AP* Chemistry
Solubility Equilibrium
*AP is a registered trademark of the College Board, which was not involved in the production of, and does not endorse, this
product.© 2008 by René McCormick. All rights reserved
SOLUBILITY EQUILIBRIA (The Solubiliy-Product Constant,
K
sp
)
We’ve got good news and we’ve got bad news…
The good news:
Solubility equilibrium is really simple.
The bad news:
You know all those solubility rules that state a substance is
insoluble
?
They are
actually a bit soluble after all.
Only the future attorneys among you read the fine print.
Soluble
is often defined as greater than 3 grams dissolving in 100 mL of water. So, there is a lot of
wiggle room for solubility up to 3 grams!
This type of equilibria deals with that wiggle room.
If you can actually see that a salt is insoluble, then the solution is actually saturated.
Saturated
solutions of salts present yet another type of chemical equilibria.
¾
Slightly soluble salts establish a dynamic equilibrium with the hydrated cations and
anions in solution.
•
When the solid is first added to water, no ions are initially present.
•
As dissolution proceeds, the concentration of ions increases until equilibrium is
established. This occurs when the solution is saturated.
•
The equilibrium constant, the
K
sp
, is no more than the product of the ions in
solution.
(Remember, solids do not appear in equilibrium expressions.)
•
For a saturated solution of AgCl, the equation would be:
AgCl(
s
)
R
Ag
+
(
aq
) + Cl
−
(
aq
)
•
The solubility product expression would be:
K
sp
= [Ag
+
][Cl
−
]
•
The AgCl(
s
) does not appear in the equilibrium expression since solids are left
out. Why? The concentration of the solid remains relatively constant.
A table of
K
sp
values follows on the next page.
Solubility Equilibria
2
Write the
K
sp
expression for each of the following reactions and find its value in the table above.
CaF
2
(
s
)
R
Ca
+2
+ 2 F
−
K
sp
=
Ag
2
SO
4
(
s
)
R
2 Ag
+
+ SO
4
2
−
K
sp
=
Bi
2
S
3
(
s
)
R
2 Bi
+3
+ 3 S
2
−
K
sp
=
DETERMINING
K
sp
FROM EXPERIMENTAL MEASUREMENTS
¾
In practice,
K
sp
values are determined by careful laboratory measurements using various
spectroscopic methods.
You've reached the end of your free preview.
Want to read all 8 pages?
- Fall '14
- STARNES
- Chemistry, Equilibrium, Solubility, Solubility equilibrium, Solubility Equilibria
