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CH1101 PChem Tutorial 2. 2012. Acid/Base Equilibria. Professor Mike Lyons.
Acid strength : the acid dissociation constant KA.
Weak Acid: ethanoic (acetic) acid CH3COOH
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It is easy to quantify the
strength of strong acids since
they fully dissociate to ions in
solution.
The situation with respect to
weak acids is more complex
since they only dissociate to a
small degree in solution.
The question is how small is
small?
We quantify the idea of
incomplete dissociation of a
weak acid HA by noting that the
dissociation reaction is an
equilibrium process and
introducing the acid dissociation
constant KA.
KA values vary over a wide range
so it is best to use a log scale.
Acid dissociation
equilibrium
H3O+ (aq) + A- (aq)
HA(aq)+H2O(l)
KC =
[H O ][A ]
+
−
3
[HA][H 2O]
K A = K C [H 2O ] =
[H O ][A ]
+
−
3
[HA]
Acid dissociation
constant
KA is a measure of the acid strength.
When KA is large there is considerable
Dissociation and the acid is strong.
When KA is small there
is a small degree of dissociation, and
the acid is weak.
pK A = − log10 K A
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The pH concept.
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The best quantitative measure
of acidity or alkalinity rests in
the determination of the
concentration of hydrated
protons [H3O+] present in a
solution.
The [H3O+] varies in magnitude
over quite a large range in
aqueous solution, typically from 1
M to 10-14 M.
Hence to make the numbers
meaningful [H3O+] is expressed
in terms of a logarithmic scale
called the pH scale.
The higher the [H3O+] , the
more acidic the solution and the
lower is the solution pH.
[H3O+]
(b)
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[H O ] = 10
+
]
− pH
3
10-7 M
0
7
Ba2+(aq) + 2 OH-
[
pH = − log10 H 3O +
1.0 M
Ba(OH)2 +H2O
pH is expressed on a numerical scale
from 0 to 14.
When [H3O+] = 1.0 M (i.e. 100M), pH = 0.
When [H3O+] = 10-14 M, pH = 14.
pH value < 7 implies an acidic solution.
pH value > 7 implies an alkaline solution.
pH value = 7 implies that the solution is
neutral.
The definition of pH involves logarithms.
Hence a change in one pH unit represents
a change in concentration of H3O+ ions by
a factor of 10.
10-14 M
14
pH
1 mol Ba(OH)2 = 2 mol OH-
[Ba(OH)2] = 0.003M
[OH-] = 2 x 0.003 = 0.006 M
[H3O+].[OH-] = Kw = 1.0 x 10-14
[H3O+] = Kw/ [OH-] = 1.0 x 10-14/0.006 = 1.66 x 10-12
pH = - log [H3O+] = -log (1.66 x 10-12) = 11.78
Hydrogen ion concentration very small hence solution will be strongly
Alkaline, hence pH value will be high.
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cHA = 0.1 M
KA = 3.5 x 10-8
α=
KA
3.5 ×10−8
≅
= 3.5 ×10−7 = 5.92 ×10−4
c
0.1
 H 3O +  = α c = 5.92 ×10−4 × 0.1 = 5.92 ×10−5 M
pH = − log  H 3O +  = − log ( 5.92 ×10−5 ) = 4.23
pH value obtained reasonable for weak acid.
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pH of an aqueous solution of sodium acetate is alkaline.
This arises because of anion hydrolysis.
Sodium acetate Na+ CH3COO- is a salt derived from reaction between a strong
base (NaOH) and a weak acid (CH3COOH).
NaCH3COO (s)
H2O
CH3COO- (aq) + H2O (l)
Anion hydrolysis reaction
Na+ (aq) + CH3COO- (aq)
CH3COOH (aq) + OH- (aq)
Acetate ion is a strong conjugate base.
Large tendency to abstract proton from
Neighbouring water molecules to generate OH-
Generated hydroxyl ions result in increase in solution pH
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