Chapter 7

Acids and Bases

Acids and Bases

Arrhenius Definitions－ Acids produce hydrogen ion in aqueous, and bases produce hydroxide ions.

Brønsted-Lowry Definitions－ An acid is a proton (H+) donor, and a base is a proton acceptor.

Lewis Acid-Base Definitions－ An acid as an electron pair acceptor and a base as an electron donor.

An Acid is Dissolved in Water

HA(aq)+H2O(l) H3O+(aq)+A-(aq)

acid base conjugate acid

conjugatebase

3[ ][ ] [ ][ ]

[ ] [ ]

: acid dissociation constant

a

a

H O A H AK

HA HA

K

Conjugate Acid-Base Pair

Conjugate acid-base pair consists of two substances related to each other by the donating and accepting of a single proton.

Water as an Acid and a Base

Water is the common amphoteric substance.Kw=[H3O+][OH-]=1×10-14 ion-product constant

pH=-log[H+]

2H2O(l) H3O+(aq)+OH-(aq)

Calculating the pH of Weak Acid Solutions

HF H++F-

1.0MBefore 0 0Equilibrium (1-x)M xM xM

24 2 4

2 4 4 2

2

7.2 10 7.2 10 ( 1)1

7.2 10 7.2 10 2.7 10

log(2.7 10 ) 1.57

a

xK x if x

x

x x

pH

The 5% Rule The validity of an approximation should

always be check. The 5% rule should be used to evaluate which

approximation are reasonable.

%.%.

.For HF,

%%[HA]

x

-

72100001

1072

5100

2

0

The approximation is acceptable.

The pH of a Mixture of Weak Acids

HNO2 is assumed to be the dominant producer

of H+.

HCN(aq) H+(aq)+CN-(aq) Ka=6.2X10-10

HNO2(aq) H+(aq)+NO2-(aq) Ka=4.0X10-4

H2O(l) H+(aq)+OH-(aq) Kw=1.0X10-14

4 2

2

[ ][ ]4 10

[ ]a

H NOK

HNO

HNO2(aq) H+(aq)+NO2-(aq)

5 0 0

5-x x x2 2

4 2

2

2 0

4 10 4.5 105 5

Using the 5% rule

4.5 10100% 100% 0.9%

[ ] 5

a

x xK x

x

x

HNO

2

-

- 2 -10

- -8

[ ] 4.5 10 1.35

To calculate [CN ]

[ ][CN ] (4.5 10 )[CN ]6.2 10

[HCN] 1

[CN ]=1.4 10 ( )

a

H pH

HK

M

Percent Dissociationamount dissociation

Percent Dissociation= 100%initial concentration

HC3H5O3 H++C3H5O3-

0.1 0 00.1-x x x

3

3 34

3.7% 100% 3.7 10 ( )0.1

(3.7 10 )(3.7 10 )1.4 10

0.1a

xx M

K

Polyprotic AcidsDiprotic Acids

H2CO3(aq) H+(aq)+HCO3-(aq)

HCO3-(aq) H+(aq)+CO3

2-(aq)

1

2

73

2 3

2113

3

[ ][ ]4.3 10

[ ]

[ ][ ]5.6 10

[ ]

a

a

H HCOK

H CO

H COK

HCO

The Concentration of Species of Polyprotic Acid As a Function of pH

21312

3

212

21312

3

311

21312

3

23

0

210

2

212

0

22

][][

][][

][

][][

][

1

aaa

aa

aaa

a

aaa

TTT

T

KKOHKOH

KKα

KKOHKOH

OHKα

KKOHKOH

OHα

ααα

C

][M , α

C

][HM , α

C

M][Hα

][M][HMM][HC

Polyprotic Acid－ H3PO4

a3a2a1a2a12

a13

a3a2a13

a3a2a1a2a12

a13

a2a12

a3a2a1a2a12

a13

2a1

1

a3a2a1a2a12

a13

3

0

3210

T

34

3T

24

2T

421

T

430

34

244243T

KKK][HKK][HK][H

KKKα

KKK][HKK][HK][H

][HKKα

KKK][HKK][HK][H

][HKα

KKK][HKK][HK][H

][Hα

1αααα

C

][POα ,

C

][HPOα ,

C

]PO[Hα ,

C

]PO[Hα

][PO][HPO]PO[H]PO[HC

Calculate the equilibrium concentration of the different species in a 0.1 M phosphoric acid solution at pH=3.

(M)103.3103.30.1C][PO

103.3

(M)109.6109.60.1αC][HPO

109.6

(M)109.20.920.1αC]PO[H

92.0

(M)108.3108.30.1]PO[H

103.8

15143POH

-34

143

652POH

-24

52

21POH

-42

1

320POH43

20

43

43

43

43

C

Acid-Base Properties of Salts Salt that produce neutral solutions

Salts that consist of the cations of strong

bases and the anions of strong acids have no

effect on [H+] when dissolved in water.

The aqueous solutions of salts such as KCl,

NaCl, NaNO3 and KNO3 are neutral.

Acid-Base Properties of Salts Salt that produce basic solutionsC2H3O2

-(aq)+H2O(l) HC2H3O2(aq)+OH-(aq)

2 3 2

2 3 2

2 3 2 2 3 2

2 3 2 2 3 2

1410

5

[ ][ ]

[ ]

[ ][ ] [ ][ ]

[ ] [ ]

1 105.6 10

1.8 10

b

a b w

wb

a

HC H O OHK

C H O

H C H O HC H O OHK K K

HC H O C H O

KK

K

Acid-Base Properties of Salts Salt that produce acidic solutions

NH4+(aq) NH3(aq)+H+(aq)

14103

54

[ ][ ] 1 105.6 10

[ ] 1.8 10w

ab

NH H KK

NH K

Water Contributes to the Very Dilute H+ Concentration

HA(aq) H+(aq)+A-(aq)

H2O(l) H+(aq)+OH-(aq)

0

[ ][ ]

[ ][ ]

[ ]

[ ] [ ] [ ] charge balance

[ ] [ ] [ ] material balance

w

a

K H OH

H AK

HA

H A OH

HA HA A

0 0

0

2

2

0

[ ] [ ] [ ] [ ] [ ][ ]

[ ] [ ] [ ] [ ] [ ] [ ][ ]

[ ] [ ][ ][ ][ ]

[ ][ ] [ ]

[ ]

[ ]

[ ][ ]

[ ]

w

w

w

a

w

w

w

KH A OH A H

H

KHA HA A HA HA H

H

KH H

HH AK

HA KHA H

H

H K

H KHA

H

+ 2 + 2 + 2

+ 2 2

+ 20 0

0

Assume that the condition

[H ] [H ] [H ]

[ ] [ ][ ] [H ]

[ ] [H ] [ ][ ] [ ]

[ ]

w w

w

a

w

K K

KH H

H xK

HA HA xKHA H

H

If [H+]2>>Kw, the full equation reduces to

the typical expression for a weak acid.