1. No category

Insert picture from
First page of chapter
Chapter 16
Acids and Bases
Copyright McGraw-Hill 2009
1
16.1 Brønsted Acids and Bases
• Acid – proton donor
• Base – proton acceptor
• Conjugate base – what remains of the
acid after the donation of a proton
• Conjugate acid – newly formed
protonated species (base)
Copyright McGraw-Hill 2009
2
Conjugate Acid-Base Pairs
Copyright McGraw-Hill 2009
3
Copyright McGraw-Hill 2009
4
16.2 The Acid-Base Properites of
Water
• Autoionization of water – water is
amphoteric, can act as an acid or base
or
Copyright McGraw-Hill 2009
5
For pure water at 25 oC


K w  [H3O ][OH ]
7
7
K w  (1 x 10 ) x (1 x 10 )  1 x 10
14
Relative amounts determine if the solution is
acid, base, or neutral.
• When [H3O+] = [OH], the solution is neutral.
• When [H3O+] > [OH], the solution is acidic.
• When [H3O+] < [OH], the solution is basic.
Copyright McGraw-Hill 2009
6
16.3 The pH Scale
pH  log[H  ]
or
pH  log[H 3O  ]
or

[H3O ]  10
pH
pH Meter: Laboratory
Measurement of pH
Copyright McGraw-Hill 2009
7
Relative amounts determine if the solution is
acid, base, or neutral.
• When pH = 7, the solution is neutral.
• When pH < 7.00, the solution is acidic.
• When pH > 7.00, the solution is basic.
Copyright McGraw-Hill 2009
8
Copyright McGraw-Hill 2009
9
Copyright McGraw-Hill 2009
10
1.What is the pH of a solution that has a
hydronium ion concentration of 6.5 x 105M?
2. What is the hydronium ion concentration of
a solution with pH 3.65?
Copyright McGraw-Hill 2009
11
1.
pH  log[H 3O  ]
5
pH  log[6.54 x 10 ]
pH  4.19
2.

pH

3.65
[H3O ]  10
[H3O ]  10
[H3O  ]  2.2x10 4
Copyright McGraw-Hill 2009
12
Relations among pH, [OH], pOH, and Kw

pOH  log[OH ]

[OH ]  10
pOH
K w  [H3O  ][OH  ]
pK w  pH  pOH  14.00
Copyright McGraw-Hill 2009
13
Copyright McGraw-Hill 2009
14
1. What is the pOH of a solution that has a
hydroxide ion concentration of 4.3 x 102M?
2. What is the hydroxide ion concentration of a
solution with pOH 8.35?
Copyright McGraw-Hill 2009
15
1.
pOH  log[OH  ]
2
pOH  log[4.3 x 10 ]
pOH  1.37
2.
[OH ]  10pOH
[OH ]  108.35
[OH  ]  4.5x10 9
Copyright McGraw-Hill 2009
16
16.4 Strong Acids and Bases
• Ionization or dissociation goes to completion –
not considered an equilibrium process
• Concentration of ions in solution is determined
by the stoichiometry of the reaction only
– [H3O+] is equal to starting concentration of the
acid
– [OH] is equal to starting concentration of the
base (stoichiometry must be considered)
• Only a few acids and bases are strong
Copyright McGraw-Hill 2009
17
Copyright McGraw-Hill 2009
18
Copyright McGraw-Hill 2009
19
What is the pH of a 0.057 M solution of HBr?
Copyright McGraw-Hill 2009
20
HBr completely ionizes, and the [H3O+] is
equal to the initial concentration of HBr.
HBr (aq) +
H2O (l)
H3O (aq) + Br (aq)
pH  log[H 3O  ]
pH  log[0.057]
pH  1.24
Copyright McGraw-Hill 2009
21
What is the pOH of a solution of 0.034 M
solution of Ca(OH)2?
Copyright McGraw-Hill 2009
22
Ca(OH)2 completely ionizes, and the [OH] is
equal to twice the initial concentration of
Ca(OH)2.
Ca(OH)2 (aq)
2+
Ca (aq) + 2OH (aq)

2
mol
OH
[OH ]  0.034 M Ca(OH)2 x
1 mol Ca(OH)2
[OH  ]  0.068 M
pOH  log[OH  ]
pOH  log[0.068]
pOH  1.17
Copyright McGraw-Hill 2009
23
16.5 Weak Acids and Acid
Ionization Constants
• Weak acids ionize only to a limited extent
in water.
• The degree to which a weak acid ionizes
depends on
– the concentration of the acid
– the equilibrium constant for the
ionization called the acid ionization
constant, Ka.
Copyright McGraw-Hill 2009
24
For a generic weak monoprotic acid
[H3O ][A  ]
Ka 
[HA]
or
[H ][A  ]
Ka 
[HA]
Copyright McGraw-Hill 2009
25
Copyright McGraw-Hill 2009
26
Format for solving problems of weak acids
using an equilibrium table
• Fill in initial concentrations
• Determine concentration changes in terms of x
• Determine equilibrium concentrations in terms of initial
concentrations (Ci) and x
• Substitute into the Ka expression and solve for x
Copyright McGraw-Hill 2009
27
( x )( x )
Ka 
Ci  x
which produces a quadratic expression of the form:
x  K a x  K aCi  0
2
If the initial concentration and Ka allow, the expression
can be simplified by assuming that x is insignificant
when compared to Ci.
Ci  x  Ci
( x )( x ) x 2
Ka 

Ci
Ci
Copyright McGraw-Hill 2009
28
Testing the approximation – it is acceptable to
use this shortcut if the calculated value of x is
less than 5 percent of the initial acid
concentration.
x
5% 
x100
Ci
Copyright McGraw-Hill 2009
29
Calculate the pH at 25°C of a 0.18 M solution
of a weak acid that has Ka = 9.2 x 106.
Copyright McGraw-Hill 2009
30
0.18 M
−x
0.18 M − x
9.2 x 10
6
0
0
+x
+x
x
x
( x )( x )

0.18 M  x
Use the approximation since Ka is small compared to Ci.
9.2 x 10
6
2
x

0.18 M
Copyright McGraw-Hill 2009
31
9.2 x 10 6
x2

0.18 M
1.3 x 10 3 M  x
Check the approximation:
1.3 x 10 3 M
x100  0.72%
0.18 M
0.72%  5%
Approximation is valid.
pH  log(1.3 x 10
3
M )  2.89
Copyright McGraw-Hill 2009
32
16.6 Weak Bases and Base
Ionization Constants
• Weak bases ionize only to a limited extent
in water.
• The degree to which a weak base ionizes
depends on
– the concentration of the base
– the equilibrium constant for the
ionization called the base ionization
constant, Kb.
Copyright McGraw-Hill 2009
33
For a generic weak base
[HB  ][OH  ]
Kb 
[B]
Solving problems involving weak bases requires the
same approach as for weak acids. Solving for x in a
typical weak base problem gives the hydroxide ion
concentration rather than the hydronium ion
concentration.
Copyright McGraw-Hill 2009
34
Copyright McGraw-Hill 2009
35
Determine the Kb of a weak base if a 0.50 M
solution of the base has a pH of 9.59 at
25°C.
Copyright McGraw-Hill 2009
36
pH  pOH  14.00
9.59  pOH  14.00
pOH  4.41
[OH ]  10pOH

[OH ]  10
4.41
5
 3.89 x 10 M
Copyright McGraw-Hill 2009
37
0.050 M
− 3.89x10-5 M
0.050 M*
0M
0M
+3.89x10-5 M +3.89x10-5 M
3.89x10-5 M
3.89x10-5 M
* 0.050 M - (3.89 x 10-5 M )  0.050 M
[HB  ][OH  ]
Kb 
[B]
(3.89x10 5 M )2
Kb 
 3.0 x 10 9
0.50 M
Copyright McGraw-Hill 2009
38
16.7 Conjugate Acid-Base Pairs
• The conjugate base of a strong acid, is an
example of a weak conjugate base.
• The conjugate base of a weak acid, is an
example of a strong conjugate base.
• Conversely, a strong base has a weak
conjugate acid and a weak base has a
strong conjugate acid.
Copyright McGraw-Hill 2009
39
Reciprocal Relationship Between the Strength of an Acid
or Base and the Strength of its Conjugate
Quantitative Relationship Between Ka and Kb
Kb  K a  K w
Copyright McGraw-Hill 2009
40
Determine Kb of the benzoate ion (C6H5COO).
Copyright McGraw-Hill 2009
41
Benzoate ion is the conjugate base of benzoic acid.
H2O (l)
C6H5COOH (aq) +
H3O (aq) + C6H5COO (aq)
K a  6.5 x 10 5
C6H5COO (aq)
+
H2O (l)
OH (aq) + C6H5COOH (aq)
Kw
Kb 
Ka
1.0 x 10 14
10
Kb 

1.5
x
10
6.5 x 10 5
Copyright McGraw-Hill 2009
42
16.8 Diprotic and Polyprotic Acids
• Diprotic and polyprotic acids undergo
successive ionizations, losing one proton
at a time
• The conjugate base in the first ionization is
the acid in the second ionization.
• Each ionization has a Ka associated with
it.
• Ionization constants are designated by the
step in the ionization involved.
Ka1 , Ka2 , etc.
Copyright McGraw-Hill 2009
43
Copyright McGraw-Hill 2009
44
Calculate the concentrations of H2C2O4,
HC2O4 , C2O42, and H+ ions in a 0.20 M
oxalic acid solution at 25°C.
Copyright McGraw-Hill 2009
45
H2C2O4 (aq)
K a 1  6.5 x 10-2
H (aq) + HC2O4 (aq)
H2C2O4 (aq)
H (aq) + HC2O4 (aq)
0.20 M
−x
0.20 M − x
+x
+x
x
x

[HC 2O 4 ][H  ]
x2
K a1 

 6.5x10 2
[H2C2O 4 ]
0.20 M  x
x 2  6.5 x 10 2 x  1.3 x 10 2  0
x  0.086 and  0.151
[H+]=0.086 M
[HC2O4+]=0.086 M
[H2C2O4]=0.20 M − 0.086 M = 0.11 M
Copyright McGraw-Hill 2009
46
HC2O4 (aq)
K a 2  6.1 x 10 5
H (aq) + C2O4 (aq)
H (aq) + C2O4 (aq)
HC2O4 (aq)
0.086 M
0.086 M
−y
0.086 M − y
K a2
+y
0.086 M + y
+y
y
[C2O42- ][H ] (0.086  y)(y) (0.086 M )(y)
5




6
.
1x10
0.086 M  y
0.086 M
[HC2O4  ]
y  6.1x10 5 M
[H+] = 0.086 M
[HC2O4] = 0.086 M
[C2O42] = 6.1 x 105 M
Copyright McGraw-Hill 2009
47
16.9 Molecular Structure and Acid
Strength
• The strength of an acid is measured by its
tendency to ionize.
HX
H + X
• Two factors influence the extent to which
the acid undergoes ionization.
– the strength of the H X bond
– polarity of the H X bond
Copyright McGraw-Hill 2009
48
Types of Acids
• Hydrohalic Acids – binary acids formed
between hydrogen and the halogens
– the predominant factor in determining
the strength of the hydrohalic acids is
bond strength
HF << HCl < HBr < HI
Copyright McGraw-Hill 2009
49
Copyright McGraw-Hill 2009
50
• Oxoacids - contain hydrogen, oxygen,
and a central, nonmetal atom
– contain one or more O H bonds
– an electronegative or a high oxidation
state central atom, will attract electrons,
causing the O H bond to be more
polar
– high electronegativity and oxidation
result in stronger acids
Copyright McGraw-Hill 2009
51
To compare oxyacid strengths
• Oxoacids having different central atoms
that are from the same group of the
periodic table and that have the same
oxidation number.
– acid strength increases with increasing
electronegativity of the central atom
Example:
Copyright McGraw-Hill 2009
52
• Oxoacids having the same central atom
but different numbers of oxygen atoms
– strength increases with increasing
oxidation number of the central atom
Example:
Copyright McGraw-Hill 2009
53
• Carboxylic acids – organic acids with the
structure
O
R C O H
– Ionize to produce a carboxylate anion
O
O
R C O
R C O
– Increasing stability of the carboxylate
anion increases acidity
• Stability increases with the number of
electronegative groups in the R group
Copyright McGraw-Hill 2009
54
Indicate which is the stronger acid: (a) HBrO3
or HBrO4; (b) H2SeO4 or H2SO4.
Copyright McGraw-Hill 2009
55
a)These two acids have the same central atom but
differ in the number of attached oxygen atoms. In a
group such as this, the greater the number of
attached oxygen atoms, the higher the oxidation
number of the central atom and the stronger the
acid.
HBrO4 > HBrO3
b) In a group with different central atoms, we must
compare electronegativities. The electronegativities
of the central atoms in this group decrease as
follows: S>Se.
H2SO4 > H2SeO4
Copyright McGraw-Hill 2009
56
16.10 Acid-Base Properties of Salt Solutions
• Salt solutions are affected by salt
hydrolysis, in which ions produced by the
dissociation of a salt react with water to
produce either hydroxide ions or
hydronium ions—thus impacting pH.
• Basic salt solutions - an anion that is the
conjugate base of a weak acid reacts with
water to produce hydroxide ion.
Copyright McGraw-Hill 2009
57
• Acidic salt solutions
– When the cation of a salt is the conjugate
acid of a weak base, a solution of the salt
will be acidic.
For example:
Copyright McGraw-Hill 2009
58
– Small, highly charged metal ions are hydrated
by water molecules and polarization of one of
the O-H bonds can produce hydrogen ions.
Copyright McGraw-Hill 2009
59
Determine the pH of a 0.25 M solution of
pyridinium nitrate (C5H5NHNO3) at 25°C.
[Pyridinium nitrate dissociates in water to
give pyridinium ions (C5H5NH+), the conjugate
acid of pyridine (Kb = 1.7 x 109), and
nitrate ions (NO3 ).]
Copyright McGraw-Hill 2009
60
C5H5NH (aq) + H2O(l)
C5H5N (aq) + H3O (aq)
K w 1.00 x 10 14
6
Ka 


5
.
9
x
10
Kb
1.7 x 10 9
[C 5H6N][H 3O ]
Ka 
[C 5H6NH ]
C5H5NH (aq) + H2O(l)
C5H5N (aq) + H3O (aq)
0.25 M
−x
0.25 − x M
Copyright McGraw-Hill 2009
+x
+x
x
x
61
C5H5N (aq) + H3O (aq)
C5H5NH (aq) + H2O(l)
0.25 M
−x
0.25 − x M
+x
+x
x
x
x2
x2
Kb 

 5.9 x 106
(0.25 M  x) (0.25 M )
x  (5.9 x 106 )(0.25 M )  1.2 x 10 3 M
1.2 x 10 3 M
Check:
x 100  0.49%
0.25 M
[H3O ]  1.2 x 103 M
pH  l og(1.2 x 10
3
M )  2.92
Copyright McGraw-Hill 2009
62
• Neutral salt solutions
– A salt composed of the cation of a strong
base and the anion of a strong acid
produces a neutral solution.
– These ions do not hydrolyze in water.
For example:
NaCl or KNO3
Copyright McGraw-Hill 2009
63
Summary
Copyright McGraw-Hill 2009
64
• Salts in Which Both the Cation and the
Anion Hydrolyze
–pH depends on the relative strengths
of the conjugate acid and base
• When Kb > Ka, the solution is basic.
• When Kb < Ka, the solution is acidic.
• When Kb = Ka, the solution is neutral
or nearly neutral.
For example: NH4NO2
Copyright McGraw-Hill 2009
65
Predict whether 0.10 M solutions of the
following are acidic, basic or nearly neutral.
a) C5H5CNHCl
b) KF
Copyright McGraw-Hill 2009
66
a) C5H5CNHCl
acidic
C5H5NH (aq) + H2O(l)
C5H5N (aq) + H3O (aq)
b) KF
basic
F (aq) + H2O(l)
HF (aq) + OH (aq)
Copyright McGraw-Hill 2009
67
16.11 Acid-Base Properties of
Oxides and Hydroxides
Copyright McGraw-Hill 2009
68
• Basic metallic oxides react with water to
form metal hydroxides.
• Acidic nonmetallic oxides react with water
to form acids.
Copyright McGraw-Hill 2009
69
• Basic and Amphoteric Hydroxides
– All the alkali and alkaline earth metal
hydroxides, except Be(OH)2, are basic.
– Be(OH)2, as well as Al(OH)3, Sn(OH)2,
Pb(OH)2, Cr(OH)3, Cu(OH)2, Zn(OH)2, and
Cd(OH)2, are amphoteric and insoluble in
water.
– React with acids
– React with bases
Copyright McGraw-Hill 2009
70
16.12 Lewis Acids and Bases
• Lewis base as a substance that can
donate a pair of electrons.
• Lewis acid is a substance that can accept
a pair of electrons.
• A Lewis acid-base reaction is one that
involves the donation of a pair of electrons
from one species to another.
• Most general definition of acids and bases
Copyright McGraw-Hill 2009
71
A Lewis Acid-Base Reaction
coordinate covalent bond
Copyright McGraw-Hill 2009
72
Key Points
• BrØnsted acids and bases
– Conjugate acid
– Conjugate base
• Acid-base properties of water
– Autoionization of water
– Kw
– Relation between hydronium and hydroxide
ion concentrations
Copyright McGraw-Hill 2009
73
• The pH scale
– Defining equation
– pOH scale
– Relation among pH, pOH and Kw
• Strong acids and bases
• Weak acids and acid ionization constants
– Definition of weak acids
– Ka, the acid ionization (equilibrium) constant
– Method to solve weak acid equilibrium
problems
Copyright McGraw-Hill 2009
74
• Weak bases and base ionization constants
– Definition of weak bases
– Kb, the base ionization (equilibrium) constant
– Method to solve weak base equilibrium problems
• Conjugate acid-base pairs
– Strength of acid-base pairs
– Relation between Ka and Kb for conjugate pairs
• Diprotic and triprotic Acids
– Successive ionization steps
– Associated Ka values
– Solving equilibrium problems
Copyright McGraw-Hill 2009
75
• Molecular structure and acid strength
– Hydrohalic acids
– Oxoacids
– Carboxylic acids
• Acid-base properties of salt solutions
– Basic salts
– Acidic salts
– Neutral salts
– Complex salts in which both ions hydrolyze
Copyright McGraw-Hill 2009
76
• Acid-base properties of oxides and
hydroxides
– Oxides
• Metal
• Nonmetal
– Basic and amphoteric hydroxides
• Lewis acids and bases
Copyright McGraw-Hill 2009
77