chapter 8 acid-base equilibria -...

2/25/2003 OFB Chapter 8 1

Chapter 8Acid-Base Equilibria

• 8-1 Brønsted-Lowry Acids and Bases

• 8-2 Water and the pH Scale• 8-3 The Strengths of Acids and

Bases• 8-4 Equilibria Involving Weak

Acids and Bases• 8-5 Buffer Solutions• 8-6 Acid-Base Titration Curves• 8-7 Polyprotic Acids• 8-8 Lewis Acids and Bases


Acid and Base Definitions

1) Arrhenius (Section 4.3)• Acids are H+ donors• Bases are OH- donors

2) Broadened Definition (Section 4.3)

• Acids are substances that increase [H+]

• Bases are substances that increase [OH-]

3) Brønsted-Lowry (Section 8.1)••

4) Lewis (Section 8.8)••



• Brønsted-Lowry– Acids

– Bases

– Conjugate Base -

– Conjugate Acid


Acid-Base Equilibria

Brønsted-Lowry Acids and Bases

A Brønsted-Lowry acid is a substance that can donate a hydrogen ion.

A Brønsted-Lowry base is a substance that can accept a hydrogen ion.

In the Brønsted-Lowry Acid and Base concept, acids and bases occur as conjugate acid-base pairs.


Conjugate Base - subtract an H+ from the acidConjugate Acid add H+ to the base

Examples

1. is the conjugate base of H2O

2. is the conjugated base of H3O+ (called the hydronium ion)

3. is the conjugated acid of OH-

4. (or shown as H+) is the conjugate acid of H2O

• Conjugate means to join, unite or marry


H2O

- H+ + H+

OH-conjugate

base of H2O

conjugate acid of H2O

H3O+

+ H+- H+

H2OWater is the conjugate acid

of OH-

Water is the conjugate base

of H3O+

Hydroxide ion

Hydronium ion

Conjugate Base - subtract an H+ from the acidConjugate Acid add H+ to the base


CH3CO2H + H2O ↔ H3O+ + CH3CO2-

Pairs 1 2 2 1

Acetic Acid

Point of View #1

acid base Conjugate acid of H2O

Conjugate base of

CH3CO2H


Point of View #2

acid baseConjugate base of H3O+

Conjugate acid of

CH3CO2-


basebaseacid acid

Acetate Ion


H2O + H2O ↔ H3O+ + OH-

Pairs 1 2 2 1basebaseacid acid

Autoionization of H2O

Point of View #2

acid baseConjugate base of H3O+

Conjugate acid of

OH-

H2O + H2O ↔ H3O+ + OH-

Point of View #1

acid base Conjugate acid of H2O #2

Conjugate base of H2O #1

H2O + H2O ↔ H3O+ + OH-# 1 # 2


Exercise 8-1:

Trimethylamine (C3H9N) is a soluble weak base with a foul odor (it contributes to the smell of rotten fish). Write the formula of its conjugate acid.

2/25/2003 OFB Chapter 8 10

Nomenclature

When H+ is hydrated it is H3O+

and called a hydronium ion.

Often H3O+ is written in a simpler notation H+

111.7°

+

2/25/2003 OFB Chapter 8 11

Amphoterism - an ion or molecule can act as an acid or base depending upon the reaction conditions

H2O + NH3 ↔ NH4+ + OH-

1.) Water in NH3 serves as an acid

2.) Water in acetic acid serves as a base

H2O + CH3CO2H ↔ H3O+ + CH3CO2-

2/25/2003 OFB Chapter 8 12

3.) Acetic Acid is also amphoteric, if in the presence of a strong acid serves as a base

H2SO4 + CH3CO2H ↔ CH3CO2H2+ + HSO4

-

Amphoterism - an ion or molecule can act as an acid or base depending upon the reaction conditions

2/25/2003 OFB Chapter 8 13

Water

[H3O+][OH-]

[H2O]2

Autoionization of water:

2 H2O(l) ⇔ H3O+(aq) + OH-(aq)

KW = 1.0 × 10-14

(at 25oC)

2/25/2003 OFB Chapter 8 14

Strong Acids and Bases

A strong acid is one that reacts essentially completely with water to produce H3O+(aq).

Hydrochloric acid (HCl) is a strong acid:

HCl (aq) + H2O(l) → H3O+(aq) + Cl-(aq) (reaction essentially complete)

Dissolving 0.10 mol of HCl in enough water to make 1.0 L of solution gives a final concentration of 0.10 M for H3O+(aq).

[H3O+][OH-] = KW

2/25/2003 OFB Chapter 8 15

Strong Acids and BasesA strong base is one that reacts essentially completely with water to produce OH-(aq) ions.

Sodium hydroxide (NaOH) is a strong base:

Others are NH2- (amide ion) and H- (Hydride ion)

NaOH(s) → Na+(aq) + OH-(aq) (reaction essentially complete)

Dissolving 0.10 mol of NaOH in enough water to make 1.0 L of solution gives a final concentration of 0.10 M for OH-(aq).

[H3O+][OH-] = KW

2/25/2003 OFB Chapter 8 16

The pH Function

pH = -log10[H3O+]

pH < 7 acidic solution

[H3O+] > [OH-]

pH = 7 neutral solution

[H3O+] = [OH-]

pH > 7 basic solution

[H3O+] < [OH-]

2/25/2003 OFB Chapter 8 17

The pH Function

EXAMPLE 8-3

Calculate the pH (at 25oC) of an aqueous solution that has an OH-(aq) concentration of 1.2 × 10-6 M.

2/25/2003 OFB Chapter 8 18

If converting from pH to [H+]

[H+] = 10 -pH = 10 -X

pH = -log10[H3O+]

2/25/2003 OFB Chapter 8 19

Recall

[H3O+][OH-] = KW = 10 -14

pH + pOH = pKw = 14

2/25/2003 OFB Chapter 8 20

Exercise page 8-4:

Calculate [H3O+] and [OH-] in saliva that has a pH of 6.60 at 25oC.

The pH Function

2/25/2003 OFB Chapter 8 21

Strengths of Acids and Bases

• Strong acids1.2.3.4.5.6.7.

[HA]]][AO[H

O][HA][H]][AO[HK

3

2

3a

−+

−+

=

=

HA + H2O ↔ H3O+ + A –HA = generic acid

2/25/2003 OFB Chapter 8 22

a10a KlogpK −=

Table 8-2

Scale of pKa

-11

To

+14

2/25/2003 OFB Chapter 8 23

2/25/2003 OFB Chapter 8 24


• Base Strength– strong acids have weak

conjugate bases– weak acids have strong

conjugate bases

2/25/2003 OFB Chapter 8 25

Indicators

A soluble compound, generally an organic dye, that changes its color noticeably over a fairly short range of pH.Typically, a weak organic acid that has a different color than its conjugate base.

HIn(aq) + H2O(l) ⇔ H3O+(aq) + In-(aq)

[H3O+][In-]

[HIn]= Ka

2/25/2003 OFB Chapter 8 26

2/25/2003 OFB Chapter 8 27

Methyl Red

Bromothymol blue

Phenolphtalein

2/25/2003 OFB Chapter 8 28

2/25/2003 OFB Chapter 8 29

IndicatorsExercise 8-5:

Rainwater that has leached through a pile of building materials and puddled underneath turns bromothymol blue from yellow to blue, but it leaves phenolphthalein colorless. Estimate the pH of this water.

2/25/2003 OFB Chapter 8 30

Equilibria Involving Weak

Acids and Bases

Weak acids Ka < 1

i.e., pKa > 0

H3O+ (hydronium ion)

Ka = 1

pKa =0

HA + H2O↔ H3O+ + A-

HA is a weak acid,

Ka < 1 or pKa > 1

2/25/2003 OFB Chapter 8 31

2/25/2003 OFB Chapter 8 32

2/25/2003 OFB Chapter 8 33

Problem: Calculate pH and the fraction of CH3CO2H ionized at equilibrium. Assume 1.0M CH3CO2H initially


Init. conc.

∆ conc.

Equil. conc.

==+

O]H][HCO[CH]CO][CHO[H K

223

-233

a

2/25/2003 OFB Chapter 8 34

2-8 Table from 1.8x10K 5a

−=

A trick to solving:

Assume that y is small (less than 5% of the initial conc.)

y)(1.0y

O]H][HCO[CH]CO][CHO[H K

2

223

-233

a −==

+

2/25/2003 OFB Chapter 8 35

Fraction CH3CO2H ionized at equilibrium

Problem: Calculate pH and the fraction of CH3CO2H ionized at equilibrium. Assume 1.0M CH3CO2H initially

2/25/2003 OFB Chapter 8 36

Weak Bases

Similarly NH3 acts as a weak base in H2O

Kb = 1.8 x 10-5

H20 + NH3 ↔ NH4+ + OH-

acid1 base2 acid2 base1

O]][H[NH]][OH[NH K

23

-4

b

+

= 1

2/25/2003 OFB Chapter 8 37

Init. conc.

∆ conc.

Equil. conc.

As before, you can calculate a pH at equilibrium. Assume 0.01M NH3initially. Calculate the pH of the resulting solution

H20 + NH3 ↔ NH4+ + OH-

53 b

3

-4

b

1.8x10NHfor K

y)(0.01y2

][1][NH]][OH[NH K

−

+

=

−==

2/25/2003 OFB Chapter 8 38

4--

52 b

3

-4

b

4.15x10][NH4 ][OH y

1.8x10 yK

y0.01y2

][1][NH]][OH[NH K

===

==

−==

+

−

+

== −+

][OHKw]O[H3

2/25/2003 OFB Chapter 8 39

• Hydrolysis is a term applied to reactions of aquated ions that change the pH from 7

• When NaCl is placed in water, the resulting solution is observed to be neutral (pH = 7)• However when sodium acetate (NaC2H3O2) is dissolved in water the resulting solution is basic• Other salts behave similarly, NH4Cl and AlCl3 give acid solutions.• These interactions between salts and water are called hydrolysis

2/25/2003 OFB Chapter 8 40

Despite the special term, hydrolysis, there is no reason to treat hydrolysis in a special manner.

It is still a Brønsted-LowryAcid and Base Reaction

2/25/2003 OFB Chapter 8 41

Example problem:

Suppose a 0.1 mole solution sodium acetate is dissolved in 1 liter of water. What is the pH of the solution?

Init. conc.

∆ conc.

Equil. conc.

1.Find Kb

2.Find [OH-]

3.Find [H+]

4.Find pH

2/25/2003 OFB Chapter 8 42

Example problem:

What is the pH of the solution?CH3CO2

- + H2O ↔ CH3CO2H + OH-

Init. conc. 0.1M 0 ~0

∆ conc. - y + y + y

Equil. conc. 0.1– y y y

1. Find Kb

2. Find [OH-]

3. Find [H+]

4. Find pH

6

45

14

a

wb

2

-23

-23

b

7.5x10][OHy

5.6x10 table1.8x10

1.0x10KK K

y0.1y

][1]CO[CH]H][OHCO[CH K

−−

−−

−

==

===

−==

Ka x Kb = Kw

8.89 )(1.39x10log- pH

x103.17.5x101x10

][OHKw]O[H

9-10

96

14

3

==

=== −−

−

−+

2/25/2003 OFB Chapter 8 43

Raise pH

Lower pH

Anions

Cations

ResultHydrolysis of

Non-Hyrolyzed Ions (a few)7 Anions, not hydrolyzed

Cl-, Br-, I-, HSO4-, NO3

-, ClO3-,

ClO4-

10 Cations, not hydrolyzed

Li+, Na+, K+, Rb+, Sc+, Mg++, Ca++, Sr++, Ba++, Ag+

2/25/2003 OFB Chapter 8 44

Raise pH

Lower pH

Anions

Cations

ResultHydrolysis of

Can predict pH of some salts (relative pH)Na3PO4 is basic

(a non hydrolyzed cation and a hydrolyzed anion)

FeCl3 is acidic

(a hydrolyzed cation and a non hydrolyzed anion)

2/25/2003 OFB Chapter 8 45


• Buffer Solutions: important in biochemical and physiological processes

• Organisms (and humans) have built-in buffers to protect them against large changes in pH.

• Buffers any solutions that maintain an approximately constant pH despite small additions of acids or bases

2/25/2003 OFB Chapter 8 46

Buffers any solutions that maintain an approximately constant pH despite small additions of acids or bases

Human blood (pH=7.4) is maintained by a combination of CO3

-2, PO4-3 and protein

buffers, which accept H+

Death = 7.0 < pH > 7.8 = Death

How Do Buffers Work?

2/25/2003 OFB Chapter 8 47

How Do Buffers Work?

• H+ depends on Ka and the ratio of acid to salt.

• Thus if both conc. HA and A-

are large then small addictions of acid or base don’t change the ratio much

formula K rearranged][A

[HA]K]O[H

a

a3 −+ =

2/25/2003 OFB Chapter 8 48

• H+ depends on Ka and the ratio of acid to salt.

• Thus if both conc. HA and A- are large then small addictions of acid or base don’t change the ratio much

Try Example 8-11

2/25/2003 OFB Chapter 8 49

2/25/2003 OFB Chapter 8 50

2/25/2003 OFB Chapter 8 51

8-8 Lewis Acids

• Extremely important for Organic Chemistry

• Recall from Chapter 3 that main group elements in a molecule are surrounded by 8 electrons (the Octet Rule).

• Hydrogen by 2 electrons

• Carbon is 6C or 1s2 2s2 2p2

• Hydrogen is 1H or 1s1

2/25/2003 OFB Chapter 8 52

CH HCH4

C

H

H

HHmethane

H

H

Covalent bonds

Lewis Structures

2/25/2003 OFB Chapter 8 53

O HH OH HO

HHwater

N H

H

H NH

H

HNH3

ammonia

Lone pairLewis Structures

2/25/2003 OFB Chapter 8 54

B F

F

F B

F

F

F

boron triflouride

Electron deficient

Lewis Structures

2/25/2003 OFB Chapter 8 55


• Lewis Acids-Bases– Lewis Acid is any species that

accepts electron pairs• Called electrophiles (electron

seeking)• e.g., Boron, Al, Group III

elements

• BF3, Mg2+, H+, H3O+

BH3, SnCl4, CH3+, AlCl3, HCl

2/25/2003 OFB Chapter 8 56


• Lewis Acids-Bases– Lewis Base is any species that

donates electrons through coordination to its lone pair

• Called Nucleophiles (nucleus seeking)• e.g., Group V, VI, VII elements

• Br -1, Cl -1, F -1, H -1, HO -1, :NH3, H2O::

R R

O

RN

R

R

AminesCarbonyls

R O H

Alcohols

2/25/2003 OFB Chapter 8 57

O

H

H H + OH HOH +

Lewis Acid Lewis Base

HO H

H3C

O

O

acetic acid

H +

H3C

O

O-

acetate ion

+ NH4+

ammonium ion

:NH3

ammonia

Lewis Acid Lewis Base Lewis Base Lewis Acid

2/25/2003 OFB Chapter 8 58


• Examples/Exercises8-1, 8-2, 8-3, 8-4, 8-6, 8-7, 8-8,8-9, 8-10, 8-11, 8-16

• Problems5, 9, 11, 12, 26, 31,

42, 43, 44, 46

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