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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
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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.
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Strong Acid Dissociation (makes 100 % ions)
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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
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Chapter FifteenPrentice-Hall ©2002 Slide 16 of 31• http:\\asadipour.kmu.ac.ir.......44 slid
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•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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•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.
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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.
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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
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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
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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.
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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.
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OH
H
OH
HO
Self ionization reaction of water:
+O
HH
H
OHOHOH2Q 32
C)25(at10][OH]O[HK -143w
H
+
+-
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• T ↑ Kw ↑
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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
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The pH Scale
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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?
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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
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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.
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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
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The Lewis Theory
• A third Lewis example is the autoionization of water.
base acid
OH OH OH OH -322
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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
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Chapter FifteenPrentice-Hall ©2002 Slide 42 of 31• http:\\asadipour.kmu.ac.ir.......44 slid
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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
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