《电分析化学》（英文版）Lecture 4 The metal electrode

Electroanalytical Chemistry Lecture 4 Why Electrons Transfer

Electroanalytical Chemistry Lecture #4 Why Electrons Transfer?

The metal electrode Ef= Fermi level highest occupied electronIc energy level in a metal E

The Metal Electrode EF E Ef = Fermi level; highest occupied electronic energy level in a metal

Why Electrons Transfer Reduction Oxidation F E redox E E redox E F .Net flow of electrons from m.Net flow of electrons from to solute solute to m Ef more negative than e E redox f more positive than ered emore cathodic emore anodic g emore reducin more oxidizing

Why Electrons Transfer EF Eredox E F 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 inetics of electron Transfer Consider 0+ne=R Assume o andr are stable soluble Electrode of 3rd kind (i.e inert no competing chemical reactions occur

The Kinetics of Electron Transfer Consider: O + ne- = R Assume: O and R are stable, soluble Electrode of 3rd kind (i.e., inert) no competing chemical reactions occur kR ko

Equilibrium for this Reaction is Characterised byu The nernst equation Ecell Eo-(rt/nF)In(Cr co) where R=[R] in bulk solution Co *=OJ in bulk solution So, Ecell is related directly to [o] and [r]

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 no net current flows i e △E=0→i=0 However, there will be a dynamic equilibrium at electrode surface 0+e=R R-ne=o both processes will occur at equal rates so no net change in solution composition

Equilibrium (cont’d) At equilibrium, no net current flows, i.e., E = 0  i = 0 However, there will be a dynamic equilibrium at electrode surface: O + ne- = R R - ne- = O both processes will occur at equal rates so no net change in solution composition

Current Density Since i is dependent on area of electrode We normalize currents and examine I=j/A we call this current density So at equilibrium i=0=in H/A=-ICA= IA Which we call the exchange current density Note: by convention ia produces positive current

Current Density, I Since i is dependent on area of electrode, we “normalize currents and examine I = i/A we call this current density So at equilibrium, I = 0 = iA + iC  ia /A = -i c /A = IA = -Ic = Io which 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 ho large much simultaneous ox/red electron transfer(ET → inherently fast E「( kinetics) Io small little simultaneous ox/red electron transfer(E o sluggish ET reaction( kinetics)

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 (kinetics) Io small  little simultaneous ox/red electron transfer (ET)  sluggish ET reaction (kinetics)

Summary: Equilibrium Position of equilibrium characterized electrochemically by 2 parameters egbm-equilibrium potential, Eo o exchange current density

Summary: Equilibrium Position of equilibrium characterized electrochemically by 2 parameters: Eeqbm - equilibrium potential, Eo Io - exchange current density

How Does I vary with E? Lets consider case 1: at equilibrium case 2: at E more negative than Eeabm case 3: at E more positive than Eeabm

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