• 7-3 Acid and Base
• Monoprotic acid:
• an acid that donates one H+ ion per
molecule.
• HBr → H+ + Br -
• Diprotic acid: an acid that donates
two H+ ion per molecule
• H2SO4 + H2 O → H3O+ + HSO4•
• HSO4- + H2O → H3O+ + SO42-
• Polyprotic acid: an acid that
donates three or more H+ ion per
molecule.
H3PO4 + H2O → H3O+ + H2PO4-
H2PO4- + H2O → H3O+ + HPO4 2HPO4 2- + H2O → H3O+ + PO4 3-
• Polyprotic can yield a higher
concentration of hydronium ions than
their given concentration.
H3PO4 (aq) + 3 H2O(l)
PO43- (aq) + 3H3O+
(aq)
0.100 M
0.100 M
0.300 M
Strength of Acids and Bases
• Strength is defined in terms of the
degree of dissociation of the acid or
base into ions. For example:
• HX + H2O
• BOH
→
→
H3O+ + X-
B+ + OH-
• A strong acid or base is defined as
one which completely or nearly
completely dissociates into ions in
solution.
• A weak acid or base is defined as
one which only a small amount
dissociates into ions in solution.
• Keq =
[H3O+ ][X- ]
[HX]
• strong acid: a very high Keq
(favours the product).
• weak acid: a very low Keq
(favours the reactant).
• Note: The equilibrium
expression for an acid is referred
to as a Ka; the constant for a
base is a Kb.
• Note: A polyprotic acid would
have a different Ka for each step;
the value of the Ka would
decrease for each step.
Strength vs. Concentration:
• Strength is a measure of the degree of
dissociation of an acid or base.
• Concentration is a measure how many
moles of acid or base are present per litre
of solution.
Comparing Strength of Acids:
• The strength of an acid is measured
by its Ka value. The higher the Ka,
the stronger the acid.
• If two acids are put in solution the
strongest one will dominate; it will act as
the acid, forcing the weaker acid to act
as a base. For example:
•
Oxalic acid and Citric acid
• HOOCCOOH
•
H 3C 6 H 5O 7
H+ + HOOCCOOH + + H 2C 6H 5O 7 -
• Because oxalic acid is higher on the table it is the
strongest acid; it will go in the forward direction and
will force citric acid to act as the base and go in the
reverse direction.
• HOOCCOOH
H+ + HOOCCOO-
• H+ + H2C6H5O7 H3C6 H5O7
___________________________________________
HOOCCOOH + H2C6H5O7- H3C6 H5O7 +HOOCCOOacid
base
acid
base
Ionization of Water and Kw
• Acids and bases act in aqueous
solution. There is a relationship
between the hydronium ion and the
hydroxide ion in solution.
• The equation for the dissociation of
water is:
H2O(l)
→
H3O+ (aq) + OH- (aq)
• The equilibrium constant for the
dissociation of water is
Keq = [H3 O+ ][OH- ]
• The Keq for water, like that of acids
and bases has a special notation;
Kw.
• At most temperatures the Kw of water
is 1.00 x 10-14
Kw = [H3O+ ][OH- ]
Kw = 1.00 x 10-14
• In a neutral solution the concentration
of both hydronium and hydroxide ions
is
1.00 x 10-7 M
• If the hydronium ion concentration is
greater than 1.00 x 10-7 M the solution
is acidic;
• If it is less than that value, the solution
will be basic (since the concentration of
hydronium ions is less than 1.00 x 10-7
M, the hydroxide ion concentration will
be greater than 1.00 x 10-7 M).