CK-12-Chemistry Intermediate

21.2. The pH Concept http://www.ck12.org

a transfer of a hydrogen ion from one molecule to the other. The products are a positively charged hydronium ion
and a negatively charged hydroxide ion.

H 2 O(l)+H 2 O(l)⇀↽H 3 O+(aq)+OH−(aq)

Theself-ionizationof water is the process in which water ionizes to hydronium ions and hydroxide ions. As with
other aqueous acid-base reactions, the process is often simplified to show the ionization of just one water molecule
into a hydrogen ion and a hydroxide ion.

H 2 O(l)⇀↽H+(aq)+OH−(aq)

Either equation is adequate, though the first is more accurate, since hydrogen ions in aqueous solution will always
be attached to water molecules. Further discussion of acids and acid ionizations in this book will primarily show
hydrogen ions in aqueous solution as H+, but keep in mind that this is just a commonly used abbreviation for the
more accurate hydronium structure.

The Ion-Product of Water

The self-ionization of water occurs to a very limited extent. In other words, the equilibrium position strongly favors
the reactant water molecule. As with other equilibrium reactions, we can write an equilibrium expression for the
self-ionization process. The equilibrium constant is referred to as the ion-product for water and is given the symbol
Kw.

Kw= [H+][OH−]

Theion-product of water (Kw)is the mathematical product of the concentrations of hydrogen ions and hydroxide
ions. Note that H 2 O is not included in the ion-product expression because it is a pure liquid.

The value of Kwis very small, which is consistent with a reaction that heavily favors the reactants. At 25°C, the
experimentally determined value of Kwin pure water is 1.0× 10 −^14.

Kw= [H+][OH−] = 1.0× 10 −^14

This equation is true for all dilute aqueous solutions at 25°C. As we will see, the relative concentrations of H+and
OH−are different in acidic and basic solutions. However, the product of the two concentrations must remain equal
to 1.0× 10 −^14.

In pure water, the concentrations of hydrogen and hydroxide ions are equal to one another. Any aqueous solution
in which [H+]=[OH−] is said to be neutral. To find the molarity of each ion in a neutral solution, simply take the
square root of Kw.

H+ = [OH−] = 1.0× 10 −^7 M

For any neutral solution at 25°C, each of these ions has a concentration of 1.0× 10 −^7 M.

Anacidic solutionis a solution in which the concentration of hydrogen ions is greater than the concentration of
hydroxide ions. For example, hydrogen chloride ionizes to produce H+and Cl−ions upon dissolving in water.

HCl(g)→H+(aq) + Cl−(aq)

This increases the concentration of H+ions in the solution. According to Le Châtelier’s principle, the equilibrium
represented by H 2 O(l)⇀↽H+(aq)+OH−(aq)is forced to the left, toward the reactant. As a result, the concentration
of the hydroxide ion decreases.

Abasic solutionis a solution in which the concentration of hydroxide ions is greater than the concentration of
hydrogen ions. Solid potassium hydroxide dissociates in water to yield potassium ions and hydroxide ions.