Half Cell Potentials Measured
vs. Standard H2 electrode (SHE)
Standard Hydrogen Electrode
Zn
Electrode
What is E°1/2(reduc) ?
What is E°1/2 (oxid)?
E0cell = E°1/2(reduc) + E°1/2(oxid) =
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E0cell for Cu/Zn Cell
CELL POTENTIAL
What is E°cell for the following reaction?
Al(s) + Cu+2(aq) ! Al+3(aq) + Cu(s)
What is E°1/2(red) ?
What is E°1/2(ox)?
E0cell = E°1/2(red) + E°1/2(ox) =
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Oxidizing and Reducing Agents
Oxidizing and Reducing Agents
Br2
O2
Fe+2
The more positive E°red :
Na+
The more negative E°red :
Which one of these is the best oxidizing
agent? Put these in order of increasing
ability to oxidize.
??? Which is most easily reduced?
Will F2 oxidize H2
or Li?
Strategy: write out half cell potentials for
REDUCTION
Br2 (l) + 2 e- ! 2Br -(aq)
1.06V
O2(g) + 4H+(aq) + 4 e- ! 2H2O (l)
1.23V
Fe+2(aq) + 2 e- ! 2Fe (s)
-0.44V
Na+(aq) + 2 e- ! Na (s)
-2.71V
Will Ni2+
oxidize Al(s)?
A species which is higher and to the left on the
table of standard reduction potentials will
spontaneously oxidize a species that is lower to
the right in the table.
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Sample Problem:
Sample Problem Continued:
Which is the best Reducing agent?
Which is the best Oxidizing agent?
Red agent gets _________; write ________
Ox agent gets ________; write ___________
Cl-(aq)
Cl-(aq)
Fe(s)
Fe(s)
Fe3+(aq)
Fe3+(aq)
Fe2+(aq)
Fe2+(aq)
Cl2(g)
Cl2(g)
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Is it spontaneous?
RELATIONSHIP BETWEEN
"G AND EMF
The standard cell potential (E°cell) for the reaction
below is 0.89V. What is the value of "G° for the
reaction?
"G = -nFE
2Cr(s) + 3 Sn+4(aq) !
At Standard State: "G° = -nFE°
2 Cr+3(aq) + 3 Sn+2(aq)
n = number of electrons transferred in a
balanced redox reaction
Faraday’s Constant :
F
= 96,485 coulomb/mole e“1 Faraday”
1 coulomb = 1 Amp-sec
1 J = 1 Amp-sec-V = 1 coulomb-V
1 F = 1 Faraday = 96,485 J/V-mole eCH112 LRSVDS Redx !
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Connecting "G, Keq, and E°
Effect of Concentration
Standard concentrations:
1M solution, 1 atm gas pressure
"G° = -2.303 RT log Keq
"G° = -n F E°
What if concentrations are different?
E° = 2.303 RT log Keq
nF
"G = "G o + RT ln Q and "G = #nFE
"nFE = "nFE o + RT ln Q
E° = 2.303 RT log Keq
nF
!
R = 8.314 J/K-mole
F = 96,485 J/V-mole e-
!
So, at 25°C = 298K:
E° =
(0.0592) log Keq
n
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!
RT
ln Q
nF
0.02568
E = Eo "
ln Q @ 298K
n
E = Eo "
For electrochemical cell at
equilibrium: "G = 0
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FYI: Effect of Concentration on ECell
E = Eo "
Example
What is the half cell potential of the Ag/
Ag+ redox couple (E0 = +0.799 V) in a
1 M NaCl solution that contains solid
AgCl (Ksp = 1.1 x 10-10)?
RT
ln Q
n!
For a half reaction:
aA + bB + n e- ! cC + dD
c
d
[C] [D]
a
b
[A] [B]
Q =
!
E 1 = E o1 "
!
2
2
ln Q = 2.303 log Q
2.303RT
= 0.0592
F
0.0592
log Q at 298K
n
!
!
For an overall reaction:
gG + hH ! jJ + kK
j
k
J ] [K ]
[
Q =
g
h
[G] [H]
E cell = E !cell "
!
0.0592
log Q at 298K
n
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Explore the Same Reaction:
Al(s) + 3Ag+(aq) ! Al3+(aq) + 3Ag(s)
Sample Problem
A voltaic cell has the following reaction
1) What will happen to the cell emf (Ecell) if the
size of the Al electrode is increased?
Al(s) + 3Ag+(aq) ! Al3+(aq) + 3Ag(s)
What will happen to the cell emf (Ecell) if water is
added to the anode compartment?
a)
b)
c)
2)! What will happen to the cell emf (Ecell) if AgNO3
is added to the cathode compartment
increasing the quantity of Ag+(aq), but not
changing its concentration?
Ecell will increase
Ecell will decrease
This will have no effect on Ecell
3) What will happen to the cell emf (Ecell) if HCl is
added to the AgNO3 solution?
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Concentration Cell:
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