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Redox Reactions and
Electrochemistry
Chapter 19
Cell Potentials
Ecell
 = Ered
 (cathode) − Ered
 (anode)
= +0.34 V − (−0.76 V)
= +1.10 V
Oxidizing and Reducing Agents
• The strongest oxidizers
have the most positive
reduction potentials.
• The strongest reducers
have the most negative
reduction potentials.
• Remember that the
oxidant occurs on the
left side of the equation,
and the reductant
occurs on the right side
of the equation
Oxidizing and Reducing Agents
The greater the
difference between
the two halfreaction potentials,
the greater the
voltage of the cell.
Free Energy
G for a redox reaction can be found by
using the equation
G = −nFE
where n is the number of moles of
electrons transferred, and F is a
constant, the Faraday.
1 F = 96,485 C/mol = 96,485 J/V-mol
Free Energy
Under standard conditions,
G = −nFE
Nernst Equation
• Remember that
G = G + RT ln Q
• This means
−nFE = −nFE + RT ln Q
Nernst Equation
Dividing both sides by −nF, we get the
Nernst equation:
RT
ln Q
E = E −
nF
or, using base-10 logarithms,
2.303 RT
log Q
E = E −
nF
Nernst Equation
At room temperature (298 K),
2.303 RT
= 0.0592 V
F
Thus the equation becomes
0.0592
log Q
E = E −
n
Nernst
E=
E0
0.0592 V
log Q
n
Concentration Cells
• Notice that the Nernst equation implies that a cell could be created that
has the same substance at both electrodes.
E=
E0
0.0592 V
log Q
n
• For such a cell, Ecell would be 0, but Q would not.
• Therefore, as long as the concentrations are different, E will
not be 0.
Concentration Cells
Ion concentration and emf in the human heart: variation of the electrical
potential caused by changes of ion concentrations in the pacemaker cells of
the heart
Concentration Cells
Electrocardiography: measuring voltage changes during heartbeats at the
surface of the body
Applications of
Oxidation-Reduction
Reactions
Batteries
Portable, self-contained electrochemical power source; vary greatly in both
size and in the electrochemical reaction used to generate electricity
Batteries
Galvanic cell, or a series of combined galvanic cells, that can be used as a
source of direct electric current at a constant voltage
Batteries
Different applications require batteries with different properties
• The battery required to start a car must be
capable of delivering a large electrical current
for a short period of time
• The battery that powers a heart pace-maker
must be very small and capable of delivering a
small but steady current over an extended time
period
• Some batteries are primary cells, meaning they
cannot be recharged
• Some batteries are secondary cells, meaning
they can be recharged from an external power
source after their emf has dropped
Batteries
Dry cell
Leclanché cell
Anode:
Cathode:
Zn (s)
2NH+4 (aq) + 2MnO2 (s) + 2e-
Zn (s) + 2NH4 (aq) + 2MnO2 (s)
Zn2+ (aq) + 2eMn2O3 (s) + 2NH3 (aq) + H2O (l)
Zn2+ (aq) + 2NH3 (aq) + H2O (l) + Mn2O3 (s)
19.6