Electroanalytical Chemistry------Lecture_5.ppt

Electroanalytical Chemistry
Lecture #4
Why Electrons Transfer?
The Metal Electrode
EF
E
Ef = Fermi level; highest occupied electronic energy level in a metal
Why Electrons Transfer
EF
Eredox
EF
Eredox
Net flow of electrons from M to solute
Ef more negative than Eredox
more cathodic
more reducing
Reduction
Oxidation
Net flow of electrons from solute to M
Ef more positive than Eredox
more anodic
more oxidizing
E
E
The ics of Electron Transfer
Consider:O + ne- = R
Assume:
O and R are stable, soluble
Electrode of 3rd kind (., inert)
peting chemical reactions occur
kR
ko
Equilibrium for this Reaction is Characterised by...
The Nernst equation:Ecell = E0 - (RT/nF) ln (cR*/co*)
where: cR* = [R] in bulk solution co* = [O] in bulk solution
So, Ecell is related directly to [O] and [R]
Equilibrium (cont’d)
At equilibrium, current flows, .,E = 0  i = 0
However, there will be a dynamic equilibrium at electrode surface:O + ne- = RR - ne- = Oboth processes will occur at equal ratesso change in position
Current Density, I
Since i is dependent on area of electrode, we “normalize currents and examineI = i/A we call this current density
So at equilibrium, I = 0 = iA + iC ia/A = -ic/A = IA = -Ic = Iowhich we call the exchange current density
Note: by convention iA produces positive current
Exchange Current Density
Significance?
Quantitative measure of amount of electron transfer activity at equilibrium
Io large  much simultaneous ox/red electron transfer (ET)  inherently fast ET (ics)
Io small  little simultaneous ox/red electron transfer (ET)  sluggish ET reaction (ics)
Summary: Equilibrium
Position of equilibrium characterized electrochemically by 2 parameters:
Eeqbm - equilibrium potential, Eo
Io - exchange current density
How Does I vary with E?
Let’s consider:
case 1: at equilibrium
case 2: at E more negative than Eeqbm
case 3: at E more positive than Eeqbm