Chapter 7 Electrochemistry

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Chapter 7 Electrochemistry
7.12 Basic principal and application of
electrolysis
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Inert anode
Oxidation of species in solution
Anodic reaction
Active dissolution
Active anode
Passivation and conversion
Electrolysis reaction
Anodization
reduction of species in solution
cathodic reaction
reduction of oxide/conversion layer
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1. Cathode reaction
1) Order of liberation
Suppose a solution in an electrolytic cell
containing Ag, Cu2, H, and Pb2 of 1 molarity.
If the potential is initially very high and is
gradually turned down, in which order will the
metals be plated out onto the cathode?
Ag e? ? Ag ?? Ag/Ag 0.799 V Cu2 2e? ? Cu ?? Cu2/Cu 0.337 V
2H 2e? ? H2 ?? H/H2 0.000 V Pb2 2e?? Pb ?? Pb2/Pb -0.126 V
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For liberation of metal, the overpotential is
usually very low, and the reversible potential
can be used in stead of irreversible potential.
For evolution of gas, the overpotential is
relatively large, therefore, the overpotential
should be taken into consideration.
0.337 ?? Cu2/Cu
Ag, Cu2, H, and Pb2 will liberates at 0.799
V 0.337 V 0.000 V -0.126 V, respectively
without consideration of overpotential
Overpotential of hydrogen liberation on Cu is 0.6
V, on Pb is 1.56 V
-0.126 ?? Pb2/Pb
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Potential sweep polarization curve
The liberation order and the residual
concentration of the ions upon negative shift of
potential of cathode
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2) Application
1) Separation of metal 2) Quantitative and
qualitative analysis 3) Electroplating of single
metal and alloy 4) Electrolytic metallurgy 5)
Electrorefining of metal 6) Electrosynthesis
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2. Anode reaction
1) Reaction on inert anode
When inert material such as Platinum and graphite
was used, the species in the solution discharge
on the electrode in the order of liberation
potential.
F? lt Cl? lt Br? lt I?
Henri Moissan 1906 Noble Prize France 1852/09/28
1907/02/20 Investigation and isolation of the
element fluorine
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2) Reaction of active anode
(1) Active dissolution (2) Anodic
passivation (3) Anodic oxidation
We usually judge the reaction based on Porbaix
diagram
(1) Active dissolution At pH4 and low
current density, active dissolution occurs.
Fe ? Fe2 2e?
Pourbaix diagram of iron-water system
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(2) Anodic passivation At pH 12 and high
potential, upon polarization, compact thin layer
of Fe3O4 forms and passivation of iron takes
place. 3Fe 4H2O 8e?
? Fe3O4 8 H
Passivation curve of iron
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(3) Anodic oxidation
Anodic oxidation of aluminum
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SEM photograph of the AAM
Cross-section
top surface