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Chapter 8 & 9
Acids, Bases & Buffers
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Chapter 8 Introducing Acids & Bases• Water pH (Acid rain) in the USA in 2001
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Conjugate acids & bases
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Relation between [H+], [OH-], and pH?
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8-3 Strengths of acids & bases
Strong Acids and BasesStrong 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
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Carboxylic Acids Are Weak Acids and Carboxylic Acids Are Weak Acids and Amines Are Weak BasesAmines Are Weak Bases
(8-6)
P.175
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8-3 Strengths of acids & bases
• Ask yourself at p.178
Carboxylic Acids are Weak Acids and Amines are Weak Bases
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Metal Ions with Charge 2 ≧Metal Ions with Charge 2 ≧ Are Weak Acids Weak Acids
A proton can dissociate from M(H2O)wn+ to reduce
the positive charge on the metal complex.
P.177
Relation Between Ka and Kb
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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)?
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8-5 Tools for Dealing with Weak Acids and Bases
• pK the negative logarithm of an equilibrium constant
Weak Is Conjugate to WeakWeak 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
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P.182
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:
•
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P.182
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8-6 Weak-Acid Equilibrium
•
•
•
P.182
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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
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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
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9.2 The Henderson-Hasselbalch eqn
HAA n whepKpH
acid
baselogpKpH
HA
AlogpHpK
HA
AlogHlog logK
HA
A HK AHHA
-
a
a
a
a
a(aq)(aq)(aq)
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If pH = pKa, [HA] = [A-]If pH < pKa, [HA] > [A-]If pH > pKa, [HA] < [A-]
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9-3 A Buffer in Action• Example: find the pH of a buffer solution at p. 198
• Effect of adding acid to a buffer
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9-4 Preparing Buffers
• Example at p. 202
• In the real life p. 203
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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.
Procedure: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
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9.5 Buffer capacity -1
The amount of H+ or OH-
that buffered solution can
absorb without a significant
change in pH
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.
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9.5 Buffer capacity -2
0.67
0.06
0.04
HA
A1
0.05
0.05
HA
A
:B
0.9965.01
4.99
HA
A1
5.00
5.00
HA
A
:A
H mol 0.01
H mol 0.01
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
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9.5 Buffer capacity -3
3) [A-] / [HA] ratio the pH of a buffered soln.
solnnew 50.5% soln original
49.5HA
A100
0.01
1.00
HA
A
0.98HA
A1.00
1.00
1.00
HA
A
2% :C
H mol 0.01
H mol 0.01
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Table 9-2 Structures and pKa values for common buffers
P.205
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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
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9.6 How indicators work -2
1)
2)
3)
-
HIn
-
HIn
In & HIn of colorthe of ncombinatio
InHIn
1pKpH
In of colorthe show 10
HIn
In
1-pKpH
HIn of colorthe show 10
In
HIn
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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)
3) Not all indicators change color at the same pH. (Table 9.3)
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Table 9-3
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Two different sets of colors