1497 heru che trelkim4
TRANSCRIPT
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Dr. Heru Setyawan
Jurusan Teknik Kimia FTI ITS
Teknik Reaksi Elektrokimia
Kuliah 4:A More Detailed View of Interfacial Potential Differences
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Reference Electrodes
Many reference electrodes other than the NHE and the SCE have
been devised for electrochemical studies in aqueous and nonaqueoussolvents.
There are experimental reasons for the choice of a reference electrode.
System Ag/AgCl/KCl (saturated, aqueous) has a smaller
temperature coefficient of potential than an SCE and can be builtmore compactly.
Mercurous sulfate electrode Hg/Hg2SO4/K2SO4 (saturated,aqueous) may be used when chloride is not acceptable.
A quasireference electrode(QRE) is often employed for a nonaqueoussolvent due to the difficulty in finding a reference electrode that doesnot contaminate the test solution with undesirable species.
Usually just a metal wire, Ag or Pt used with the expectation that in experiments where there is
essentially no change in the bulk solution, the potential of this wire, although unknown, will not change during
a series of measurements.
The actual potential of QRE vs. a true reference electrode must becalibrated before reporting potentials with reference to the QRE.
Ferrocene/ferrocenium (Fc/Fc+) couple is recommended as acalibrating redox couple, since both forms are soluble and stable inmany solvents, and since the couple usually shows nernstian
behavior.
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Reference Electrodes
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The Physics of Phase Potentials
So far, discussion about thermodynamics consideration doesnt require to
advance a mechanistic basis for the observable differences in potentials across
certain phase boundaries.
However, it is difficult to think chemically without a mechanistic model;
It is helpful to consider the kinds of interaction between phases that could
create these interfacial differences.
Consider two questions:
1. Can we expect the potential within a phase to be uniform?
2. If so, what governs its value?
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The Physics of Phase Potentials
The potential, defined as the work required to bring a unit positive charge,
without material interactions, from an infinite distance to point (x,y,z), can be
expressed as
( ) ldEzyxzyx
=
,,
,,
E= electric field strength vector, V/m
dl = an infinitesimal tangent to the path in
direction of movement.
The difference in potential between points (x,y,z) and (x,y,z):
( ) ( ) ldEzyxzyx zyxzyx
= ',','
,,,,',','
In general,Eis not zero every where between two points and the integral does not vanish.
For conducting phases: When no current passes through, there is no net movement of charge carriers, so the
electric field at all interior points must be zero.
If it were not, the carriers would move in response to it to eliminate the field.
The difference in potential between any two points (Eq. 2.2.2) in the interior of thephase must also be zero under these conditions equipotential volume
= inner potentialorGalvani potentialof the phase
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The Physics of Phase Potentials
Why does the inner potential have the value that it does?
Any excess charge that might exist on the phase itself a test charge would
have to work against the coulombic field arising from that charge.
Can arise from miscellaneous fields resulting from charged bodies outside the
sample.
Alterations in charge distributions inside or outside the phase will change the
potential.
Differences in potential arising from chemical interactions between phases
have some sort ofcharge separation.
Where is the location of any excess charge on a conducting phase?
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The Physics of Phase Potentials
The Gauss law from elementary electrostatics:If we enclose a volume with an imaginary surface (a Gaussian surface), we will
find that the net charge q inside the surface is given by
= SdEq0
0 = permittivity of free space or electric constant (8.85419 10-12 C2N-1 m-1)
dS = an infinitesimal vector normal outward from the surface
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The Physics of Phase Potentials
The way in which phase potentials are established: Changes in potential of a conducting phase can be affected
by altering the charge distributions on or around the
phase.
If the phase undergoes a change in its excess charge, its
charge carries will adjust such that the excess becomes
wholly distributed over an entire boundary of the phase.
The surface distribution is such that the electric field
strength within the phase is zero under null-current
conditions.
The interior of the phase features a constant potential, .
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Interaction Between Conducting Phases
MSqq =
M
- S
= interfacial potential difference
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Electrochemical Potentials
The electrochemical potential for species i with chargeziin phase (Butler &
Guggenheim):
Chemical potential, defined as:
ni = number of moles ofi in phase
Thus, the electrochemical potential would be:
The electrochemical free energy, G, differs from the chemical potential, G, by the
inclusion of effects from the large scale electrical environment.
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Properties of the Electrochemical Potential
For an uncharged species: i
= i
For any substance: i = i
0 + RTln ai, where i
0 is the
standard chemical potential, and ai is the activity of
species iin phase .
For a pure phase at unit activity (e.g., solid Zn, AgCl, Ag,
or H2 at unit figicity): i = i
0.
For electrons in a metal (z= -1): e
= e0
- F
. Activityeffects can be disregarded because the electron
concentration never changes appreciably.
For equilibrium of species ibetween phases and : i
=
i.
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Reactions in a Single Phase
is constant everywhere and exerts no effect on a
chemical equilibrium within a single conducting phase.
terms drop out of relations involving electrochemical
potentials, and only chemical potentials will remain.
Example: Acid-base equilibrium
This requires that
-OAcHHOAc + +
-OAcHHOAc += +
FF ++=+ -OAcHHOAc
-OAcHHOAc +=
+
Reactions Involving Two Phases Without Charge
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Reactions Involving Two Phases Without Charge
Transfer
)(solution,ClAg)(crystal,AgCl
-
sc +
+
Solubility equilibrium:Considering separate equilibria involving Ag+ and Cl- in solution and solid:
s
Ag
AgCl
Ag ++=
sCl
AgClCl =
AgCl
Cl
AgCl
Ag
AgCl
AgCl - += +
s
Cl
s
Ag
0AgClAgCl - += +
Expanding
ss
Cl
0s
Cl
ss
Ag
0s
Ag
0AgCl
AgCl FlnFln - ++++= ++ aRTaRT
Ksp = solubility product
sp
s
Cl
s
Ag
0s
Cl
0s
Ag
0AgCl
AgCl lnln- KRTaaRT =+= ++
Rearranging
The equilibrium is unaffected by the potential difference across the interface.
This is a general feature of interface reactions without transfer of charge.
When charge transfer occurs, the interfacial potential difference strongly affects the
chemical process can be used to probe or alter the equilibrium position.
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Formulation of a Cell Potentials
At equilibrium(2.2.22)
But
Expanding 2.2.22
where
(Nernst equation for the cell)
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Liquid Junction Potentials
Potential Difference at an Electrolyte-Electrolyte Boundary
Cu/Zn/Zn2+/Cu2+/Cu
E= (Cu ) (Cu ) + ( )
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Types of Liquid Junctions
Classification of liquid junctions (Lingane):
1. Two solutions of the same electrolyte at different concentrations
(Fig. 2.3.2a).
2. Two solutions at the same concentration with different electrolytes
having an ion in common (Fig. 2.3.2a).3. Two solutions not satisfying conditions 1 or 2 (Fig. 2.3.2a).
C d f N b d M bili
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Conductance, transference Numbers, and Mobility
When an electric current flows in an electrochemical cell, the current is carried in
solution by the movement of ions.
Transference (Transport) numbers: the fractions of current carried by positive ion
and negative ion.
1i i=
t
+ = H+
- = Cl-
Electroneutrality
C d f N b d M bili
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Conductance, transference Numbers, and Mobility
Transference numbers are determined by the details of
ionic conduction, which are understood mainly through
measurements of either the resistance to current flow in
solution or its reciprocal, the conductance, L.
L = A/l
Mobility, ui: the limiting velocity of the ion in an electric
field of unit strength.
= conductivity A = surface area
l = distance