Acids & Bases
Lesson 3
Levelling Effect &
Ionization of Water (Kw), Ka and Kb intro.
Review of
“Strong & Weak” Acids
and Bases
Strong acids and bases
• They 100% ionize in solution.
• The ionization is NOT at equilibrium 
- one-way reactions.
-In other words, reverse rxns do not occur at all
Strong acids and bases
TOP SIX STRONG ACIDS
HClO4
HI
HBr
HCl
HNO3
H2SO4
Sulphiric acid is diprotic, however, it is only strong for the first dissociation! In fact, we will always only use the first
dissociation and neglect any second ones. The first one will be the most significant.
Weak acids and bases
• They DON’T 100% ionize in solution.
• The ionization is at equilibrium
-two-way reactions.
-In other words, reverse rxns do occur.
• We use Ka and Kb
Something to think about…
Since we do not have Ka or Kb values for
strong acids and bases…How do we get [OH-]
or [H+] ???
There is something called “ The Levelling
Effect of water”
The Levelling Effect
Of water
Consider these aqueous solutions
of a strong and a weak acid
1 M HCl produces 1 M H3O+ and 1 M Clwhereas,
1 M HF produces 0.97 M HF + 0.03 M H3O+ + 0.03 M F-
What does this indicate?
There is NONE of the strong acid left in the
products, it has ALL ionized.
No un-dissociated HCl.
The concentration of H3O is EXACTLY the
same as the starting concentration of HCl.
Let us look at 2 strong acids
now
1 M HCl produces 1 M H3O+ and 1 M Cl1 M HClO4 produces 1 M H3O+ and 1 M ClO4 They both have exactly the same strengths in water,
they have been levelled off by water.
The Levelling Effect
All strong acids in water are equally strong.
Remember that strong means they ionize 100%
in water.
In other words, in a solution of a strong acid:
• No molecules of the strong acid itself remain; they all
become the hydronium ion and the anion left.
• [strong acid] = [H3O+]
The Levelling Effect
H3O+ is the strongest acid that can exist in
an undissociated form in water solution.
All strong acids ionize 100% to form H3O+.
The only way you can tell which strong acid is “stronger” is to react them in a nonaqueous (not H2O) solvent.
Ex: HClO4 + H2SO4  H3SO4+ + ClO4It is found that HClO4 donates a proton to H2SO4, not the other way around, so HClO4 is a stronger acid
than H2SO4. This is not important in Chemistry 12.
This would not happen in a water solution…in H2O, they would both form H3O+.
The Levelling Effect
The strongest base which can exist in high concentrations in water
solution is OHOne of The two stronger bases below will react with water completely
to form OH-.
Ex:
OR…
O2- + H2O  OH- + OHO2- + H2O  2OH-
In other words, in a solution of a strong base:
• No molecules of the strong base itself remain; they all
become the hydroxide ion and the cation left.
• [strong base] = [OH-]
Ionization of water
Water ionizes to a small degree.
H2O(l)
⇄
H+
+
OH-
2H2O(l)
⇄
H 3 O+
+
OH-
1.0
x 10-14
Kw
=
[H+][OH-]
=
@ 250 C
[H+] and [OH-] are inversely related as Kw is constant at 250 C
Ionization of water
⇄
H2O(l)
Kw
=
H+
[H+][OH-]
=
+
OH-
1.0
x 10-14
Always assume the temperature is 25 ºC and that it is an
aqueous solution to use this equation above.
1.
Calculate the [H+] and [OH-] in 1.0 M HCl(aq).
HCl 
1.0 M
H+
+
1.0 M
we know that [H+][OH-]
[1.0][OH-]
[OH-]
Cl1.0 M
=
=
=
1.0
1.0
1.0 x
x 10-14
x 10-14
10-14
Note that in acid solutions, the [H+] is large and the [OH-] is small.
2.
Calculate the [H+] and [OH-] in 1.0 M Ba(OH)2(aq)
Ba(OH)2

1.0 M
[H+][OH-]
[H+][2.0]
[H+]
Ba2+
+
1.0 M
=
=
=
1.0
1.0
5.0
2OH2.0 M
x 10-14
x 10-14
x 10-15 M
In a basic solution the [OH-] is very large and the [H+] is very
small!
3.
Calculate the [H+] and [OH-] in H2O.
⇄
H2 O
H+
+
x
x
[H+][OH-]
=
1.0
x 10-14
x2
=
1.0
x 10-14
[H+]
=
1.0
[OH-]
=
OH-
x 10-7 M
Note that in pure water that the [H+] and the [OH-] are equal!
Summary
• A neutral solution is a solution having
[H3O+] = [OH-]
• An acidic solution is a solution having
[H3O+] > [OH-]
• A basic solution is a solution having
[H3O+] < [OH-]
Ka
Acid ionization reaction of a WEAK acid in
water.
Ka is called the acid ionization constant
Ex:
Ka
• The greater the value of Ka, the stronger the
acid.
• For a STRONG ACID, there is no Ka since
the acid will ionize 100% and the [ ] of the
unionized acid will be zero.
Kb
Base Ionization reaction of a WEAK base in
water.
Kb is called the base ionization constant.
Ex:
Kb
• The greater the value of Kb, the stronger the
base.
• We don’t have Kb values in the table, but
we have a way to get Kb from the known
Ka values.
Relationship between Ka and Kb
Experimentally, it is found that the acid ionization equation
NH4+ + H2O ⇄ NH3 + H3O+
Ka= [NH3][H3O+]
[NH4+ ]
Also, the base ionization equation
NH3 + H2O ⇄ NH4+ + OHKb
[NH4+][OH-]
[NH3]
Relationship between Ka and Kb
Both equations involved NH3 and NH4+ which are conjugates! You might suspect
there is a relationship. Let us multiply them together and find out…
Conclusion: For a conjugate pair, Ka x Kb = Kw
This equation will always allow you to find Kb from Ka values in the table
whenever you want!
Relationship between Ka and Kb
Find the Ka value for: H2PO4- and the Kb HPO4-2
Relationship between Ka and Kb
Find the Ka value for: H2PO4- and the Kb HPO4-2
Kb(HPO42-)
=
Kw
Ka(H2PO4-)
=
1 x 10-14
6.2 x 10-8
=
1.6 x 10-7
You can calculate the Kb by:
using the Ka value from the table
rearrange the equation below
Ka
FINAL:
x
Kb
Kb
=
=
1.0
Kw
Ka(conjugate)
x 10-14
@ 25 oC
Calculate the Kb for CN-.
Kb(CN-)
=
Kw
Ka(HCN)
=
1 x 10-14
4.9 x 10-10
=
2.0 x 10-5
Homework:
Hebden p.125 #24, 25
p. 127 #28,30
p. 128 #31,32,33,34
p. 130 #35(a,c,e), 36, 37