We can use standard electrode potentials and the Nernst equation to calculate The potential obtainable from a galvanic cell or the potential required to operate An electrolytic cell.
E
cell= E
right– E
leftCalculating Potentials of Electrochemical Cells
There are three distinct regions in the titration of iron(II) with standard cerium(IV)
1.Before the equivalence point, where the potential is dominated by the analyte redox pair.
2.At the equivalence point, where the potential at the indicator electrode is the average of their conditional potential.
3.After the equivalence point, where the potential was
determined by the titratant redox pair.
Consider the Titration Reaction (essentially goes to completion):
Ce
4+is added with a buret to a solution of Fe
2+
Pt electrode responds to relative concentration of Fe
3+/Fe
2+& Ce
4+/Ce
3+
Calomel electrode used as reference
E
o= 0.767 V Indicator half-reactions at Pt electrode:
K ≈ 10
16E
o= 1.70 V
Before the Equivalence Point
Each aliquot of Ce
4+creates an equal number of moles of Ce
3+and Fe
3+
Excess unreacted Fe
2+remains in solution
Amounts of Fe
2+and Fe
3+are known, use to determine cell voltage.
Residual amount of Ce
4+is unknown
Before the Equivalence Point
E
o= 0.767 V
Use iron half-reaction relative to calomel reference electrode:
241 0
05916 0
767
0 2 3 .
Fe log Fe
. .
E
] [
] [
Potential of
calomel
electrode
At the Equivalence Point
Enough Ce
4+has been added to react with all Fe
2+- Primarily only Ce
3+and Fe
3+present
- Tiny amounts of Ce
4+and Fe
2+from equilibrium
From Reaction:
- [Ce
3+] = [Fe
3+] - [Ce
4+] = [Fe
2+]
Both Reactions are in Equilibrium at the Pt electrode
[ ]
] [
3 2
05916 0
767
0 Fe
log Fe .
. E
[ ]
] [
4 3
05916 0
70
1 Ce
log Ce .
.
E
At the Equivalence Point
Don’t Know the Concentration of either Fe
2+or Ce
4+
Can’t solve either equation independently to determine E
+
Instead Add both equations together
[ ]
] [
3 2
05916 0
767
0 Fe
log Fe .
.
E
[ ]
] [
4 3
05916 0
70
1 Ce
log Ce .
. E
[ ]
] [
] [
] [
4 3 3
2
05916 0
05916 0
70 1 767 0
2 Ce
log Ce Fe .
log Fe .
. .
E
Rearrange
[ ]
] [
] [
] [
4 3 3
2
05916 0
47 2
2 Ce
Ce Fe
log Fe .
. E
Add
Equivalence-point voltage is independent of the concentrations and volumes of the reactants
After the Equivalence Point
Opposite Situation Compared to Before the Equivalence Point
Equal number of moles of Ce
3+and Fe
3+
Excess unreacted Ce
4+remains in solution
Amounts of Ce
3+and Ce
4+are known, use to determine cell voltage.
