chapter 4 -Electrochemistry

7/29/2019 chapter 4 -Electrochemistry

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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

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chapter 4 -Electrochemistry