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Chapter 8 & 9. Acids, Bases & Buffers. Chapter 8 Introducing Acids & Bases. Water pH (Acid rain) in the USA in 2001. Conjugate acids & bases. P172. Relation between [H + ], [OH - ], and pH?. P174. 8-3 Strengths of acids & bases. Strong Acids and Bases
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Chapter 8 & 9 Acids, Bases & Buffers
Chapter 8 Introducing Acids & Bases • Water pH (Acid rain) in the USA in 2001
8-3 Strengths of acids & bases Strong Acids and Bases • Common strong acids and bases are listed in Table 8-1. • A strong acid or strong base is completely dissociated in aqueous solution. (8-4) (8-5) P.175
Carboxylic Acids Are Weak Acids and Amines Are Weak Bases (8-6) P.175
8-3 Strengths of acids & bases Carboxylic Acids Are Weak Acids and Amines Are Weak Bases • Ask yourself at p.178
Metal Ions with Charge ≧2 Are Weak Acids A proton can dissociate from M(H2O)wn+ to reduce the positive charge on the metal complex. Relation Between Ka and Kb P.177
8-4 pH of strong Acids & Bases • Example at p.180 • The pH of 4.2 x 10-3 M HClO4 ? • The pH of 4.2 x 10-3 M KOH? • Can we dissolve base in water and obtain an acidic pH (<7)?
8-5 Tools for Dealing with Weak Acids and Bases • pK the negative logarithm of an equilibrium constant Weak Is Conjugate to Weak • The conjugate base of a weak acid is a weak base. The conjugate acid of a weak base is a weak acid. P.181
Using Appendix B • Acid dissociation constants appear in Appendix B. Each compound is shown in its fully protonated form. • Pyridoxal phosphate is given in its fully protonated form as follows: P.182
Fraction of Dissociation • Figure 8-4 compares the fraction of dissociation of two weak acids as a function of formal concentration. • acid increase as it is diluted. P.184
Chapter 9 Buffers Buffered solution resists changes in pH when small amounts of acids or base are added or when dilution occurs. pH dependence of the rate of a particular enzyme-catalyzed reaction. The rate near pH 8 is twice as the rate at pH 7 or 9
If pH = pKa, [HA] = [A-]If pH < pKa, [HA] > [A-]If pH > pKa, [HA] < [A-]
9-3 A Buffer in Action • Example: find the pH of a buffer solution at p. 198 • Effect of adding acid to a buffer
9-4 Preparing Buffers • Example at p. 202 • In the real life p. 203
Preparing a Buffer in Real Life • Suppose you wish to prepare 1.00 L of buffer containing 0.100 M tris at pH 7.60. When we say 0.100 M tris, we mean that the total concentration of tris plus tris H+ will be 0.100M. 1. Weigh out 0.100 mol tris hydrochloride and dissolve it in a beaker containing about 800 mL water and a stirring bar. 2. Place a pH electrode in the solution and monitor the pH. 3. Add NaOH solution until the pH is exactly 7.60. The electrode does not respond instantly. 4. Transfer the solution to a volumetric flask and wash the beaker and stirring bar a few times. Add the washings to the volumetric flask. 5. Dilute to the mark and mix. P.202
9.5 Buffer capacity -1 Buffer capacity measures how well a solution resists changes in pH when acid or base is added. The greater the buffer capacity, the less the pH changes. The amount of H+ or OH-that buffered solution can absorb without a significant change in pH
9.5 Buffer capacity -2 • 2) Magnitudes of [HA] and [A-] • the capacity of a buffered soln. • Ex : soln A : 5.00 M HOAc + 5.00 M NaOAc • soln B : 0.05 M HOAc + 0.05 M NaOAc • pH change when 0.01 mol of HCl(g) is added
9.5 Buffer capacity -3 3) [A-] / [HA] ratio the pH of a buffered soln.
Table 9-2 Structures and pKa values for common buffers P.205
9.6 How indicators work -1 1) Usually a weak organic acid or base that has distinctly different colors in its nonionized & ionized forms. HIn(aq) H+(aq) + In-(aq)pKHIn nonionized ionized form form
9.6 How indicators work -2 1) 2) 3)
9.6 How indicators work -3 2) The useful pH range for indicator is pKHIn ± 1 (Fig 10.3) encompass the pH at equivalence point (titration curve) • Not all indicators change color at the same pH. (table 9.3)
