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End Lecture 8
If only the salt s cation affects
pH, the solution is acidic
2
Is NH4Cl acidic or basic in water?
Start Lecture 9
Acid Base Reactions
Lewis Acid Base
Acid-Base Properties of Salts
Syllabus Learning Outcomes : 6, 7
Dr. Michael Love (1)
© 2007
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If only the salt s cation affects
pH, the solution is acidic
3
Is NH4Cl acidic or basic in water?
NH4Cl(aq) → NH4+(aq) + Cl-(aq)
Answer: (a) React anion, Cl-, with H2O
Cl- + H2O →
HCl
+ OHbase
acid
acid
base
Because Cl- is the conjugate base of a very
strong acid (HCl), Cl- does not affect pH.
Calculate the pH of 0.10 M Na2CO3, Kb=2.1 x 10-4.
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NH4Cl(aq) → NH4+(aq) + Cl-(aq)
Answer: (b) React cation, NH4+, with H2O
NH4+ + H2O → NH3 + H3O+
acid
base
base
acid
NH4+ is a weak acid, and NH3 is a weak base.
NH4+ gives an acidic solution
or Ka(NH4+) > Kb(Cl-) so solution is acidic
Cl- gives a neutral solution, no effect on pH
If only the salt s anion affects
pH, the solution is basic
If only the salt s cation affects
pH, the solution is acidic
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If only the salt s anion affects
pH, the solution is basic
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Calculate the pH of 0.10 M Na2CO3, Kb=2.1 x 10-4.
Answer: (a) Cation: Na+ + H2O → neutral
(b) Anion: CO32- + H2O → weak base
CO32- + H2O ↔
HCO3- + OHbase
acid
acid
base
Step 1. Set up ICE table for weak base
[CO32-]
[HCO3-]
[OH-]
initial
change
equilib
1
If only the salt s anion affects
pH, the solution is basic
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Calculate the pH of 0.10 M Na2CO3, Kb=2.1 x 10-4.
Answer: (a) Cation: Na+ + H2O → neutral
(b) Anion: CO32- + H2O → weak base
CO32- + H2O ↔
HCO3- + OHbase
acid
acid
base
Step 1. Set up ICE table for weak base
[CO32-]
[HCO3-]
[OH-]
initial
0.10
0
0
change
-x
+x
+x
equilib 0.10 - x
x
x
If neither the salt s cation nor
anion affects pH, the solution is
neutral, pH=7
If only the salt s anion affects
pH, the solution is basic
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Calculate the pH of 0.10 M Na2CO3, Kb=2.1 x 10-4.
Step 2. Solve equilibrium expression
[HCO3-][OH-]
x2
Kb= 2.1x10-4 = ―――――― = ―――
[CO32-]
0.10-x
100*Kb<0.1, so 0.10-x ≈ 0.10, giving
x= [HCO3-]= [OH-]= 0.00458M
pOH= 2.33, and pH= 11.66
0.1M Na2CO3 is a basic solution.
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Is 0.1M NaCl acidic or basic in water?
If neither the salt s cation nor
anion affects pH, the solution is
neutral, pH=7
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Is 0.1M NaCl acidic or basic in water?
Answer: (a) We did Na+ and Cl−, before.
Neither Na+ nor Cl− affect pH (Na+ from
NaOH and Cl− from HCl).
0.1M NaCl is neutral in water.
If both the salt s cation and
anion affect pH, the larger of Ka
and Kb determine pH
Is 0.1M NH4Bz acidic or basic in water?
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If both the salt s cation and
anion affect pH, the larger of Ka
and Kb determine pH
12
Is 0.1M NH4Bz acidic or basic in water?
Answer: Look up Ka(NH4+) and Kb(Bz−)
(a) Ka for NH4+ = 5.6x10-10
(b) Kb for Bz− = 1.6x10-10
Ka(NH4+) > Kb(Bz−)
NH4Bz makes weakly acidic aqueous
solutions.
2
Lewis acids and bases react
H
―
H
18
2+
H3N―Cu―NH3
―
+
N―
H
4H―
H
NH3
―
―
―
H
H
3+
2+
―
3
Fe―OH2
―
+
―
Fe
Cu
NH3
OH2
:
3+
: O―
N―
H
H―
H
Lewis acids & bases react
2+
H3N―Zn―NH3
―
Base
:
Acid
+
Metal ions form coordinate covalent
bonds and complex ions
―
+
―
Zn
―
:
NH3
H
― ―
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Metal ions form coordinate covalent
bonds and complex ions
N―
H
4H―
H
+
N―
H
H―
H
―
H
+
H
Lewis acids & bases react
2+
:
―
H
Lewis acids & bases react
+
H ―O
―
:O
+
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Ammonium ion has a coordinate covalent
bond
:
+
:
H
H
―
―
hydronium ion has a coordinate covalent
bond
H
―
:
―
N―
H
H―
H
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Lewis acids & bases react
H
+
N―
H
H―
H
•  New bond (Coordinate
covalent bond) forms
using electron pair
H
from the Lewis base.
H―
H
B―
•  Geometry changes
after reaction (VSEPR
N―
model).
H
H―
H
―
―
H ―B
:
Lewis base: a substance
that donates an electron
pair
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H ―B
―
Lewis acid: a substance
that accepts an electron
pair
13
―
Define Lewis acid & Lewis
base
H
NH3
As a result, copper ion (a Lewis acid) can
react with ammonia (a Lewis base).
OH2
3
Lewis acids & bases react in
biology
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Many water-containing
complex ions hydrolyze
(undergo hydrolysis) to give acidic
solutions.
•  The heme group in hemoglobin
interacts with O2 and CO.
•  The Fe ion in hemoglobin is a Lewis
acid
•  O2 and CO can act as Lewis bases
•  CO has a strong (irreversible) binding
constant with hemoglobin, whereas O2
binds more weakly and reversibly.
[Cu(H2O)4]2+ + H2O →
[Cu(H2O)3(OH)]+ + H3O+
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Al(OH)3(s) + 3 H+ → Al3+ + 3 H2O
Cu2+―OH2 bond weakens this O―H bond
allowing
another H2O to
take H+,
(hydrolysis)
:
+
―
: O―
Here Al(OH)3 is a Brønsted base.
Al(OH)3(s) + OH- → Al(OH)4-
Al3+ ß :OH−
Here Al(OH)3 is a Lewis acid.
:
―
H
H
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Brønsted and Lewis theories explain
amphoteric nature of some metal hydroxides.
Hydrolysis makes solutions of Fe3+, Al3+,
Cu2+, and Pb2+ acidic.
−
2+ ― O :
Cu
H
H
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Complex ion formation can
dissolve a precipitate
AgCl(s) ↔
Ag+
+
Cl-
Ag+ + 2NH3 ↔ Ag(NH3)2+
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Ksp = 1.8 x
10-10
Kform = 1.6 x 107
――――――――――――――――
AgCl(s) + 2NH3 ↔ Ag(NH3)2+ + Cl-
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CH3CO2H is an Acid
1. Electronegativity of O atoms takes e−,
weakening O―H bond.
2. H atom in O—H can be taken by H2O to
form H3O+.
Knet=
: O― H
―
+
H
:
O
−
CH3― C―O
=
Knet = Ksp*Kform = 2.9x10-3, so some precipitate
dissolves
H
4
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CH3CO2H
Trichloroacetic acid
10-5
NO3-
Ka = 0.3
Least basic
• Trichloroacetic acid is stronger than
acetic acid because electronegative Cl
in -CCl3 stabilizes ―O− anion more than
the H in -CH3.
Basisity oxoanions é as #Oê
Least basic
ClO3-
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CCl3CO2H
Acetic acid
Ka = 1.8 x
Basisity oxoanions  as − charge 
CO32-
PO43Most basic
As charge  , water interaction  , basicity 
These are both Bronsted and Lewis bases
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Most basic
<
ClO2-
<
ClO-
As # oxygens é , strength of acid é,
basicity conjugate base ê. Recall that
HClO4 is a strong acid.
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