Outline:
4/9/07
due Wednesday…
Worksheet #11 answers posted…
 Final exam in 3 weeks…
 Special seminar Tuesday/Thursday
CAPA 18
Today: Continue Chapter 19
Nernst Equation ( and DG)
Coulomb Calculations
 Redox Applications
Spontaneous?
o
 DG = nF

Relation to DG
Definitions:
o = Electromotive force (at STP)
Coulomb = unit of charge
e = 1.6021019 Coulomb
1 mol of e = 6.0221023 . 1.602  1019
= 9.6485  104 Coulomb
F =Faraday Const. = 96,485 C/mol
DG, o , and equilibrium....
DG = DGo + RTlnQ.....
Then:  = o  (RT/nF) lnQ
 Since
 Which
is often written as:
Then:  = o  (0.05916/n) log Q
(@298K) (Nernst Equation)
 What
good is this?
Can calculate  at non-standard
conditions!
Example
What is the voltage produced by the
following cell ?
o
MnO4 +8H+ +5e  Mn2+ +4H2O 1.51
O2(g) + 4H+ + 4e  2 H2O
1.23
at pH=7.00, pO2 =0.20 atm,
[MnO4] = [Mn2+] = 0.10 M ?
 First
balance redox equation for cell....
 Set up expression for Q
 Solve in Nernst equation
Set up redox equation for cell:
 Which
equation gets reversed?
MnO4 +8H+ +5e  Mn2+ +4H2O 1.51
2 H2O  O2 + 4H+ + 4e 1.23
 What
is n?
n=20
MnO4 +8H+ +5e  Mn2+ + 4H2O 4
2 H2O  O2 + 4H+ + 4e
5
4MnO4 +12H+  4Mn2+ +5O2 + 6H2O
 What
is o?
+0.28V
What is Q?

Products/reactants :
[Mn2+]4 pO25
[H+]12 [MnO4]4
= 3.2  1080
What is ?

 = o  (0.05916/n) log Q
= 0.28  0.24
= 0.04 V
Using the Nernst equation….
  = o  (0.05916/n) log Q
Example :
What is the voltage produced by the
concentration cell shown with two
copper electrodes. One is immersed in
1M CuSO4 and the other in 0.001M?
What is
o
 for
this cell? = 0.00 V
Only the Q part of the Nernst
equation is non-zero…..
 =  o  (0.05916/n) log Q
What is Q?

Products/reactants :
[Cu2+] (less concentrated)
= 1  103
[Cu2+] (more concentrated)
What is ?

 =  o  (0.05916/n) log Q
= 0.00  (0.05916/2) 3
= 0.089 V
Worksheet #12
 A galvanic
cell is constructed from a
silver-silver iodide electrode and a Zn
wire in 0.005 M solution of ZnCl2.
What is the cell potential?
 First, what is the galvanic cell?
 Balance redox equation for cell....  o
 Set up expression for Q….prods/rcts
 Solve with Nernst equation
 =  o  (0.05916/n)logQ
Set up redox equation for cell:
 Which
equations?
 Which equation get reversed?
AgI(s) + e  Ag(s) + I
0.152
Zn (s)  Zn2+ + 2e
+0.762
 What is n?
n=2
2AgI(s) + Zn(s)  2Ag(s) + Zn2+ + 2I
 What
is o?
+0.610 V
What is Q?

Products/reactants :
[Zn2+] [I]2
= (0.005)(1)2
1
= 5 × 103
What is ?

 =  o  (0.05916/n)logQ
= 0.610 + 0.068 = 0.678 V
Problem 2

You need to know:
o
N2O(g) + 6H+ + H2O + 4e-  2NH3OH+ 0.050 V
O2(g) + 2H+ + 2e-  H2O2
Answer: + 0.745 V
+0.695 V
More definitions:
 Current
= charge/time
1 Ampere = 1 Coulomb / second
Calculational Example:
 A lead-acid
battery:
PbO2+HSO4+2H++Pb 2PbSO4 +2H2O
Assume 250 g of PbO2 and that this
battery supplies 6 Amps until it dies;
how long will it last?
Quiz #7
 Put
away your books & papers
 When you are finished, turn your
quiz into me