Chapter 7 Electrochemistry
§7.12 Basic principal and application of electrolysis
Electrolysis:
The chemical reactions which accompany the
passage of a current through an electrolytic solution.
Electrode
Positive electrode Negative electrode
Anode Cathode
Reaction Oxidation Reduction
Electrode and reaction:
1. Cathode reaction
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 m
etals be plated out onto the cathode?
1) Order of liberation
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
For evolution of gas, the overpotential is relatively large, therefore, the overpotential should be taken into consideration.
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
0.337
⊖ Cu2+/Cu
-0.126
⊖ Pb2+/Pb
0.799
⊖ Ag+/Ag
0.000
⊖ H+/H2
For liberation of metal, the overpotential is usually very low, and the reversible potential can be used in stead of irreversible potential.
a(Ag+) = 1.510-8
0.799 V
a(Cu2+) = 2.210-16
0.337 V
a(Pb2+) = 3.310-49
-0.126 V -1.56 V
The liberation order and the residual concentration of the ions upon negative shift of potential of cathode
Potential sweep: polarization curve
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
⊖ Cu2+/Cu = 0.337 V; ⊖ Zn2+/Zn = -0.763 V;
When Zn begins to plate out, the residual concentration of Cu2+ in the solution can be calculated according to:
2+ 2+ 2+Cu /Cu Cu /Cu Culn
RTa
nF y
(1) Separation of metal
-0.763 = 0.337 + 0.05916lg aCu2+
CCu2+ aCu2+ = 2.5410-19 mol·dm-3
When Zn begins to deposit, Cu has deposited completely.
When the difference between liberation potential of two metals is larger than 0.2 V, the two metal can be separated completely.
(2) Quantitative and qualitative analysis
Polarograph
Polarographic wavenDC
I d (max)
C
CII
s
dd 1(max)
Dropping mercury cathode
N2
A
+
Hg anode
Cu2+
Tl+
E1/2Imax
Polarograph
Jaroslav Heyrovský1959 Noble Prize
Czechoslovakia
1890/12/20 ~ 1967/03/27
Polarography
Progress of the sensitivity of polarogr
aphy
1935: 10-2 ~ 10-5 mol·dm-3
1957: 10-8 ~ 10-9 mol·dm-3
1957: 210-10 mol·dm-3
At present: 1010~1012 moldm-3
(3) Electroplating of single metal and alloy
Anode: Ag Ag++ e
Cathode: Ag+ + e Ag
A silver-plated teapot
Alloy electroplating:
Zn-Fe, Cu-Zn
Composite electroplating:
Ni-PTFE, Ni-Diamond
Electroplating of non-metals:
Plastic, wood, flowers
⊖ Cu2+/Cu = 0.337 V; ⊖ Zn2+/Zn = -0.763 V;
When Zn begins to plate out, the residual concentration of Cu2+ in the solution is
Principle of alloy deposition
Brass can’t deposit from the solution containing Cu2+ and Zn2+.
⊖ Cu(CN)3/Cu = 1.03 V; ⊖ Zn(CN)4
2/Zn = 1.12 V;
< 0.2 V
Zn co-deposits with Cu and form brass. When tin is added, a alloy coating with gold luster can be plated out.
2+ 2+
19 -3
Cu Cu2.54 10 mol dmc a
(4) Electrolytic metallurgy
Many most active metals, such as Li, Na, K, Mg, Ca, Al, Ti, rare earth metal, etc. can be only produced electrochemically.
Electroreduction of aluminum
Charles Martin Hall (1863-
1914), who first produced
metal aluminum cheaply by
electrolysis of molten mixture
of Al2O3/Na3AlF6.
Production of metal sodium
The man who discovered the largest number of elements
Davy, on knowing the electrolysis of water by Nicholson a
nd Carlisle, set out his element finding trip using electrolys
is as his powerful tools, he discovered 8 elements including:
K, Na, Ma, Ca, Sr, Ba, B, and Si.
Titanium
(5) electrorefining of metal
From 95% to 99.99%, which is suitable for electric usage.
CuZnAgAu
Cu2+
Zn2+
Cu2+
Cu2+
Cu2+
Cu2+
Industrial electrorefining of copper
(6) Electrosynthesis
Advantages of Electrochemical Synthesis
1) The oxidative or reductive ability can be easily adjusted.
2) The most powerful oxidation or reduction methods.
3) Without introduction of impurities.
2. Anode reaction
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 < Cl < Br < I Henri Moissan1906 Noble PrizeFrance1852/09/28 ~ 1907/02/20 Investigation and isolation of the element fluorine
1) Reaction over inert anode
(1) Active dissolution;
(2) Anodic passivation
(3) Anodic oxidation
2) Reaction over active anode
Pourbaix diagram of iron-water system
(1) Active dissolution:
At pH=4 and low current density, active dissolution occurs.
Fe Fe2+ + 2e
Fe2+
Fe2O3
Fe
pH
/ V
2 4 6 8 10 12 140
Fe3O4
Fe3+
FeO22
We usually judge the reaction
based on Porbaix diagram
(2) Anodic passivation:
At pH= 12 and high potential,
upon polarization, dense thin la
yer of Fe3O4 forms and passivat
ion of iron takes place.
3Fe + 4H2O – 8e
Fe3O4 + 8 H+
Passivation curve of iron
Fe2+
Fe2O3
Fe
pH
/ V
2 4 6 8 10 12 140
Fe3O4
Fe3+
FeO22
Active dissolution
passivation
Trans-passivation
Anodic oxidation of aluminum
(3) Anodic oxidation
t / h
E /
V
Barrier layer
Porous layer
Initiation of pores
SEM photograph of the AAM
top surface
Cross-section
Application of anodic alumina membrane (AAM)
1) Coloring of aluminum and aluminum alloys
2) Corrosion protection
3) Template synthesis of nanomaterials.
Nanomaterials synthesized using AAM template
Nanotubes
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