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Strong Acids & Bases –
- Autoionization of Water -
1


a strong acid ionizes completely in water to form
hydrogen ions, percent ionization is greater than
99% (we assume 100% in calculations)
Example: HCl completely ionizes in water
100%
HCl(g)  H+(aq) + Cl-(aq)
H2O(l)
2



Common strong acids:
hydrochloric acid,
hydrobromic acid,
sulfuric acid, nitric acid,
phosphoric acid
monoprotic acid is an
acid that possesses
only one ionizable
(acidic) proton
diprotic acid possesses
two or more ionizable
protons
3


a strong base (Arrhenius definition) dissociates
completely in water to release hydroxide ions
all hydroxides of Group 1 elements are strong
bases (only one mole of hydroxide ions are
formed when one mole of Group 1 hydroxides
are dissolved in water)
100%
NaOH(s)  Na+(aq) + OH-(aq)
H2O(l)
4


these metal hydroxides are all highly soluble in
water
all hydroxides of Group 2 elements are also bases
when dissolved in water (two moles of hydroxide
ions are formed when one mole of Group 2
hydroxides are dissolved in water)
100%
Ba(OH)2(s)  Ba+(aq) + 2OH-(aq)
H2O(l)

these metal hydroxides are only slightly soluble in
water (main reason used in medicine – milk of
magnesia, Mg(OH)2(aq))
5

Experiments have revealed that some water
molecules react with each other to produce
hydronium, H3O+(aq), and hydroxide, OH-(aq), ions
according to the following equation
H2O(l)+H2O(l)  H3O+(aq) + OH-(aq)

conductivity is slight, an equilibrium is formed
between hydronium and hydroxide ions, and water
molecules are greatly favoured
6


the collision that forms hydronium and
hydroxide ions are very rare (need the right
energy and orientation)
when this reaction does occur, it is called the
autoionization of water (since water molecules
ionize one another).
7

chemists often omit the water molecule that
carries the H+ ion for convenience
H2O(l)  H+(aq) + OH-(aq)

water molecules do NOT spontaneously
dissociate into H+ and OH- ions, but rather,
they are formed by the ionization process in
which a proton is transferred from one
molecule to another
8

water equilibrium also obeys equilibrium law
Keq = [H+(aq)][OH-(aq)]
[H2O(l)]
9


the concentration of
water molecules in pure
water and in dilute
aqueous solutions is
essentially CONSTANT
and equal to 55.6 mol/L
therefore, a new
constant, which
incorporates both the
constant value of [H2O(l)]
and the equilibrium
constant, can be
calculated called the ion
[H+(aq)][OH-(aq)] = K
[H2O(l)]
[H+(aq)][OH-(aq)] = K[H2O(l)]
[H+(aq)][OH-(aq)] = Kw
product constant for
water, Kw
10

note the 1:1 ratio of hydrogen ions and
hydroxide ions, therefore the concentration of
hydrogen and hydroxide ions in pure water must
be EQUAL
[H+(aq)] = [OH-(aq)] = 1.0 x 10-7 mol/L

At SATP,
Kw = [H+(aq)][OH-(aq)]
Kw = (1.0 x 10-7 mol/L) (1.0 x 10-7 mol/L)
Kw = 1.0 x 10-14
11


autoionization occurs in all aqueous solutions
In all aqueous solutions at SATP,
[H+(aq)][OH-(aq)] = Kw =1.0 x 10-14

What makes a solution neutral, acidic or basic?
In neutral solutions
[H+(aq)] = [OH-(aq)]
In acidic solutions
[H+(aq)] > [OH-(aq)]
In basic solutions
[H+(aq)] < [OH-(aq)]
12


Kw is valid at SATP, however at different
temperatures it will change (equilibrium
constant is temperature dependent)
the ion product constant for water, Kw, can be
manipulated to solve for either hydrogen ion or
the hydroxide ion concentration
Since
Kw = [H+(aq)] [OH-(aq)]
then
[H+(aq)] =
and
[OH-(aq)] =
Kw__
[OH-(aq)]
Kw__
[H+(aq)]
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p. 537 Practice
UC # 4, 5, 6
p. 540 Practice
UC # 8, 9, 10
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