Solubility equilibria Chemistry 201 NC State University Lecture 17
Solubility Equilibria
• What is the role of metal ions in solution pH?
• How do solution conditions affect solubility?
• Can precipitation be used to separate ions?
Text : Sections 8.1 - 8.3
Role of metal ions
Metal ions in solution can act as weak acids as
Shown in the Figure. These are called complex ions.
Hydrated metal ion Ka
Fe(H2O)63+ 6 x 10-3
Al(H2O)63+ 1 x 10-5
Cu(H2O)62+ 3 x 10-8
Zn(H2O)62+ 1 x 10-9
Ni(H2O)62+ 1 x 10-10
Role Hydrated metal ions Acidity constants
Solubility Equilibria
Consider the solubility of NaCl in water. The
ions Na+ and Cl- are in equilibrium with the solid
NaCl(s).
The equilibrium constant is K = 36. This applies
only to the ions and not to the solid. The solid
does not have concentration.
Solubility Equilibria
Since only the ions contribute to the equilibrium
constant, we can write,
The equilibrium constant is product of two
solubilities, and hence has the name solubility
product. Since K = 36, this means that we have
6 M [Na+] and [Cl-] in a saturated solution of
NaCl.
Given these Ksp’s:
– AgBr : 5.0 x 10-13
– Ag2S : 6.3 x 10-50
– Ag3PO4 : 2.6 x 10-18
What is the molar solubility of each compound?
Calculating Solubility Equilibria
Solution: the solubility equilibrium is
and the product is
The reaction leads to x moles of Ag+ and Br-.
Assuming a solid of AgBr is in equilibrium with
the ions in solution, calculate the concentration
of Ag+ and Br-. Ksp= 5.0 x 10-13
Calculating Solubility Equilibria
Solution: the solubility equilibrium is
and the product is
The reaction leads to x moles of Ag+ and S2-.
Assuming a solid of Ag2S is in equilibrium with
the ions in solution, calculate the concentration
of Ag+ and S2-. Ksp= 6.3 x 10-50
Calculating Solubility Equilibria
Assuming a solid of Ag3PO4 is in equilibrium with
the ions in solution, calculate the concentration
of Ag+ and PO43-. Ksp= 2.6 x 10-18
Calculating Solubility Equilibria
Solution: the solubility equilibrium is
and the product is
The reaction leads to x moles of Ag+ and PO43-.
Assuming a solid of Ag3PO4 is in equilibrium with
the ions in solution, calculate the concentration
of Ag+ and PO43-. Ksp= 2.6 x 10-18
Calculating Solubility Equilibria
What is the solubility of AgBr in 0.02 M aqueous
NaBr?
Calculating Solubility Equilibria in the presence of a common ion
Solution: the solubility equilibrium is
Make a reaction table
Substitute into the solubility product expression and
solve for x.
What is the solubility of AgBr in 0.02 M aqueous
NaBr?
Calculating Solubility Equilibria in the presence of a common ion
Species Ag+ Br-
Initial 0.0 0.02
Final x 0.02+x
Substitute into the solubility product expression and
solve for x.
What is the solubility of AgBr in 0.02 M aqueous
NaBr?
Calculating Solubility Equilibria in the presence of a common ion
Step 1. Write down the solubility equilibrium.
Step 2. Determine the hydroxide ion concentration.
Step 3. Substitute into the solubility product expression.
Step 4. Compare the value to the tablulated Ksp.
Will 0.01 M Mg2+ precipitate at pH 4?
Calculating Solubility Equilibria of metal hydroxides
Will 0.01 M Mg2+ precipitate at pH 4?
Calculating Solubility Equilibria of metal hydroxides
For this reaction Ksp = 5.61 x 10-12. Since the product of
the concentrations is less than this value the hydroxide will not precipitate. Here is a related, but harder problem.
At what pH will 0.01 M Mg2+ precipitate?
The comparison of concentrations in the solubility
product with the tabulated Ksp is an example of
a free energy relationship.
At equilibrium we have a saturated solution so
Thus, if Q > Ksp then DG > 0 and the solution
reaction,
is not spontaneous.
Justification for treatment of Ksp
Can 0.01 M Pb2+ be separated from 0.01 M Mg2+
at pH 9?
Calculating Solubility Equilibria of metal hydroxides
The answer to this question resides in the relative
Ksp for the hydroxides of each of these ions.
If one is completely insoluble then they can be
separated.
Can 0.01 M Pb2+ be separated from 0.01 M Mg2+
at pH 9?
Calculating Solubility Equilibria of metal hydroxides
Can 0.01 M Pb2+ be separated from 0.01 M Mg2+
at pH 9?
Calculating Solubility Equilibria of metal hydroxides
Step 1. Calculate the [OH-].
Can 0.01 M Pb2+ be separated from 0.01 M Mg2+
at pH 9?
Calculating Solubility Equilibria of metal hydroxides
Step 1. Calculate the [OH-].
Step2. Calculate the solubility products.
Can 0.01 M Pb2+ be separated from 0.01 M Mg2+
at pH 9?
Calculating Solubility Equilibria of metal hydroxides
Step 1. Calculate the [OH-].
Step2. Calculate the solubility products.
The answer in this case is yes, Pb(OH)2 can be
separated since it will precipitate, while Mg(OH) 2
remains in solution.
For polyprotic ions, the solubility product will depend
on the state of ionization of the ions.
For example, CaCO3 in the ocean is sparingly
soluble. However, if the CO32- concentration is
reduced by the equilibrium,
then CaCO3 will begin to dissolve. This equilibrium
is particularly important since coral reefs and diatoms
are composed of CaCO3.
pH-dependent equilibria
CaCO3 has a solubility product of Ksp = 3.36 x 10-9.
This is a moderately small Ksp, which means that
CaCO3 should precipitate if there is sufficient
Ca2+ and CO32-.
The equilibrium is
Precipitation of CaCO3
Step 1. Write the Ksp
Step2. Calculate the solubility.
What is the concentration of Ca2+ and CO32- in
a saturated solution at pH 7?
pH-dependent equilibria
The concentration of Ca2+ and CO32- in the world’s
oceans are 0.01 M and 3.1 x 10-4 M, respectively.
Calcium carbonate in the oceans
The concentration of Ca2+ and CO32- in the worlds
oceans are 0.01 M and 3.1 x 10-4 M, respectively.
Will CaCO3 precipitate?
Calcium carbonate in the oceans
Step 1. Calculate the standard free energy
Step2. Calculate the free energy of CaCO3 in sea.
The concentration of Ca2+ and CO32- in the worlds
oceans are 0.01 M and 3.1 x 10-4 M, respectively.
Will CaCO3 precipitate?
Calcium carbonate in the oceans
Step 2. Calculate the free energy (cont’d)
Since DG > 0 the reaction is NOT spontaneous as
written, formation of solution. Therefore, the
spontaneous process is precipitation.
The concentration of Ca2+ and CO32- in the worlds
oceans are 0.01 M and 3.1 x 10-4 M, respectively.
Will CaCO3 precipitate?
Calcium carbonate in the oceans
Simpler approach. Just calculate the reaction
quotient and compare it to the solubility product.
The conclusion is the same. CaCO3 will precipitate.
But, wait a minute… why hasn’t it already done so?