METAL-ELECTROLYTE INTERFACE

Metals are solid

contain many defects in their crystal structure

inhomogeneities in their chemical composition

May be polycrystalline, monocrystalline or amorphous

Electrolytes have many types of intermolecular forces like

Ion-ion interaction

Ion-solvent interaction

Solvent-solvent interaction

An interface is created when a metal (solid) electrode is

introduced into an electrolyte solution

This interface becomes electrified due to an electron

exchange between the electrode and the ions in solution

potential difference over the metal-electrolyte interface is

generated.

an electrical field leads to the redistribution of mobile ions

and water dipole molecules in the region near the electrode.

Metal-electrolyte interface

This potential difference across the metal-electrolyte

interface is called electrode potential.

The origin of electrode potential

Chemical potential of ith component in a mixture is given

by

where oi is the chemical potential of ion i in a hypothetical solution with molality mo in which ion-ion interaction are insignificant

Chemical potential of i is defined as free energy change for 1

mole of i in an infinite amount of the mixture, i.e.

i

o

ii aRT ln

ji nnTPi

in

G

,,

Total free energy in the mixture is given by

If reaction takes place in a large excess of mixture with

insignificant changes in mole fractions of reactants and

products, then free energy change of reaction is

i is the stoichiometric number of reactants.

At equilibrium rG equal zero

i

iinG

i

iirG

Chemical potential of component i present in two

mixtures/solutions in contact with chemical equilibrium

across the interface is given by:

If above equality is not fulfilled, then spontaneous reaction

takes place across the interface.

Free energy of i is lowered when it is transferred from one

phase to another.

)()( III ii

When metal (M)is in contact with its ions (Mz+) in solution then the

following equilibrium is expected:

where (m) refer to the metal

For example

chemical reaction in either sides is needed to make chemical potential

for copper equal in the two phases

Such reaction consumes or generates electrical charges

Potential difference is formed between the two phases

m

z

aqS zeMM

)(

0

)(

)(

2

)(

0

)( 2 maqs eCuCu

The favorable direction of the reaction depends on chemical potential

If then Cu will dissolve (a)

Solution near Cu metal becomes positively charged

Double layer near metal surface is created

potential difference across metal/solution interface will prevent

further dissolution

the same is true when (b)

Both cases are represented by the figure

)(2

)()( maqseCuCu

)(2

)()( maqseCuCu

(a) (b)

With potential difference at metal/solution interface the

free energy will be

where i(I)and I(II) are the electrical potential in the interior of phases (I) and (II)

ziFI the electrical work needed to bring one mole of ions with z charge from infinity to the region of where the

potential I exists

I is called Galvani potential

)()()()( IIFzIIIFzI iiiiii

is the electrochemical potential and defined as:

For an electrochemical equilibrium then

For the reaction we write:

-ve sign because electrons have negative charge

iii

o

iiii FzaRTFz ln~

0~

i

ii

)(

2

)(

0

)( 2 maqs eCuCu

ee

o

eCuCu

o

CuCuCuCuFaRTFaRTFaRT 2ln2ln0ln 222

o

i

~

Potential on LHS for metal i.e. M Potential on RHS for solution i.e. solActivities of metal atoms and its electrons are constant

The difference between two potentials across interface is

and called Galvani potential solM

By rearrangement

This equation is for one interface

o is Galvani standard potential

how much changes if activity increased 10 times?

Cu

Cuo

Cu

Cu

o

Cu

o

e

o

Cu

a

a

F

RT

a

a

F

RT

F

2

22

ln2

ln22

2

question

If two different metal/solution interfaces are connected, how the

potential difference is expressed?