1497 Heru Che TRElkim4

7/28/2019 1497 Heru Che TRElkim4

1/19

Dr. Heru Setyawan

Jurusan Teknik Kimia FTI ITS

Teknik Reaksi Elektrokimia

Kuliah 4:A More Detailed View of Interfacial Potential Differences


2/19

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.


3/19

Reference Electrodes


4/19

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?


5/19


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


6/19


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?


7/19


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


8/19


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, .


9/19

Interaction Between Conducting Phases

MSqq =

M

- S

= interfacial potential difference


10/19


11/19

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.


12/19

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.


13/19

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


14/19

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.


15/19

Formulation of a Cell Potentials

At equilibrium(2.2.22)

But

Expanding 2.2.22

where

(Nernst equation for the cell)


16/19

Liquid Junction Potentials

Potential Difference at an Electrolyte-Electrolyte Boundary

Cu/Zn/Zn2+/Cu2+/Cu

E= (Cu ) (Cu ) + ( )


17/19

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


18/19

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


19/19

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

Documents

1497 Heru Che TRElkim4