Presentation is loading. Please wait.

Presentation is loading. Please wait.

Introduction to electrochemistry - Basics of all techniques -

Similar presentations

Presentation on theme: "Introduction to electrochemistry - Basics of all techniques -"— Presentation transcript:

1 Introduction to electrochemistry - Basics of all techniques -

2 Electrochemistry is the study of phenomena at electrode-solution interfaces

3 (a) Galvanic and (b) electrolytic cells

4 Two quite different aspects of the field of electrochemistry

5 Reactions and electrodes
The overall chemical reaction taking place in a cell is made up of two independent half-reactions, which describe the real chemical changes at the two electrodes. Most of the time one is interested in only one of these reactions, and the electrode at which it occurs is called the working (or indicator) electrode, coupled with an electrode that approaches an ideal nonpolarizable electrode of known potential, called the reference electrode. In experiments, the current is passed between the working electrode and an auxiliary(or counter) electrode. Three electrodes are frequently placed in three compartments separated by a sintered-glass disk.

6 Three-electrode cell and notation for the different electrodes

7 Reference electrode ● The potential of the working electrode is monitored relative to a separate reference electrode, positioned with its tip near the working electrode. ● The internationally accepted primary reference is the standard hydrogen electrode (SHE) or normal hydrogen electrode (NHE), which is Pt/H2(a=1)/H+(a=1,aqueous) ●By far the most common reference is the saturated calomel electrode (SCE), which is Hg/Hg2Cl2/KCl(sat’d in water) Its potential is V vs. NHE.

8 Scope of electrochemistry
Introduction Introduction Investigation of chemical phenomena associated with a charge transfer reaction To assure electroneutrality two (or more) half-reactions take place in opposite directions (oxidation/reduction) If the sum of free energy changes at both electrodes is negative electrical energy is released  battery If it is positive, external electrical energy has to be supplied to oblige electrode reactions  electrolysis

9 Factors affecting electrode reaction rate
In general, the electrode reaction rate is governed by rates of processes such as: Mass transfer (e.g., from the bulk solution to the electrode surface). (2) Electron transfer at the electrode surface. (3)Chemical reactions preceding or following the electron transfer. (4)Other surface reactions. ◆ The magnitude of this current is often limited by the inherent sluggishness of one or more reactions called rate-determining steps.

10 Conditions for electrochemical experiments
Reproducible experimental conditions must be given Interfering side effects must be avoided as Migration effects High solution resistance -these effects can be minimised by adding an inert supporting electrolyte (around 1 mol/L) Undefined or large diffusion layer A complete study of the electrode process requires the measurement of kinetic as well as thermodynamic parameters.

11 Faradaic and nonfaradaic processes
Charges (e.g., electrons) are transferred across the electrode-solution interface and causes oxidation or reduction to occur. Since these reactions are governed by Faraday’s law, they are called faradaic processes. Under some conditions, processes such as adsorption and desorption can occur, and the structure of the electrode-solution interface can change with changing potential or solution composition, these processes are called nonfaradaic processes.

12 Capacitance and charge of an electrode
The behavior of the electrode-solution interface is analogous to that of a capacitor. When a potential is applied across a capacitor, charge will accumulate on its electrode plates. At a given potential there will exist a charge on the metal electrode, qM, and a charge in the solution, qs. At all times, qM=-qs. At a given potential the electrode-solution interface is characterized by a double-layer capacitance, Cd, typically in the range of 10 to 40μF/cm2.


14 The nature of electrode reactions
Electrode reactions are heterogeneous and take place in the interfacial region between electrode and solution  diffusion layer The charge separation at each electrode is represented by a capacitance the difficulty of charge transfer by a resistance The electrode can act as (1) a source of electrons (cathode)  reduction ,(2) a sink of electrons transferred from species in solution (anode)  oxidation The amount of electrons transferred is related to the current flowing between the two electrodes

15 Thermodynamics and kinetics
The potential at which a reduction or oxidation takes place (measured relative to the normal hydrogen electrode) is given by the Nernst equation i : stoichiometric numbers: positive for reduced species, negative for oxidised species E0 : standard electrode potential ci : concentration (ai has to be applied if activity coefficient is not 1) E = E0 – (RT/nF) i ln ci

16 Thermodynamics and kinetics
●The concentration of species at the electrode interface depends on its mass transport coefficient kd and ●The rate of the electrode reaction is expressed by the standard rate constant k0 which is the rate when E = E0 reversible reaction  k0 >> kd irreversible reversible reaction  k0 << kd, an overpotential  has to be applied additionally to overcome this kinetic barrier ●A behaviour in between these extremes is called quasireversible reaction

Download ppt "Introduction to electrochemistry - Basics of all techniques -"

Similar presentations

Ads by Google