Chapter 8 & 9

Acids, Bases & Buffers

Chapter 8 Introducing Acids & Bases• Water pH (Acid rain) in the USA in 2001

Conjugate acids & bases

Relation between [H+], [OH-], and pH?

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

Carboxylic Acids Are Weak Acids and Carboxylic Acids Are Weak Acids and Amines Are Weak BasesAmines Are Weak Bases

(8-6)

P.175

8-3 Strengths of acids & bases

• Ask yourself at p.178

Carboxylic Acids are Weak Acids and Amines are Weak Bases

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

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 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

 

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:

•

P.182

8-6 Weak-Acid Equilibrium

•

•

•

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

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)

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. 

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

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. 

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

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

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)

-

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

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)

Table 9-3

Two different sets of colors