Acids and Bases. Autoionization of Water and the pH Scale H 2 O( l ) H 3 O + ( aq )OH - ( aq ) + +

Acids and Bases

Autoionization of Water and the pH Scale

H2O(l) H2O(l)

H3O+(aq) OH-(aq)

+

+

The nature of the hydrated proton.

Brønsted-Lowry Acid-Base Definition

An acid is a proton donor, any species which donates a H+.

A base is a proton acceptor, any species which accepts a H+.

+O H

H

H

H

H

H + N+O

H

N H

H

H

+H H

+ -O H

H

OH

H

O HH + +O H H

Strong acid: HA(g or l) + H2O(l) H2O+(aq) + A-(aq)

The extent of dissociation for strong and weak acids

The extent of dissociation for strong and weak acids

Weak acid: HA(aq) + H2O(l) H2O+(aq) + A-(aq)

The Acid-Dissociation Constant

Weak acids dissociate very slightly into ions in water.

Strong acids dissociate completely into ions in water.

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

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

Kc >> 1

Kc << 1

Kc = [H3O+][A-]

[H2O][HA]

Kc[H2O] = Ka =[H3O+][A-]

[HA]

stronger acid higher [H3O+]

larger Ka

smaller Ka lower [H3O+]

weaker acid

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Kc = [H3O+][OH-]

[H2O]2

Kc[H2O]2 = [H3O+][OH-]

The Ion-Product Constant for Water

Kw =

A change in [H3O+] causes an inverse change in [OH-].

= 1.0 x 10-14 at 250C

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

In an acidic solution, [H3O+] > [OH-]

In a basic solution, [H3O+] < [OH-]

In a neutral solution, [H3O+] = [OH-]

The relationship between [H3O+] and [OH-] and the relative acidity of solutions.

[H3O+] [OH-]Divide into Kw

ACIDIC SOLUTION

BASIC SOLUTION

[H3O+] > [OH-] [H3O+] = [OH-] [H3O+] < [OH-]

NEUTRAL SOLUTION

The pH values of some familiar

aqueous solutions

pH = -log [H3O+]

Th

e re

lati

on

s a

mo

ng

[H

3O

+],

pH

, [O

H- ],

an

d p

OH

Acid/Base Activity Calculating [H3O+], pH, [OH-], and pOH

PROBLEM: In a restoration project, a conservator prepares copper-plate etching solutions by diluting concentrated HNO3 to 2.0M, 0.30M, and 0.0063M HNO3. Calculate [H3O+], pH, [OH-], and pOH of the three solutions at 250C.

Important pKa values to know

Compound Structure pKa

Alkanes (sp3)51

Alkenes44

Alkynes25

Amines33-35

Alcohols15-19

Water15.7

Carboxylic acids3-5

Special Carboxylic acids

-0.2

Strong AcidsHBrHCl

H2SO4

-9-7-5.2 (first ionization)

C H

H

H

H

H

HH

H

HH

N H

H

R

RO

H

OH H

O

ORH

O

OH

FF

F

Brønsted-Lowry Acid-Base Definition

An acid-base reaction can now be viewed from the standpoint of the reactants AND the products.

An acid reactant will produce a base product and the two will constitute an acid-base conjugate pair.

An acid is a proton donor, any species which donates a H+.

A base is a proton acceptor, any species which accepts a H+.

Proton transfer as the essential feature of a Brønsted-Lowry acid-base reaction

(acid, H+ donor) (base, H+ acceptor)

HCl H2O

+

Cl- H3O+

+

Lone pair binds H+

(base, H+ acceptor) (acid, H+ donor)

NH3 H2O

+

NH4+ OH-

+

Lone pair binds H+

The Conjugate Pairs in Some Acid-Base Reactions

Base Acid+Acid Base+

Conjugate Pair

Conjugate Pair

Reaction 4 H2PO4- OH-+

Reaction 5 H2SO4 N2H5++

Reaction 6 HPO42- SO3

2-+

Reaction 1 HF H2O+ F- H3O++

Reaction 3 NH4+ CO3

2-+

Reaction 2 HCOOH CN-+ HCOO- HCN+

NH3 HCO3-+

HPO42- H2O+

HSO4- N2H6

2++

PO43- HSO3

-+

Acid/Base Activity Identifying Conjugate Acid-Base Pairs

PROBLEM: The following reactions are important environmental processes. Identify the conjugate acid-base pairs.

(a) H2PO4-(aq) + CO3

2-(aq) HPO42-(aq) + HCO3

-(aq)

(b) H2O(l) + SO32-(aq) OH-(aq) + HSO3

-(aq)

Strengths of conjugate acid-

base pairs

HI I-

HBr Br-

HCl Cl-

H2SO4 HSO4-

H2OH3O+

H3PO4 H2PO4-

C6H5COOH C6H5COO-

CH3COOH CH3COO-

H2CO3 HCO3-

H2S HS-

NH3NH4+

C6H5OH C6H5O-

HCO3-

CO32-

CH3NH2CH3NH3+

H2O HO-

CH3CH2OH CH3CH2O-

HC CH HC C-H2 H-NH3 NH2

-CH2=CH2 CH2=CH

-CH3CH3 CH3CH2

-Acid Formula pKa Conjugate BaseEthane

Ammonia

EthanolWater

Bicarbonate ionPhenol

Ammonium ion

Carbonic acidAcetic acid

3525

Benzoic acidPhosphoric acid

Sulfuric acidHydrogen chlorideHydrogen bromideHydrogen iodide

51

38

10.33

15.715.9

4.766.36

9.24

9.95

-5.2-7

-9-8

4.19

2.1-1.74Hydronium ion

Strongerconjugate

base

Weakerconjugate

base

Weaker acid

Stronger acid

Methylammonium ion 10.64

Hydrogen sulfide 7.04

AcetyleneHydrogen

Ethylene 44

3.18 HF

Equilibrium favors reaction of the stronger acid and stronger base to give the weaker acid and weaker base

+ +CH3 CO

O HCH3 C

O

O

N HH

HN H

H

HH

ammoniumacetate ionpKa 4.76 pKa 9.24

acetic acid + sodium bicarbonate (omit Na+ )

6.36pKa= 4.76

CO2

H2O++CH3 C

O

O

C O-H

O

O

H

+CH3 CO

O H

C O-H

O

O

Acid/Base Activity Predicting the K Acid-Base Reaction“Will this deprotonate that”

PROBLEM: Predict the net direction and whether Ka is greater or less than 1 for each of the following reactions (assume equal initial concentrations of all species):

(b) H2O(l) + HS-(aq) OH-(aq) + H2S(aq)

(a) H2PO4-(aq) + NH3(aq) HPO4

2-(aq) + NH4+(aq)

In Class

a) You smell ammonia when NH3 dissolves in water

Use balanced equations showing the net direction of the reaction to describe each of the following observations.

b) The odor goes away when you add an excess of HCl to the solution in part a)

c) The odor returns when you add an excess of NaOH to the solution in part b)

Acid/Base Activity Finding the Ka of a Weak Acid from the pH of

Its Solution

PROBLEM: Phenylacetic acid (C6H5CH2COOH, simplified here as HPAc) builds up in the blood of persons with phenylketonuria, an inherited disorder that, if untreated, causes mental retardation and death. A study of the acid shows that the pH of 0.12M HPAc is 2.60. What is the Ka of phenylacetic acid?

Acid/Base Activity Determining Concentrations from Ka and

Initial [HA]

PROBLEM: Propanoic acid (CH3CH2COOH, which we simplify and HPr) is an organic acid whose salts are used to retard mold growth in foods. What is the [H3O+] of 0.10M HPr (Ka = 1.3x10-5)?

Percent HA dissociation = [HA]dissociated

[HA]initial

x 100

Polyprotic acids

acids with more than more ionizable proton

H3PO4(aq) + H2O(l) H2PO4-(aq) + H3O+(aq)

H2PO4-(aq) + H2O(l) HPO4

2-(aq) + H3O+(aq)

HPO42-(aq) + H2O(l) PO4

3-(aq) + H3O+(aq)

Ka1 =[H3O+][H2PO4

-]

[H3PO4]

Ka2 =[H3O+][HPO4

2-]

[H2PO4-]

Ka3 =[H3O+][PO4

3-]

[HPO42-]

Ka1 > Ka2 > Ka3

= 7.2x10-3

= 6.3x10-8

= 4.2x10-13

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Acid/Base Activity Calculating Equilibrium Concentrations for a Polyprotic Acid

PROBLEM: Ascorbic acid (H2C6H6O6; H2Asc for this problem), known as vitamin C, is a diprotic acid (Ka1 = 1.0x10-5 and Ka2 = 5x10-12) found in citrus fruit. Calculate [H2Asc], [HAsc-], [Asc2-], and the pH of 0.050M H2Asc.

+

CH3NH3+ OH-

methylammonium ion

Abstraction of a proton from water by methylamine

+

CH3NH2H2O

methylamine

Lone pair binds H+

BA

SE

ST

RE

NG

TH

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Acid/Base Activity Determining pH from Kb and Initial [B]

PROBLEM: Dimethylamine, (CH3)2NH, a key intermediate in detergent manufacture, has a Kb of 5.9x10-4. What is the pH of 1.5M (CH3)2NH?

Acid/Base Activity Determining the pH of a Solution of A-

PROBLEM: Sodium acetate (CH3COONa, or NaAc for this problem) has applications in photographic development and textile dyeing. What is the pH of 0.25M NaAc? Ka of acetic acid (HAc) is 1.8x10-5.

The effect of atomic and molecular properties on nonmetal hydride acidity.

6A(16)

H2O

H2S

H2Se

H2Te

7A(17)

HF

HCl

HBr

HI

Electronegativity increases, acidity increases

Bon

d st

reng

th d

ecre

ases

, ac

idity

incr

ease

s

H O I H O Br H O Cl< <

H O Cl

O

O

O<<

The relative strengths of oxoacids

H O Cl

Free Ion Hydrated Ion Ka

Fe3+

Sn2+

Cr3+

Al3+

Be2+

Cu2+

Pb2+

Zn2+

Co2+

Ni2+

Fe(H2O)63+(aq)

Sn(H2O)62+(aq)

Cr(H2O)63+(aq)

Al(H2O)63+(aq)

Be(H2O)42+(aq)

Cu(H2O)62+(aq)

Pb(H2O)62+(aq)

Zn(H2O)62+(aq)

Co(H2O)62+(aq)

Ni(H2O)62+(aq)

6 x 10-3

4 x 10-4

1 x 10-4

1 x 10-5

4 x 10-6

3 x 10-8

3 x 10-8

1 x 10-9

2 x 10-10

1 x 10-10

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Ka Values of Some Hydrated Metal Ions at 250C

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Al(H2O)5OH2+Al(H2O)63+

The acidic behavior of the hydrated Al3+ ion

H2O H3O+

Electron density drawn toward Al3+

Nearby H2O acts as base

Acid/Base Activity Predicting Relative Acidity of Salt Solutions

PROBLEM: Predict whether aqueous solutions of the following are acidic, basic, or neutral, and write an equation for the reaction of any ion with water:

(a) Potassium perchlorate, KClO4 (b) Sodium benzoate, C6H5COONa

(c) Chromium trichloride, CrCl3 (d) Sodium hydrogen sulfate, NaHSO4

Acid/Base Activity Predicting the Relative Acidity of Salt Solutions from Ka and Kb of the Ions

PROBLEM: Determine whether an aqueous solution of zinc formate, Zn(HCOO)2, is acidic, basic, or neutral.

Molecular Structure and AcidityMolecular Structure and Acidity

relative acidities the more A:(-) stable, greater the acidity of H-A

Ways to stabilize A:(-) the negative charge

ON a more electronegative atomON larger atomRESONANCE delocalized STABILIZED by inductive effectIN an orbital with more s character

+ +CH3 CO

O HCH3 C

O

O

N HH

HN H

H

HH

A:-A-H :B H-B+

within a period - the greater the electronegativity of A:(-)

the more A:(-) is stablized

the stronger the acid

Conjugate baseAcid

Molecular Structure and AcidityMolecular Structure and Acidity

pKa 38

pKa 16

pKa 51

CH3 OH3C O H

CH3 NH3C N HH H

CH3 CH3C C HH H

H H

Size of A:(-) Within a column - the larger the atom bearing the (-), the greater its stability

Molecular Structure and AcidityMolecular Structure and Acidity

CH3 O

pKa 7.0 pKa 16

+ +H3C S H H3C O HCH3 S

Resonance delocalized of charge in A-

Compare alcohol and carboxylic acid acidity

Molecular Structure and AcidityMolecular Structure and Acidity

CH3 CO

O

+ pKa = 15.9C OC

H H

HH

HH

OH

H

+

alkoxide ion

C OC

H H

HH

H OH

HH

+ pKa = 4.76OH

H

CH3 CO

OH

CH3 CO

O+ O

H

HH

resonance stabilization

curved arrows show the flow of electrons in an acid-base reaction

+ +CH3 CO

O HCH3 C

O

O

N HH

HN H

H

HH

ammoniumacetate ion

Molecular Structure and AcidityMolecular Structure and Acidity

+HSO4-C

OH3C

O

H

HORC

OH3C

OH

H

C

O+H3C

OH

H OSO3H

Molecular Structure and AcidityMolecular Structure and Acidity

molecules may have 2 or more sites that can accept a H+

e.g. carboxylic acids, esters, and amidesprotonation favored where the charge is more delocalized

which oxygen is protonated?

Molecular Structure and AcidityMolecular Structure and Acidity

resonance

octets greater contribution even with plus charge on O

C

O+ H2SO4 +HSO4

-H3C

OH

C

OH3C

OH

H

C

OH3C

O

H

HOR

C

OH3C

OH

H

C

OH3C

OH

H

C

O+ H2SO4 +HSO4

-H3C

OH

C

OH3C

OH

H

C

OH3C

O

H

HOR

C

OH3C

OH

H

C

OH3C

OH

H

Molecular Structure and AcidityMolecular Structure and Acidity

H+ on the hydroxyl

C

OH3C

O

H

H

can “write” contributing structures create & separate charge

H+ on the carbonyl

:B

C C

F

F

F

H

H

O

inductive effect, electron-withdrawing

covalent bonds transmit electronegativity (polarizing) effects push or pull shared e(-)s of adjacent atoms

Molecular Structure and AcidityMolecular Structure and Acidity

pKa: 15.9

C C

H

H

H

H

H

OH

C C

F

F

F

H

H

OH

12.4

C C

H

C

H

H

H

OH

FF

F

14.6

C C

H

C

H

H

H

OH

HC

H

F

FF

15.4

:B

C C

H

H

H

H

H

O

decreases w/ distance

inductive effect: butanoic and chlorobutanoic acids

Molecular Structure and AcidityMolecular Structure and Acidity

pKa 4.82 pKa 4.52 pKa 3.98 pKa 2.83

OH

O

OH

O

OH

O

OH

O

Cl

ClCl

Hybridization

greater the % s character with (-) the more stable the anion

Molecular Structure and AcidityMolecular Structure and Acidity

HO-H HO–

Weak Acid

alkyne

alkene

alkane

water

25

44

51

15.7

ConjugateBase pKa

H C C H H C C

HC C

H H

HC C

H H

H

HC C

H H

HH H

HC C

H HH H

Molecules as Lewis Acids

F

B

F F

H

N

H H

+

F

B

F F

H

N

H H

acid base adduct

An acid is an electron-pair acceptor.

A base is an electron-pair donor.

M2+

H2O(l)

M(H2O)42+(aq)

adduct