Download Self-Ionization of Water

Acidity, Basicity, and pH
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Water is both an acid and a base. We call this
__________________.
By the BrØnsted-Lowry definition, this means
that water can donate a proton AND accept a
proton.
H2O(l) + H2O(l)

H3O+(aq) + OH-(aq)
This process is known as the self-ionization
of water.
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The self-ionization of water makes even pure
water have ions within it.
In pure water, the two ions (acidic and basic)
must share the same concentration (1:1 ratio)
Experiments show that this concentration is
1.00x10-7 M at 25°C.
[H3O+] = [OH-] = 1.00x10-7 M
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The equilibrium constant is an expression
that relates the concentrations of species
involved in an equilibrium.
To write an equilibrium constant expression,
you must know the concentrations of the
products and the reactants at equilibrium.
Keq = [Products] / [Reactants]
**Excluding solids and liquids
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Consider the equilibrium of water:
H2O(l) + H2O(l)
2H2O(l)

H3O+(aq) + OH-(aq)
H3O+(aq) + OH-(aq)
This equation’s Equilibrium Constant is:
Keq = (1.00x10-7)(1.00x10-7) = 1.00x10-14
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This is a special equilibrium constant, known as
Kw
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Keq = (1.00x10-7)(1.00x10-7) = 1.00x10-14
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This is the Self-Ionization Constant of Water
◦ Known as Kw
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We can use this information about water and
the relationship of hydronium and hydroxide
to figure out various concentrations in
solutions
[H3O+][OH-] = Kw = 1.00x10-14
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What is [OH-] in a 3.00x10-5 M solution of
HCl?
1. HCl = a strong acid or a strong base?
I.
II.
So, is this a hydroxide conc or a hydronium conc?
Since this is a strong acid it completely
______________, implying that [HCl] = [H3O+]
2. Remember the self-ionization constant of water
I.
Kw = [H3O+][OH-] = 1.00x10-14
3. Plug in your variables and calculate what your are
asked to find.
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What is [OH-] in a 3.00x10-5 M solution of
HCl?
Kw = [H3O+][OH-] = 1.00x10-14
1.00x10-14 = (3.00x10-5M)[OH-]
1.00x10-14 = [OH-]
3.00x10-5M
[OH-] = 3.33x10-10 M
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A solution is neutral when the concentration
of hydroxide is exactly equal to the
concentration of the hydronium. pH = 7
If the conc of the hydronium is stronger than
the conc of the hydroxide than the solution
with have a pH of <7.
If the conc of the hydroxide is stronger than
the conc of the hydronium than the solution
with have a pH of >7.
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The pH is a value used to express the acidity
or alkalinity of a solution; it is defined as the
logarithm of the reciprocal of the
concentration of hydronium ions.
p = power,
H = hydrogen
pH = power of hydrogen
pH = -log[H3O+]
[H3O+] = 10-pH
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pH is a logarithmic scale, meaning an
increase in a value is an increase of 10x
i.e. A solution whose pH is 2.0 has a [H3O+]
that is 10x greater than a solution whose pH
is 3.0 and a 100x greater than a solution
whose pH is 4.0
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What is the pH of the following solutions:
◦ 0.00010 M solution of HNO3?
◦ 0.0136 M solution of KOH?
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What are the concentrations of the hydronium
and hydroxide ions in a sample of rain that
has a pH of 5.05?
Gather info:
pH = 5.05
Kw = 1.00x10-14
[H3O+] = ?
[OH-] = ?
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Plan your work
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Method #1
◦ Uses indicators
◦ Quick and simple
◦ Does not, however, give very precise results
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Method #2
◦ Uses a pH meter
◦ Very precise
◦ More complicated and expensive
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Certain dyes with change different colors in
solutions of different pH
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These dyes are known as indicators.
◦ A compound that can reversibly change color
depending on the pH of the solution or other
chemical change.
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The color after the addition of the indicator
can help determine the relative pH of the
solution.
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Litmus is a dye used as an indicator that is
soaked into filter paper
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It serves as a quick, cheap, convenient check
of a solution’s pH.
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Blue Litmus Paper – Turns red under acidic
conditions
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Red Litmus Paper – Turns blue under basic
conditions
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pH meters are a probe that is submerged into
a solution.
An electrical voltage develops between two
electrodes and the probe digitally reads out a
pH value.
The voltage is a measure of the conductivity
of the hydronium solution.
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Page 547, # 1, 2, 5, 6, 7, 8, 9, 10
Due tomorrow!!
Vocab also due on
Wednesday.