Chemistry Reference
In-Depth Information
is because electrochemistry presents a powerful
method for promoting reactions—within a potential
scan of approximately 6.0 V it offers a vast range
of opportunities—and powerful oxidants (such as
ozone) and reductants (e.g. solvated electrons, alkali
metals), as indicated in Table 19.1 of standard elec-
trode potentials
E
o
.
The benefits in the use of electrochemistry for
greener and more sustainable processes include:
sium, 106; zinc metal, 114; sodium borohydride,
230; hydrazine, 190; sodium dichromate, 505; and
potassium permanganate, 570. Electrons are the
cheapest, purest and most versatile redox agents able
to perform clean and fast reactions. Overall, electro-
chemistry can make a significant contribution to sus-
tainability by satisfying a range of targets for green
chemistry:
•
Clean synthesis
by direct oxidation and reduc-
tion
•
Enhanced atom utilisation
•
Replacement of stoichiometric reagents
:
regeneration of a wide range of redox oxidants
and reductants
•
New solvents and reaction media
: solid poly-
mers, ionic liquids, supercritical fluids, etc.
•
Water-based processes and products
: predom-
inate in electrochemistry
•
Replacements for hazardous reagents
: solu-
tion-phase oxidants and reductants can be gener-
ated in situ
•
Intensive processing
: electrochemical processes
can be intensified using ultrasonics, centrifugal
fields, etc.
•
Novel separation technologies
: electrochemical
enhancement of ion exchange, adsorption, gas-
phase separation and water filtration
•
Alternative Feedstocks
: electrochemistry in
many cases allows alternative feedstocks
•
New, safer chemicals and materials
: in situ,
on-demand reagent can be generated
•
Waste minimisation/reduction
: achieved by
reagent regeneration and material recycling.
• Mild chemical conditions
• Ease of control
• High process selectivity
• Novel chemistry available
• Mild process conditions
• Safer operation
• The electron is an inexpensive reagent
The cost of the electron (£0.08 kWh
-1
, 3.5 V; 8-10
£ kmol
-1
) compares extremely favourably with the
cheapest oxidants and reductants. Approximate costs
(£ kmol
-1
) of other commonly used reagents are:
hydrogen peroxide, 50; sodium metal, 100; magne-
Table 19.1
Standard electrochemical potentials
Electrode reaction
E
° (V) @ 25°C
S
2
O
8
2
-
+
2e
-
Æ
2SO
4
2
-
+
2.01
H
2
O
2
+
2H
+
+
2e
-
Æ
2H
2
O
+
1.7
8
Au
+
+
e
-
Æ
Au
+
1.69
2Cl
+
2e
-
Æ
2Cl
-
+
1.36
1
/
2
O
2
+
2H
+
+
2e
-
Æ
H
2
O
+
1.23
Pd
2
+
+
2e
-
Æ
Pd
+
0.99
Ag
+
+
e
-
Æ
Ag
+
0.
8
0
Fe
3
+
+
e
-
Æ
Fe
2
+
+
0.77
Cu
+
+
e
-
Æ
Cu
+
0.52
3 Electrochemistry Fundamentals
Cu
2
+
+
2e
-
Æ
Cu
+
0.34
An understanding of the chemistry and the electro-
chemistry occurring in the cell is essential for the
successful adoption of any electrochemical process.
This understanding is embodied in the knowledge
of the reaction mechanisms, thermodynamics and
kinetics of the electrode processes. Electrode reac-
tions are heterogeneous multistep processes and can
involve several species and phases: liquid, solid and
gases. The tendency for a particular species to release
or accept electrons is determined by the magnitude
of an electrode potential. The standard electrode
potential,
E
O/R
, of a redox system involving dissolved
oxidised (O) and reduced (R) species is a measure of
2H
+
+
2e
-
Æ
H
2
+
0.00
Pb
2
+
+
2e
-
Æ
Pb
-
0.13
Sn
2
+
+
2e
-
Æ
Sn
-
0.14
Mo
2
+
+
2e
-
Æ
Mo
-
0.20
Ni
2
+
+
2e
-
Æ
Ni
-
0.25
Co
2
+
+
2e
-
Æ
Co
-
0.2
8
Cd
2
+
+
2e
-
Æ
Cd
-
0.40
Fe
2
+
+
2e
-
Æ
Fe
-
0.44
Zn
2
+
+
2e
-
Æ
Zn
-
0.76
Mn
2
+
+
2e
-
Æ
Mn
-
1.19
Al
3
+
+
3e
-
Æ
Al
-
1.66
Mg
2
+
+
2e
-
Æ
Mg
-
2.36
Na
+
+
e
-
Æ
Na
-
2.71
