|
CEE
680 |
|
Fall
2011 |
Water
Chemistry
Consider
a deep lake sediment in equilibrium with 10-4 atm of oxygen.
Like many large lakes, this one has substantial deposits of manganese in
its sediments.
1. Write a balanced equation
for the oxidation of Mn(+II) to Mn(+III), and then on
to Mn(+IV) by oxygen.
2. Determine the stoichiometric
requirements of oxygen for the complete conversion of Mn(+II)
to Mn(+IV) in mg-oxygen/mg-manganese.
3. Determine the Log K for this
reaction.
4. Based on the partial
pressure of oxygen, determine pe for this system.
5. Now calculate the the ratios
of the various free-Mn species (i.e., Mn+2, Mn+3, Mn+4)
in the pore water.
6. Assume the lake contains
large deposits of Mn(OH)2(s) .
What will the total concentration of soluble manganese be in the pore
water, if all species are in redox equilibrium?
Use the following thermodynamic data. Assume the system is buffered at pH 7.0.
|
Equ# |
Half Cell Reaction |
DEo (Volts) |
|
1 |
O2(g) + 4H+ + 4e- = 2H2O |
+1.23 |
|
2 |
Mn+3 +
e- =
Mn+2 |
+1.51 |
|
3 |
Mn+4 +
e- =
Mn+3 |
+1.65 |
|
4 |
MnO4- +
8H+ +
5e- =
Mn+2 +
4H2O
|
+1.49 |
|
5 |
Fe+3 +
e- =
Fe+2 |
+0.77 |
|
Equ# |
Equilibrium |
Log K |
|
6 |
Mn+2 +
H2O =
MnOH+ +
H+ |
-10.6 |
|
7 |
Mn+2 +
3H2O =
Mn(OH)3- + 3H+ |
-35 |
|
8 |
Mn+2 +
4H2O =
Mn(OH)4-2 + 4H+ |
-48.3 |
|
9 |
Mn(OH)2
(s) =
Mn+2 + 2OH- |
-12.8 |
Due: not collected