Lesson 14.3 The Autoionization of Water
Suggested Reading

Zumdahl Chapter 14 Section 14.2 & 14.3
Essential Question

What are the properties of the autoionization of water?
Learning Objectives:.


Solve problems regarding the autoionization of water.
Solve equilibrium problems involving acidic and basic solutions.
Introduction
In aqueous solutions, two ions have dominant roles. These ions, the hydrogen
ion (H+) and the hydroxide ion (OH-), are always present in any aqueous
solution as a result of the autoionization of water, a reaction of water with
itself. Although pure water is usually considered a nonelectrolyte, precise
measurements do show a very small conduction, resulting from the
autoionization reaction. In this reaction, a proton from one H2O molecule is
transferred to another H2O molecule giving an hydronium and hydroxide ion.
This this lesson we will investigate this reaction.
Autoionization of Water
READ THIS information on the following web site. Please take detailed notes.
http://www.chem.purdue.edu/gchelp/howtosolveit/Equilibrium/Autoionization_of_Water.htm
The term Kw, introduced in the web article, is called the ion-product constant
for water. Using Kw, you can calculate the concentrations of H+ and OH- in
pure water. These ions are produced in equal amounts in pure water, so their
concentrations are equal. Let x = [H+] = [OH-]. Then, substituting into the
equation for the ion-product constant, Kw = [H+][OH-] you get, at 25∘C
1.0 x 10-14 = x2
Solving gives x = 1.0 x 10-7, which is the concentration (M) of H+ and OH- in
pure water. What do you think is the significance of the 7 in the exponent?
Watch the following YouTube Video
https://www.youtube.com/watch?v=enq6e81VvZA
Concentrations of H+ and OH- in Solutions of Strong
Acids & Bases
Recall from the previous lesson that for strong acids the concentrations of the
ions in solution can be determined using stoichiometry. Lets elaborate on that.
Suppose you dissolve 0.10 mol HCl in 1.0 L giving 0.10 M HCl. You would like
to know the concentration of H+ ion in this solution. You have H+ from two
sources, the dissolution of HCl and the autoionization of water. Using
stoichiometry you can see that the concentration of H+ from the dissolution
reaction is 0.10 M. Now consider the concentration of H+ from the
autoionization of water. In pure water the concentration is 1.0 x 10-7 but in
acid it is even smaller. You can see this by applying Le Chatelier's
principle. When you add H+ to water by adding acid, the self-ionization
reaction is driven to the left until a new equilibrium is established. Because
of this the concentration of H+ from autoionization of water is so small that it
is negligible in comparison to the acid and is usually ignored when
calculating the concentration of H+ produced by strong acids. This is not
true for extremely dilute solutions of strong acid.
Although you normally ignore the autoionization of water in calculating the
H+ concentration is a solution of strong acid, there may be times when you
want to calculate the concentration of OH- produced by the autoionization
of water. You can use the ion-product constant for this.
For example, calculate the concentration of OH- ion in 0.10 M HCl. You
substitute = [H+] = 0.10 M into the equilibrium equation for Kw (at 25∘C).
Kw = [H+][OH-]
1.0 x 10-14 = 0.10 x [OH-] solving for [OH-] gives [OH-] = 1.0 x 10-14 / 0.10
= 1.0 x 10-13
The concentration of OH- ion is 1.0 x 10-13 M.
Problems like these involving strong bases are solved in the same way,
except that we would use the ion product constant to determine the
concentration of H+ instead of OH-.
By dissolving substances in water you can alter the concentrations of H+
and OH- ions. In a neutral solution they concentrations remain equal. In an
acidic solution the concentration of H+ is greater than OH- . In a basic
solution the concentration of OH- is greater than H+.
At 25∘C we observe that
In acidic solution
In neutral solution
In basic solution
[H+] > 1.0 x 10-7
[H+] = 1.0 x 10-7
[H+] < 1.0 x 10-7