Acid–Base Equilibrium in Water
Acid–Base Equilibrium in Water
** When an acid
or base is dissolved in water, it will dissociate, or ionize According to the
equations:
** The amount of ionization being dependent on the strength of the acid or the base.
** A “strong” electrolyte is completely dissociated, while a “weak” electrolyte is partially dissociated.
** Table blow shows lists some common electrolytes, some strong and some weak:
** Hydrochloric acid is a strong acid, and in
water, its ionization is complete:
** An equilibrium constant for Equation above would have a value of infinity. The proton H+ exists in water as a hydrated ion, the hydronium ion, H3O+. Higher hydrates probably exist, particularly H9O4+. The hydronium ion is written as H3O+ for convenience and to emphasize Brønsted behavior.
** An equilibrium constant for Equation above would have a value of infinity. The proton H+ exists in water as a hydrated ion, the hydronium ion, H3O+. Higher hydrates probably exist, particularly H9O4+. The hydronium ion is written as H3O+ for convenience and to emphasize Brønsted behavior.
Acetic acid is a weak acid, which ionizes only partially
in water (a few percent):
We can write an
equilibrium constant for this reaction:
Where:
K◦a =
the thermodynamic acidity constant
a = the activity
of the indicated species.
** Salt cations
or anions may also partially react with water after they are dissociated. For
example, acetate ion is formed from dissociated acetate salts, to give HOAc.
** The
activity can be thought of as representing the effective concentration of an
ion . The effects of protons in reactions are often governed by their
activities, and it is the activity that is measured by the widely used pH meter
.
** In dilute
solutions, the activity of water remains essentially constant, and is taken as
unity at standard state. Therefore, Equation can be written as:
** Pure water ionizes
slightly, or undergoes autoprotolysis ( Autoprotolysis is the
self-ionization of a solvent to give a characteristic cation and anion)
** Again, the
activity of water is constant in dilute solutions (its concentration is
bessentially constant at ∼55.5 M), so:
where K◦w =
the thermodynamic autoprotolysis, or self-ionization, constant
** Calculations
are simplified if we neglect activity coefficients. This simplification results
in only slight errors for dilute solutions, and we shall use molar
concentrations in all our calculations. This will satisfactorily illustrate the
equilibria involved.
** Most of the
solutions we will be concerned with are rather dilute, and we will frequently
be interested in relative changes in pH (and large ones) in which case small
errors are insignificant.
** We will
simplify our expressions by using H+ in place of H3O+.
This is not inconsistent since the waters of solvation associated with other
ions or molecules (e.g., metal ions) are not generally written and H3O+ is
not an accurate representation of the actual species present; typically the
proton in dilute aqueous solution has at least four water molecules in its
solvation shell.
Molar concentration will be represented by square brackets
[ ] around the species.Simplified
equations for the above reactions are:
Ka and
Kw are the molar equilibrium constants.
Kw is
exactly 1.00 × 10−14 at 24◦C and even at 25◦C,
to a smaller number of significant figures, it is still accurately represented
as 1.0 × 10−14. The product of the hydrogen ion concentration
and the hydroxide ion concentration in aqueous solution is always equal to 1.0
× 10−14 at room temperature:
In pure water, then, the concentrations of these two species are equal since there are no other sources of H+ or OH− except H2O dissociation:
Therefore
If an acid is
added to water, we can calculate the hydroxide ion concentration if we know the
hydrogen ion concentration from the acid. But when the hydrogen ion
concentration from the acid is very small, 10−6 M or less, the
contribution to [H+] from the ionization of water cannot be
neglected.
Example: A
1.0 × 10−3 M solution of hydrochloric acid is prepared. What is
the hydroxide ion concentration?
Solution:
Since hydrochloric acid is a strong
electrolyte and is completely ionized, the H+ concentration is
1.0 × 10−3 M. Thus,
Reference: Analytical chemistry/ Seventh edition / Gary D. Christian, University of Washington, Purnendu K. (Sandy) Dasgupta, University of Texas at Arlington, Kevin A. Schug, University of Texas at Arlington.
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