chapter 4 -electrochemistry
TRANSCRIPT
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Mass Transfer in Electrolytes
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Electrochemistry
n is the branch of chemical science that deals with the
interrelation of electrical and chemical phenomena.
n From the very beginning electrochemistry covers two
main areas:
- the conversion of the energy of chemical
reactions into electricity (electrochemical power
sources)
- the transformations of chemical compounds bythe passage of an electric current (electrolysis).
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Mass Transfer in Electrolytes
n Mass transfer refers to mass in transit due to a species
concentration gradient in a mixture.
n Must have a mixture of two or more species for mass
transfer to occur.
n The species concentration gradient is the driving
potential for transfer.
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n There are three basic mechanisms of mass transport:
o Diffusion
oMigration
oConvection
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o Diffusion
n The random movement of molecules from a region of
high concentration to regions of lower concentration.
n The rate at which a molecule diffuses is dependent
upon the difference in concentration between two
points in solution, called the concentration gradient,
and on the diffusion coefficient D, which has a
characteristic value for a specific solution species at
fixed temperature.
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This movement of a chemical species under the
influence of a concentration gradient is described by
Ficks first law.
Jd,j =Djgradcj
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Migration
The movement of charged particles in response to a
local electric field is called migration.
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Convection
Movement due to changes in density at the electrode
solution interface. This occurs due to depletion or
addition of a species due to the electrochemical
reaction.
The movement of fluids is described byhydrodynamics.
it applies to electrochemistry is forced movement of
solution species by mechanical (stirring) or other
means.
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Fick's first law
relates the diffusive flux to the concentration under
the assumption of steady state.It postulates that the flux goes from regions of high
concentration to regions of low concentration, with a
magnitude that is proportional to the concentration
gradient (spatial derivative). the law is
Jd,j =Djgradcj (1)
J The diffusion flux ( measures the amount of
substance that will flow through a small area during a small
time interval )
D the diffusion coefficient or diffusivity
grad cThe concentration gradient of diffusing substance
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the diffusion of ions in solutions, Eq. (1) is obeyed
only at low concentrations of these ions. At higher
concentrations the proportionality between flux and
concentration gradient is lost (i.e., coefficient Djceases to be constant).
A possible reason for the departures from Ficks first
law is the fact that the diffusion process tends to levelchemical potentials (thermodynamic activities) rather
than concentrations of the substances involved.
Hence, the equation sometimes is written as
Jd, j =Da, j gradaj (2)
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the equation does not provide a sufficiently accurate
description of the experimental results in solutions
unless these are highly dilute, and again coefficient
Da, j is not constant when the concentration is varied.
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Limiting Diffusion Currents inElectrolytes
Diffusion processes in electrochemical systems thatare not complicated by migration and convection.
- To exclude migration, we consider the behavior
of uncharged reaction components.
- The condition of a complete absence ofconvections of the liquid can be realized.
the electrode is provided with a porous lining of
thickness and filled with the electrolyte.
In the small pores of the lining, convection of the
liquid is almost impossible.
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Diffusion layer of constant thickness.
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By powerful stirring of the solution a concentration of
the reactant that is sufficiently close to the starting
concentration can be maintained at the outer surface
of the lining.
When current flows in an electrolyte solution, the
concentration, of a reactant and/or product close to
the electrode surface will change relative to its bulkconcentration as a result of the electrode reaction.
The layer of electrolyte where the concentrationchanges occur and within which the substances are
transported by diffusion is called the diffusion layer.
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Its thickness, (the diffusion path length), depends
on cell design features and on the intensity of
convective.
The changes in surface concentrations of the
components caused by current flow have two
important effects:
- They produce a change in electrode potential
- they imply that there is an upper limit to the cell
currents when the diffusion flux attains its
limiting value.
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Ionic Transport by Migration and Diffusio
The Total Flux Equation
The equation for the total flux of ions under the
simultaneous effects of an electrostatic field E and a
concentration gradient is (the NernstPlanck
equation, 1890)
In this equation a minus sign should be used when
the direction of the diffusion flux is opposite that of the
migration flux.
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Convective Transport
Convective transport is the transport of substances
with a moving medium (e.g., the transport of a solute
in a liquid flow). The convective flux is given by
where is the linear velocity of the medium and cj is
the concentration of the substance.
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In electrolyte solutions, the convective flux is always
electroneutral because of the mediums
electroneutrality
In electrochemical cells we often find convective
transport of reaction components toward (or away
from) the electrode surface.
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Different ways of convection transport
1- Flow-by Electrodes
2- Rotating-Disk Electrode
3- Rotating RingDisk Electrode
4- Cells with Natural Convection of the Electrolyte
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1- Flow-by Electrodes
Flow of the liquid past the electrode is found in
electrochemical cells where a liquid electrolyte is
agitated with a stirrer or by pumping.
The character of liquid flow near a solid wall depends
on the flow velocity , on the characteristic length L of
the solid, and on the kinematic viscosity.
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2-Rotating-Disk Electrode
At the rotating-disk electrode (RDE), it is the solidelectrode and not the liquid that is driven; but from a
hydrodynamic point of view this difference is
unimportant.
Liquid flows, which in the figure are shown by arrows,
are generated in the solution when the electrode is
rotated around its vertical axis. The liquid flow
impinges on the electrode in the center of the rotatingdisk, then is diverted by centrifugal forces to the
periphery.
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3- Rotating RingDisk Electrode