AP Chemistry
Chapter 15 Sections 15.6 and 15.8 Notes
Solubility Equilibria and the Solubility Product, Equilibria Involving Complex Ions
Solubility Equilibria
•
is a very important phenomenon.
• We can
due to the solubility of salt and sugar in water.
• In this section, we will consider
substances and
ways to determine “How soluble?”.
• According to the
previously learned, PbCl2 and AgCl are
both
salts.
• However, if
ion is
to a solution containing both Pb2+ and Ag+ ions,
nearly all of the
is
as AgCl
any
separates
from the solution as PbCl2.
• This occurs because
is much
than
.
• To explain these differences in solubility, solubility equilibrium must be examined
quantitatively.
• When placed in water, a
of AgCl
and the
following
is established once the solution becomes saturated:
•
AgCl(s) ⇌Ag+(aq) + Cl-(aq)
• Equilibrium expression:
•
Ksp = [Ag+][Cl-]
• Ksp =
constant
• Ksp
the
of
terms for the
dissolved in a
solution of a sparingly soluble substance.
• The
of salts
with
so a value of Ksp
applies only to solutions only at the temperature at which its value was determined.
• If reactants or products have a coefficient other than 1 concentrations must be raised to that
power.
• Ksp can be obtained from a salt’s
in water – the
of
of
dissolved in one liter of its saturated
solution.
• Examples
•
can also be
(estimated) from values of
Ksp.
• Examples
• Common Ion Effect
• Suppose we stir some calcium carbonate in water long enough to
the
:
following
•
CaCO3 (s) ⇌ Ca2+ (aq) + CO32- (aq)
• Then we
to the solution a very
of calcium, like CaCl2.
• This puts Ca2+ into solution, and it
the above
.
• The
is
longer
to Ksp.
• Remember from Le Chatelier’s principle, if we add then equilibrium
.
• The above equilibrium
to the
causing CO32- to precipitate as CaCO3.
• Eventually
is
, but with a lower concentration of
CO32+ in solution.
• In this new system, there are
of Ca2+, the added CaCl2 and the
CaCO3 still in solution.
• Because Ca2+ is common to both sources, it is called a
.
• The
of the
ion
the
of CaCO3;
it is less soluble in the presence of CaCl2 (or any other soluble calcium salt) than it is in pure
water.
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This lowering of the solubility of an ionic compound by the addition of a common ion is
called the common ion effect.
The
can dramatically
the
of a
salt.
Will a Precipitate Form?
So far we have considered
in solutions.
Now we will consider the
process – the formation of a solid from a solution.
We will use the
.
Ion product (Q) is defined like expression for Ksp but uses
instead of equilibrium concentrations.
Precipitate will form
Ion product >Ksp (supersaturated)
No precipitate will form Ion product = Ksp (saturated)
No precipitate will form Ion product < Ksp (unsaturated)
pH and Solubility
The
of a solution can
a salt’s
.
General
– If the
X- is an
(HX is a weak acid) the
salt MX will show
in an
solution.
Complex Ion Equilibria
• A
surrounded by
• A ligand is simply a
• Recall a
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is a
.
species consisting of a
ion
.
is an
or
having a
that can be
to an empty orbital on the
metal ion to form a covalent bond.
Some common ligands are H2O, NH3, Cl-, and CN-.
Metal ions
one at a time in
characterized by equilibrium
constants called
or stability constants.
For example, when solutions containing Ag+ and NH3 molecules are mixed, the following
reactions take place:
Ag+ (aq) + NH3 (aq) ⇌ Ag(NH3)+ (aq) K1 = 2.1 x 103
Ag(NH3)+(aq) + NH3(aq)⇌ Ag(NH3)2+(aq)
K2 = 8.2 x 103
where K1 and K2 are the formation constants for the two steps.
In a solution containing Ag+ and NH3,
the
NH3, Ag+, Ag(NH3)+, and
+
Ag(NH3) exist at
.
When we write the
for a
, we follow two rules:
The
for the
ion is always given
, followed by the
ligands.
on the complex is the algebraic
of the
on the
The
ion and the
on the
.
For example, the formula of the complex ion of Cu2+ and H2O is written Cu(H2O)42+ with the
Cu first followed by the ligands.
The charge on the complex is 2+ because the copper ion has a charge of 2+ and the water
molecules are neutral.
Metal
that commonly
ions (or coordination compounds):
Al3+, Cu2+, Zn2+, Fe2+ (or 3+), Ni2+, Ag+. (All Curiously Colored Zebras Felt Nicely
Agreeable)
Common
: NH3, OH-, Cl-, SCN-, CN-, H2O.
Most
coordination number:
the
of the metal ion.
That means that Ag+ and NH3 → Ag(NH3)2+.
Formation of complex ions is a
to
otherwise
.
For example, in a solution with only water present, AgCl only dissociates (dissolves) slightly
to form Ag+.
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When ammonia is added, the Ag+ complexes with the ammonia, and the removal of the Ag+
from the solution as it converts to Ag(NH3)2+ pulls the AgCl dissociation equilibrium to the
right (LeChatelier’s Principle).
If sufficient ammonia is added to complex all of the silver ions, the AgCl will completely
dissolve.
AgCl(s) ⇌ Ag+ (aq) + Cl- (aq)