Chemistry 1011
TOPIC
Acids and Bases
TEXT REFERENCE
Masterton and Hurley Chapter 4.2 (Review), 13,
14.1, 15.1 (page 427), 21.2 (page589)
Chemistry 1011 Slot 5
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13.6 Acid-Base Properties of Salt
Solutions
YOU ARE EXPECTED TO BE ABLE TO:
• Account for the acidity, basicity or neutrality of salts in
terms of the hydrolysis of cations and anions
• Identify and apply the relationship between Ka and Kb
(This section will include material contained in 13.4 and
13.5 related to the acid-base properties of cations and
anions)
Chemistry 1011 Slot 5
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Hydrolysis of Salts
• All salts are strong electrolytes – this means that
they are fully ionized in dilute aqueous solution
NaCl(s)
Na+(aq) + Cl-(aq)
CH3COONa(s)
CH3COO-(aq) + Na+(aq)
NH4Cl(s)
NH4+(aq) + Cl-(aq)
• If the salts are formed from either weak acids or
bases, then their ions may react with water
(hydrolysis)
– NaCl is the salt of a strong acid and strong base
– CH3COONa is the salt of a weak acid and a strong base
– NH4Cl is the salt of a strong acid and a weak base
Chemistry 1011 Slot 5
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Hydrolysis of Salts
• CH3COONa is the salt of a weak acid (acetic acid)
and a strong base (sodium hydroxide).
• The CH3COO- ions will react with water:
CH3COO-(aq) + H2O(aq)
CH3COOH(aq) + OH- (aq)
• The CH3COO- ion is a Bronsted-Lowry base.
The solution will be basic.
Kb = [CH3COOH] x[OH-] = 5.6 x 10-10
[CH3COO-]
• Any anion derived from a weak acid will be a
weak base
Chemistry 1011 Slot 5
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Hydrolysis of Salts
• NH4Cl is the salt of a strong acid (hydrochloric acid)
and a weak base (ammonia)
• The NH4+ ions will react with water:
NH4+(aq) + H2O(aq)
NH3(aq) + H3O+ (aq)
• The NH4+ ion is a Bronsted-Lowry acid. The solution
will be acidic.
Ka = [NH3] x[H3O+] = 5.6 x 10-10
[NH4+]
• Any cation derived from a weak base will be a weak
acid
Chemistry 1011 Slot 5
5
Metal Cations as Weak Acids
• Excluding Groups 1 and 2 (alkali metals and alkaline
earths), many metal cations are weak acids
– A 0.10mol/L solution of aluminum sulfate has a pH of  3
– A 0.10mol/L solution of zinc nitrate has a pH of  5
•
These metal ions in solution are hydrated:
– Al3+ is [Al(H2O)6]3+
– Zn2+ is [Zn (H2O)4]2+
• The hydrated ions act as Bronsted-Lowry acids:
Al(H2O)63+(aq) + H2O(aq)
ca
cb
H3O +(aq) + Al(H2O)5(OH)2+( aq)
ca
cb
Chemistry 1011 Slot 5
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Anions as Weak Bases
Base
PO43CO32HPO42-
Kb
pKb
2.2 x 10-2
2.1 x 10-4
1.6 x 10-7
1.66
3.68
6.80
The larger the value of Kb, the stronger the acid
PO43-(aq) + H2O(aq)
HPO42-(aq) + OH- (aq)
• The PO43- ion is a Bronsted-Lowry base. The
solution will be basic.
Kb = [HPO42-] x[OH-] = 2.2 x 10-2
[PO43-]
Chemistry 1011 Slot 5
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Relation between Ka and Kb
• Ammonia in aqueous solution is basic:
NH3(aq) + H2O(aq)
NH4+(aq) + OH-(aq)
Kb = [NH4+] x[OH-] = 1.8 x 10-5
[NH3]
• The ammonium ion is its conjugate acid
• We can write an equation for NH4+ acting as an
acid as:
NH4+(aq) + H2O(aq)
NH3(aq) + H3O+ (aq)
Ka = [NH3] x[H3O+] = 5.6 x 10-10
[NH4+]
Chemistry 1011 Slot 5
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Relation between Ka and Kb
• Add the two equations:
H2O(aq) + H2O(aq)
H3O+(aq) + OH-(aq)
• The equilibrium constant of the resulting equation
is the product of the two equilibrium constants:
Kw = [H3O+] x [OH-] = Ka x Kb
= (5.6 x 10-10) x (1.8 x 10-5)
= 1.0 x 10-14
• This relationship can be used to determine Kb for a
weak acid from a measurement of Ka for its
conjugate acid, and vice versa
Chemistry 1011 Slot 5
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Relative Strengths of BronstedLowry Acids and Bases
Ka
Conjugate Acid
very large
6.9 x 10- 4
1.8
x 10-
5
Conjugate Base
Kb
HCl (strong acid)
H3O+
HF
CH3COOH
ClH2O
FCH3COO -
very small
NH4+
NH3
1.8
H2O
OH- (strong base)
OHO2 -
very large
1.4 x 105.6 x 10-
10
5.6 x 1010- 5
very small
10
Chemistry 1011 Slot 5
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10
11
Relative Strengths of Acids and Bases
• Bronsted-Lowry acids:
– Strong acids - stronger proton donors than H3O+
– Weak acids - weaker proton donors than H3O+ but stronger
than H2O
– Species that do not form acidic aqueous solutions
• Bronsted-Lowry acids:
– Strong bases - stronger proton acceptors than OH– Weak bases - weaker proton acceptors than OH- but stronger
than H2O
– Species that do not form basic aqueous solutions
Chemistry 1011 Slot 5
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Predicting Acid-Base Properties
of Salts
• What will be the effect of the cation on the pH of water?
• Will the cation act as a weak acid or will it be a
“spectator ion”
– If this is a salt of a weak base, the cation will be a weak acid
– If this is a salt of a strong base, the cation will be a “spectator
ion”
– If the cation is a hydrated metal ion, the solution will be a
weak acid
– If the cation is a group 1 or 2 metal ion, the cation will be a
“spectator ion”
Chemistry 1011 Slot 5
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Predicting Acid-Base Properties
of Salts
• What will be the effect of the anion on the pH of
water?
• Will the anion act as a weak base or will it be a
“spectator ion”
– If this is a salt of a weak acid, the anion will be a weak
base
– If this is a salt of a strong acid, the anion will be a
“spectator ion”
Chemistry 1011 Slot 5
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Salts Formed by Reacting a Weak
Acid with a Weak Base
• To determine whether a salt of a weak acid and
weak base will be basic or acidic, it is necessary to
compare the values of Ka and Kb to determine
relative strength
– If Ka > Kb salt will be acidic
– If Ka < Kb salt will be basic
• eg: NH4F
– KaNH4+ = 5.6 x 10-10
– KaF- = 1.4 x 10 -11
– Solution has pH 6.2
Chemistry 1011 Slot 5
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