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Introduction to electrochemistry - Basics of all techniques -

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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 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. Reactions and electrodes

6 Three-electrode cell and notation for the different electrodes

7 ● 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/H 2 (a=1)/H + (a=1,aqueous) ● By far the most common reference is the saturated calomel electrode (SCE), which is Hg/Hg 2 Cl 2 /KCl(sat’d in water) Its potential is 0.242 V vs. NHE. Reference electrode

8 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 Introduction Scope of electrochemistry

9 In general, the electrode reaction rate is governed by rates of processes such as: (1)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. Factors affecting electrode reaction rate

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 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. Faradaic and nonfaradaic processes

12 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, q M, and a charge in the solution, q s. At all times, q M =-q s. At a given potential the electrode-solution interface is characterized by a double-layer capacitance, C d, typically in the range of 10 to 40μF/cm 2. Capacitance and charge of an electrode


14 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 The nature of electrode reactions

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 E 0 : standard electrode potential c i : concentration ( a i has to be applied if activity coefficient is not 1) E = E 0 – (RT/nF)  i ln c i

16 Thermodynamics and kinetics ●The concentration of species at the electrode interface depends on its mass transport coefficient k d and ●The rate of the electrode reaction is expressed by the standard rate constant k 0 which is the rate when E = E 0 –reversible reaction  k 0 >> k d –irreversible reversible reaction  k 0 << k d, an overpotential  has to be applied additionally to overcome this kinetic barrier ●A behaviour in between these extremes is called quasireversible reaction

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