Chapter FifteenPrentice-Hall ©2002Slide 1 of 31 1 http:\\asadipour.kmu.ac.ir.......44 slides...

$: Chapter FifteenPrentice-Hall ©2002Slide 1 of 31 1 http:\\asadipour.kmu.ac.ir.......44 slides 920117.$
Chapter FifteenPrentice-Hall ©2002 Slide 1 of 31• http:\\asadipour.kmu.ac.ir.......44 slid

es• 1• 920117

Acids And Bases

The Arrhenius Theory

• acid – a substance that produces H+ ion when dissolved in water

• base - a substance that produces OH- ion when dissolved in water

• Limitation of Arrhenius Theory

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The Brønsted-Lowry Theory

• Lowry was an English scientist,.• Bronsted was a Danish scientist

• Acid – proton donor• Base – proton acceptor

• Conjugate acid and base, HA/A-, differ by one proton.

The conjugate acid of a base is the base plus the attached proton and the conjugate base of an acid

is the acid minus the proton. (HCl/Cl-)• A substance that can act either as an acid or a base

is amphiprotic. (H2PO4-)• 920117


es• 5

The BrØnsted-Lowry Theory

• Conjugate acid-base pairs are species that differ by a H+.– Some examples:

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Amphoterism

• Look at this reaction in more structural detail.

2H+

2+

ZnOHO

OOH

O

OH2

H2H2

H2 2

ZnOHO

OOH

O

O

H2H2

H2 H2

2

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O H2 NO 2 Zn NO 2 2H Zn(OH) 2-3

2-32

reaction of zinc hydroxide with nitric acid.

Amphoterism

-24

-2 Zn(OH) 2K2OH 2K Zn(OH)

• Zn(OH)2 behaves as an acid in presence of strong bases.

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2-OH-

OH-

ZnOHO

OOH

O

OH2

H2

H2

H2

ZnOHO

OOH

O

O

H

HH2

H2

In more structural detail.


es• 8

Strong Acid Dissociation (makes 100 % ions)

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es• 9

Weak Acid Dissociation(only partially ionizes)

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Ionization of Ammonia

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Strengths Of Conjugate Acid-Base Pairs• The stronger an acid, the weaker is its conjugate base.

• The stronger a base, the weaker is its conjugate acid.

• An acid-base reaction is favored in the direction from the stronger member to the weaker member of each conjugate acid-base pair.

• HCl+H2O→H3O++Cl-

• CH3COOH + H2O H3O+ + CH3COO- ?????????

• Ka and Kb values are used to compare the strengths of weak acids and bases.

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Leveling and differentiating effect

• Water has a leveling effect; when the strong acids are dissolved in water, they all completely ionize to the hydronium ion.

• HCl+H2O→H3O++Cl-

• HClO4+H2O→H3O++ClO4-

• Which one is stroger?

• Water has a differentiating effect; when the weak acids are dissolved in water, they all partially ionize to the hydronium ion.

• CH3COOH + H2O H3O+ + CH3COO-

• HCOOH + H2O H3O+ + HCOO-

Which one is stroger? Ka• 920117

Differentiating

effect

→→

→→

→

←

H2OWeak conj. bases

Leveling effect

Weak

acids

Weak conj. acid

Weak

bases

Strongest acid

Strongest base

• Strong acid

• Strong base• http:\\asadipour.kmu.ac.ir.......44 slides

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Differentiating

effect

Differentiating effect

CH3COOH

Differentiating effect

NH3

→→

NH3

Leveling effect

←←

CH3COOH

Leveling effect

Strongest acidCH3COOH2

+

Strongest base

NH2 - • 15• http:\\asadipour.kmu.ac.ir.......44

slides

• 920117


es

•Ionization constant of an acid

• For a monoprotic weak acid (HA) dissolved in water,

• HA(aq) + H2O(l) H3O+(aq) + A-(aq)

acid conjugateacidbase conjugate

base

eqbeqb

eqbeqb

eq OHHA

AOHK

2

3

cOHthatassumewillweOHOHSinceeqbinitialeqb

222 ,

cHA

AOHK

eqb

eqbeqb

eq

3 • 16

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es• 17

•Ionization constant of an acid

cHA

AOHK

eqb

eqbeqb

eq

3

eqb

eqbeqb

eqa HA

AOHcKK

3

eqb

eqbeqb

a HA

AOHK

3

• Ionization constant of the acid

MC 55.5518

1000

55.55KK eqa

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Acid And Base Ionization Constants

weak acid: CH3COOH + H2O H3O+ + CH3COO-

[H3O+][CH3COO-]

Acid ionization constant: Ka =

[CH3COOH]

weak base: NH3 + H2O NH4+ + OH-

[NH4+][OH-]

Base ionization constant: Kb =

[NH3]

Ka and Kb are the measure of the strengths of weak acids and bases.

• 920117

Acids

H –X Hydrid acids

H – O – E Oxo acids

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Representative Trends In Strengths of hydrid Acids

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S≈E & R

Relative Strengths Of hydrid Acids

H –X

in a period: (E↑↑↑↑ & R↓)• The larger the electronegativity difference between H and X,

the more easily the proton is removed and the stronger is the acid.

EN 0.4 < 0.9 < 1.4 < 1.9

Acid strength CH4 NH3 H2O HF

• The strengths of binary acids increase from left to right across a period of the periodic table.

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Relative Strengths Of Binary Acids

H –Xin a periodic group: (E↓ & R↑↑↑↑)

Anion radius: the larger the anion’s radius, the stronger the acid.Anion radius 136 < 181 < 195 < 216 HF HCl HBr HI

Acid strength Ka 6.6x10-4 < ~106 < ~108 < ~109

The strength of binary acids increase from top to bottom in a group of the periodic table.

• 920117

Strengths Of Oxoacids

H – O - X

• E≈ Two factors:

- electronegativity of the central atom (E)

- number of terminal oxygen atoms (n)• As the electronegativity of the central atom increases.

As the number of terminal oxygen atoms increases, the acid strength also increases.

E↑ S↑

n↑ S↑ ↑ ↑HOClOn

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S≈E & R

Strengths Of Oxoacids

As the electronegativity of the central atom (E) increases the acid strength increases.Electronegativity 2.5 < 2.8 < 3.0

HOI HOBr HOCl

Acid strength Ka2.3x10-11 < 2.5x10-9 < 2.9x10-8

As the number of terminal oxygen atoms increases, the acid strength also increases. O O

װ װ H-O-Cl<H-O-Cl=O<H-O-Cl=O<H-O-Cl=O

װ O

Acid strength 2.9x10-8 < 1.1x10-2 < ~1000 < ~108

• 920117

Strengths Of Oxoacids Conventional charge or Oxidation number

O O װ װ

H-O-Cl<H-O-Cl=O<H-O-Cl=O<H-O-Cl=O װ

O Oxidation number Cl +1 +3 +5 +7

Conventional charge Cl 0 +1 +2 +3 -------------------------------------------------------- O O Oװ װ װ H-O-P-H= H-O-P-O-H= H-O-P-O-H

ו ו ו H H OH

Oxidation number P +1 +3 +5Conventional charge P +1 +1 +1

• http:\\asadipour.kmu.ac.i

r.......44 slides• 920117

Strengths Of Carboxylic Acids

O

R – C – O – H R – CH2 – O - H

1)Carboxylic acids all have the -COOH group

2)Differences in acid strength must come from differences in the R group attached to the carboxyl group.

3)In general, the more that electronegative atoms are attached in the R group, the stronger the acid.

• 920117

Strengths Of Carboxylic Acids

O װ R – CH2 – O – H < R – C – O – H

In general, the more that electronegative atoms are

attached in the R group, the stronger the acid.

I-CH2CH2COOH Cl-CH2CH2COOH CH3-CHClCOOH CH3CCl2COOH

Ka 8.3x10-5 < 1.0x10-4 < 1.4x10-3 < 8.7x10-3

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Strengths Of Amines As Bases

BrNH2 < NH3 > C6H5NH2

Kb 2.5x10-8 1.8x10-5 7.4x10-10

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• Electron-withdrawing groups on the ring further diminish the basicity of aromatic amines relative to aniline

• Me2NH>MeNH2>Me3N>NH3 Why?

Amine bases:N

HHH

NRH

H

NRH

R

NRR

R

NH

Haromatic amine base

R= CH3, CH2CH3 aliphatic amine bases

ammonia

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Self-Ionization Of Water

• Even the purest of water conducts electricity. This is due to the fact that water self-ionizes, that is, it creates a small amount of H3O+ and OH-.

• 2 H2O (l) = H3O+ (aq) + OH¯ (aq)

• [H3O+] [OH¯]Keq = ————————

[H2O] (1000/18 = 55.6)

• Kw = Keq [H2O] = 1e–14 only at 25oC,

• it’s T dependent.

• 920117

OH

H

OH

HO

Self ionization reaction of water:

+O

HH

H

OHOHOH2Q 32

C)25(at10][OH]O[HK -143w

H

+

+-

• 920117

• T ↑ Kw ↑


es• 32• 920117

pH and pOH

• pH = - log[H3O+] [H3O+] = 10-pH

pOH = - log[OH-] [OH-] = 10-pOH

• pKw = pH + pOH = 14.00

• neutral solution: [H3O+] = [OH-] = 10 –7 M pH = 7.0

acidic solution: [H3O+] > 10-7 M pH < 7.0

basic solution: [H3O+] < 10-7 M pH > 7.0

• 920117

The pH Scale

• 920117

An Example

The pH of milk of magnesia, a suspension of solid magnesium hydroxide in its saturated aqueous solution, is measured to be 10.52. What is the molarity of Mg(OH)2 in its saturated aqueous solution?

• 920117

The Lewis Theory• Developed in 1923 by G.N. Lewis.

• Acids are defined as electron pair acceptors.• Bases are defined as electron pair donors.• Neutralization reactions are accompanied by

coordinate covalent bond formation.• In organic chemistry, • Lewis acids are often called electrophiles

Lewis bases are often called nucleophiles

• 920117

The Lewis Theory

• Look at this reaction in more detail paying attention to the electrons.

NH

H

H +O

H HN

H

H

H

H +

+ O

H -N

H

H

H +O

H HN

H

H

H

H +

+ O

H -

Base - it donates the electron pair

Acid - it accepts the electron pair Notice that a coordinate

covalent bond is formed on the ammonium ion.

• 920117

The Lewis Theory

• Again, a more detailed examination keeping our focus on the electrons.

Br HO

H

H

+ OH

HH

Br

+_

+Br H

O

H

H

+ OH

HH

Br

+_

+

Acid - it accepts the electron pair

Base- it donates the electron pair

covalent coordinate bond formed

• 920117

The Lewis Theory

• A third Lewis example is the autoionization of water.

base acid

OH OH OH OH -322

• 920117

The Lewis Theory

• The reaction of sodium fluoride and boron trifluoride provides an example of a reaction that is

• - only a Lewis acid-base reaction.– It does not involve H+ at all, thus it cannot be an Arrhenius

nor a Brønsted-Lowry acid-base reaction.

NaF + BF3 Na+ + BF4-

• which is the acid and which is the base؟؟؟؟

• Electrophilic replacement

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The Lewis Theory

F B

F

F F

+

_

B

F

FF

F

-

F B

F

F F

+

_

B

F

FF

F

-

Base - it donates the electron pair

Acid - it accepts the electron pair

coordinate covalent bond formed

• 920117


es• 42

Evolution of the acid-base conceptyear thinker Acid Base acid-base reaction

====================================================

1884 Arrhenius ionize ionize H+ + OH¯ HOH H+ OH¯

1923 Bronsted- Proton proton HA + B HB + A

Lowry Donor acceptor conjugation

1923 Lewis electrophil nucleophil E + Nu E:Nu

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Acid-Base Theories

• Arrhenius, Brønsted-Lowry, and Lewis Acid-Base Theories expand on one another.

• Lewis

• Brønsted-Lowry

• Arrhenius

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Acid-Base Theories

• Arrhenius, Brønsted-Lowry, and Lewis Acid-Base Theories expand on one another.

• Lewis

• Brønsted-Lowry

• Arrhenius

• 920117

Chapter FifteenPrentice-Hall ©2002Slide 1 of 31 1 http:\\asadipour.kmu.ac.ir.......44 slides...

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