odors are H2N(CH2)4NH2, known as putrescine, and H2N(CH2)5NH2, known as cadaverine. '-. Many drugs, including quinine, codeine, caffeine, and amphetamine (Benzedrine), are amines. Like other amines, these substances are weak bases; the amine nitrogen is readily protonated upon treatment with an acid. The resulting products are called add salts. If we use A as the abbreviation for an amine, the acid salt formed by reaction with hydrochloric acid can be written as AH+C1~. It is also sometimes written as A • HC1 and referred. to as a hydrochloride. Amphetamine hydrochloride, for example, is the acid salt formed by treating amphetamine with HC1: Such acid salts are much less volatile, more stable, and generally more water-soluble than the corresponding neutral amines. Many drugs that are amines are sold and administered as acid salts. Some examples of over-the-counter medications that contain amine hydrochlorides as active ingredients are shown in Figure 16.10 T. CH2— CH— HCl(fl-CH2-CH- CH3 Amphetamine hydrochloride A. Figure 16.10 Some over-the-counter medications in which an amine hydrochloride is a major active ingredient. ST. io fc sc at By using Equation 16.40, we can calculate Kb for any weak base if we know Ka for its conjugate acid. Similarly, we can calculate Ka for a weak acid if we know Kb for its conjugate base. As a practical consequence, ionization constants are often listed for only one member of a conjugate acid-base pair. For example, Appendix D does not contain Kj, values for the anions of weak acids because these can be readily calculated from the tabulated Ka values for their conjugate acids. If you look up the values for acid- or base-dissociation constants in a chemistry handbook, you may find them expressed as pKa or pKb (that is, as -log Ka or —log Kb). •'33a (Section 16.4) Equation 16.40 can be written in terms of pKa and pXf, by taking the negative log of both sides. pKa + pKb - pKw = 14.00 at 25°C [16.41] SAMPLE EXERCISE 16,16 Calculate (a) the base-dissociation constant, Kb, for the fluoride ion (F~); (b) the aciddissociation constant, Ka, for the ammonium ion Solution Analyze: We are asked to determine dissociation constants for F , the conjugate base of HF, and NH4+, the conjugate acid of NH3. Plan: Although neither F~ nor NH4+ appears in the tables, we can find the tabulated values for ionization constants for HF and NH3, and use the relationship between Kn and Kb to calculate the ionization constants for each of the conjugates. Solve: (a) Ka for the weak acid, HF, is given in Table 16.2 and Appendix D as Ka = 6.8 x 10~4. We can use Equation 16.40 to calculate Kb for the conjugate base, F : &_i!)xiS£ i. Ka 6.8 x 10~4
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Chapter 16 Text

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