Brønsted-Lowry definition of an acid: An acid is a
proton donor.
721
Brønsted-Lowry definition of an acid: An acid is a
proton donor.
Brønsted-Lowry definition of a base: A base is a
proton acceptor.
722
Brønsted-Lowry definition of an acid: An acid is a
proton donor.
Brønsted-Lowry definition of a base: A base is a
proton acceptor.
H+(aq) + OH-(aq) 
 H2O(l)
acid
base
723
Brønsted-Lowry definition of an acid: An acid is a
proton donor.
Brønsted-Lowry definition of a base: A base is a
proton acceptor.
H+(aq) + OH-(aq) 
 H2O(l)
acid
base
HCl(g) + NH3(g)
acid
base
 NH Cl

4 (s)
724
Brønsted-Lowry definition of an acid: An acid is a
proton donor.
Brønsted-Lowry definition of a base: A base is a
proton acceptor.
H+(aq) + OH-(aq) 
 H2O(l)
acid
base
 NH Cl

4 (s)
HCl(g) + NH3(g)
acid
base
Note in this latter example, there is no water present.
725
Strong and Weak Acids:
726
Strong and Weak Acids:
+
HBr(g) + H2O(l) 
H
O
+
Br
(aq)
 3 (aq)
727
Strong and Weak Acids:
+
HBr(g) + H2O(l) 
H
O
+
Br
(aq)
 3 (aq)
[H3O ][Br - ]
Kc 
[HBr][H2O]
728
Strong and Weak Acids:
+
HBr(g) + H2O(l) 
H
O
+
Br
(aq)
 3 (aq)
[H3O ][Br - ]
Kc 
[HBr][H2O]
Kc is large – the concentration of HBr at equilibrium
is very small.
729
Strong and Weak Acids:
+
HBr(g) + H2O(l) 
H
O
+
Br
(aq)
 3 (aq)
[H3O ][Br - ]
Kc 
[HBr][H2O]
Kc is large – the concentration of HBr at equilibrium
is very small.
The size of Kc can be used to indicate the strength
of an acid, but there is a more convenient way to
represent data on acid strengths, which will be
treated shortly.
730
+
HF(g) + H2O(l) 
H
O
+
F
(aq)
 3 (aq)
731
+
HF(g) + H2O(l) 
H
O
+
F
(aq)
 3 (aq)
[H3O ][F- ]
Kc 
[HF][H2O]
732
+
HF(g) + H2O(l) 
H
O
+
F
(aq)
 3 (aq)
[H3O ][F- ]
Kc 
[HF][H2O]
Kc for this acid is small. This means the
concentration of H3O+ at equilibrium is small.
733
+
HF(g) + H2O(l) 
H
O
+
F
(aq)
 3 (aq)
[H3O ][F- ]
Kc 
[HF][H2O]
Kc for this acid is small. This means the
concentration of H3O+ at equilibrium is small.
HF(aq) is a weak acid.
734
+
HF(g) + H2O(l) 
H
O
+
F
(aq)
 3 (aq)
[H3O ][F- ]
Kc 
[HF][H2O]
Kc for this acid is small. This means the
concentration of H3O+ at equilibrium is small.
HF(aq) is a weak acid.
Note that a weak acid can be corrosive. HF(aq) is a
very corrosive acid – it attacks glass.
735
Strong and weak bases.
NH2- + H2O(l) 
 NH3(aq) + OH (aq)
736
Strong and weak bases.
NH2- + H2O(l) 
 NH3(aq) + OH (aq)
amide ion
737
Strong and weak bases.
NH2- + H2O(l) 
 NH3(aq) + OH (aq)
amide ion
Kc for this reaction is large, indicating the amide ion
is a strong base.
738
Strong and weak bases.
NH2- + H2O(l) 
 NH3(aq) + OH (aq)
amide ion
Kc for this reaction is large, indicating the amide ion
is a strong base.
NH3(g) + H2O(l)
 NH +
+
OH

4 (aq)
(aq)
739
Strong and weak bases.
NH2- + H2O(l) 
 NH3(aq) + OH (aq)
amide ion
Kc for this reaction is large, indicating the amide ion
is a strong base.
NH3(g) + H2O(l)
 NH +
+
OH

4 (aq)
(aq)
Kc for this reaction is small, indicating that NH3 is a
weak base.
740
Amphoteric compounds: Compounds that can
function as either acids or bases, depending on the
other substances present.
741
Amphoteric compounds: Compounds that can
function as either acids or bases, depending on the
other substances present.
Example: HCO3-
742
Amphoteric compounds: Compounds that can
function as either acids or bases, depending on the
other substances present.
Example: HCO3-
HCO3-(aq) + H3O+(aq) 
 H2CO3(aq) + H2O
743
Amphoteric compounds: Compounds that can
function as either acids or bases, depending on the
other substances present.
Example: HCO3-
HCO3-(aq) + H3O+(aq) 
 H2CO3(aq) + H2O
base in this reaction
744
Amphoteric compounds: Compounds that can
function as either acids or bases, depending on the
other substances present.
Example: HCO3-
HCO3-(aq) + H3O+(aq) 
 H2CO3(aq) + H2O
base in this reaction
2HCO3-(aq) + OH-(aq) 
CO
3 (aq) + H2O

745
Amphoteric compounds: Compounds that can
function as either acids or bases, depending on the
other substances present.
Example: HCO3-
HCO3-(aq) + H3O+(aq) 
 H2CO3(aq) + H2O
base in this reaction
2HCO3-(aq) + OH-(aq) 
CO
3 (aq) + H2O

acid in this reaction
746
Amphiprotic: A substance that can function as an
acid or a base.
747
Conjugate acid-base relationships
748
Conjugate acid-base relationships
Consider the equilibrium:
HCN(aq) + H2O


H3O+(aq) + CN-(aq)
749
Conjugate acid-base relationships
Consider the equilibrium:

+
HCN(aq) + H2O
H
O
+
CN
3 (aq)
(aq)

conjugate acid-base pair
750
Conjugate acid-base relationships
Consider the equilibrium:
HCN(aq)
conjugate acid-base pair

+
+ H2O
H
O
+
CN
3 (aq)
(aq)

conjugate acid-base pair
751
Conjugate acid-base relationships
Consider the equilibrium:
HCN(aq)
conjugate acid-base pair

+
+ H2O
H
O
+
CN
3 (aq)
(aq)

conjugate acid-base pair
The formulas of any conjugate acid-base pair differ
only by one H+. CN- is a base, because of the
reaction:
752
H3O+(aq) + CN-(aq) 
 HCN(aq) + H2O
753
H3O+(aq) + CN-(aq) 
 HCN(aq) + H2O
stronger stronger
weaker weaker
acid
base
acid
base
754
H3O+(aq) + CN-(aq) 
 HCN(aq) + H2O
stronger stronger
weaker weaker
acid
base
acid
base
The conjugate acid-base relationship can be written
in generic format as:
755
H3O+(aq) + CN-(aq) 
 HCN(aq) + H2O
stronger stronger
weaker weaker
acid
base
acid
base
The conjugate acid-base relationship can be written
in generic format as:
acid1 + base2


acid2 + base1
756
H3O+(aq) + CN-(aq) 
 HCN(aq) + H2O
stronger stronger
weaker weaker
acid
base
acid
base
The conjugate acid-base relationship can be written
in generic format as:
acid1 + base2


acid2 + base1
757
H3O+(aq) + CN-(aq) 
 HCN(aq) + H2O
stronger stronger
weaker weaker
acid
base
acid
base
The conjugate acid-base relationship can be written
in generic format as:

acid1
H

+ base2 
acid2 + base1

H
758
759
Self-Dissociation of Water
760
Self-Dissociation of Water
Water dissociates to give very low concentrations of
hydronium and hydroxide ions:
761
Self-Dissociation of Water
Water dissociates to give very low concentrations of
hydronium and hydroxide ions:
+
H2O + H2O 
H
O
+
OH
3 (aq)
(aq)

762
Self-Dissociation of Water
Water dissociates to give very low concentrations of
hydronium and hydroxide ions:
+
H2O + H2O 
H
O
+
OH
3 (aq)
(aq)

The position of this equilibrium lies strongly on the
left (at 25 oC):
763
Self-Dissociation of Water
Water dissociates to give very low concentrations of
hydronium and hydroxide ions:
+
H2O + H2O 
H
O
+
OH
3 (aq)
(aq)

The position of this equilibrium lies strongly on the
left (at 25 oC):

Kc 
[H3O ][OH ]
2
 3.25x 1018
[H2O]
764
At equilibrium and at 25 oC, the concentrations are
[H3O+ ] = [OH-] = 1.00 x 10-7 M, and [H2O] = 55.5 M.
765
At equilibrium and at 25 oC, the concentrations are
[H3O+ ] = [OH-] = 1.00 x 10-7 M, and [H2O] = 55.5 M.
Because the concentration of H3O+ is so small, the
concentration of H2O is essentially constant. That is,
it is almost a pure liquid. The preceding expression
for Kc can be simplified. We can write:
766
At equilibrium and at 25 oC, the concentrations are
[H3O+ ] = [OH-] = 1.00 x 10-7 M, and [H2O] = 55.5 M.
Because the concentration of H3O+ is so small, the
concentration of H2O is essentially constant. That is,
it is almost a pure liquid. The preceding expression
for Kc can be simplified. We can write:
Kc[H2O]2  [H3O][OH-]
767
At equilibrium and at 25 oC, the concentrations are
[H3O+ ] = [OH-] = 1.00 x 10-7 M, and [H2O] = 55.5 M.
Because the concentration of H3O+ is so small, the
concentration of H2O is essentially constant. That is,
it is almost a pure liquid. The preceding expression
for Kc can be simplified. We can write:
Kc[H2O]2  [H3O][OH-]
Now set Kw = Kc[H2O]2 , so that at 25 oC
768
At equilibrium and at 25 oC, the concentrations are
[H3O+ ] = [OH-] = 1.00 x 10-7 M, and [H2O] = 55.5 M.
Because the concentration of H3O+ is so small, the
concentration of H2O is essentially constant. That is,
it is almost a pure liquid. The preceding expression
for Kc can be simplified. We can write:
Kc[H2O]2  [H3O][OH-]
Now set Kw = Kc[H2O]2 , so that at 25 oC
Kw  [H3O][OH-]  1.00 x 10-14
769
At equilibrium and at 25 oC, the concentrations are
[H3O+ ] = [OH-] = 1.00 x 10-7 M, and [H2O] = 55.5 M.
Because the concentration of H3O+ is so small, the
concentration of H2O is essentially constant. That is,
it is almost a pure liquid. The preceding expression
for Kc can be simplified. We can write:
Kc[H2O]2  [H3O][OH-]
Now set Kw = Kc[H2O]2 , so that at 25 oC
Kw  [H3O][OH-]  1.00 x 10-14
The w subscript refers to water.
770
Criteria for Acidic, Basic, and neutral
solutions
771
Criteria for Acidic, Basic, and neutral
solutions
Neutral solution: [H3O+] = [OH-]
772
Criteria for Acidic, Basic, and neutral
solutions
Neutral solution: [H3O+] = [OH-]
Acidic solution: [H3O+] > [OH-]
773
Criteria for Acidic, Basic, and neutral
solutions
Neutral solution: [H3O+] = [OH-]
Acidic solution: [H3O+] > [OH-]
Basic solution:
[H3O+] < [OH-]
774
The pH concept
775
The pH concept
To avoid awkward numbers such as 9.2 x 10-11 when
dealing with H3O+ concentrations, the following
concept is employed:
776
The pH concept
To avoid awkward numbers such as 9.2 x 10-11 when
dealing with H3O+ concentrations, the following
concept is employed:
pH = - log[H+]
777
The pH concept
To avoid awkward numbers such as 9.2 x 10-11 when
dealing with H3O+ concentrations, the following
concept is employed:
pH = - log[H+]
This is actually shorthand for
pH = - log([H+]/M)
778
The exact definition of pH is:
pH  - log(aH )

779
The exact definition of pH is:
pH  - log(aH )

aH is called the activity of the hydrogen ion.

780