lecture 1.pptx
- 1. Electrochemistry (2) Electrochemical Cell Three Electrodes Electrochemical Cell • It consists of three electrodes immersed in a conductive electrolyte. • i. Working electrode. An Electrode which occurs on its surface reduction or oxidation. • ii. Reference electrode. An electrode with a constant electrochemical potential. • iii. A counter electrode. A current-carrying electrode that completes the cell circuit.
- 2. • WE RE • CE •
- 5. • A potentiostat is an electronic instrument that measures and controls the voltage difference between a working electrode and a reference electrode. It measures the current flow between the working and counter electrodes. • This instrument is fundamental to modern electrochemical studies for investigations of reaction mechanisms related to redox chemistry and other chemical phenomena. Potentiostats control most electroanalytical experiments. They can be used to test for electrochemically active compounds and microbes in solution, and thus have applications in many areas, such as: • Environmental monitoring • Drug testing • Identifying contaminants/toxins in water • Identifying toxins/ingredients in food
- 6. • Electrode reaction An electrode reaction is a heterogeneous chemical process involving the transfer of electrons to and from a surface of the electrode. The electrode reaction may be an anodic process whereby a species is oxidized by the loss of electrons to the electrode or cathodic process whereby a species is reduced by the gain of electrons from the electrode .
- 7. Reduction and Oxidation at Electrode Surface • By driving the electrode to more negative potentials the energy of the electron is raised to a level high enough to occupy vacant states on species in the electrolyte. In this case a flow of electrons from electrode to solution( a reduction current) occurs (figure 2a). • Imposing a more positive potential, electrons on solutes in the electrolyte will find a more favorable energy on the electrode and will transfer there , their flow from solution to electrode, is an oxidation current (Fig. 2b).
- 10. Figure 2. representation of (a) reduction and (b) oxidation process of species A in solution. The molecular orbitals (M.O) of species A shown are the highest occupied MO and the lowest vacant M.O. As shown , these correspond in an approximate way to the Eo𝐬 of the A/A- and A+/A couples, respectively. The illustrated system could represent an aromatic hydrocarbons (e.g. 9,10-dipheny- lanthracene) in an aprotic solvent (e.g acetonitrile) at a platinum electrode.
- 11. • Faradic and non faradic processes: there are two types of processes occur at electrodes. The first involves the transfer of charges e.g. electrons across the metal-solution interface , this electron transfer process causes oxidation or reduction to occur. Since these reactions are governed by faraday’s law ,i.e. the amount of chemical reaction caused by the current is proportional to the amount of electricity passed , they are called faradaic processes.
- 12. Under some conditions at a given electrode solution interface and within a range of potentials the second process is non faradaic process i.e. no charge transfer reactions because such reactions are thermodyna- mically or kinetically unfavorable Polarization and over potential The departure of the electrode potential (or cell potential) from the reversible value (Nernstain or equilibrium) upon passage of faradaic current is called polarization. The larger this departure is, the larger the extent of polarization. We have seen that an ideal polarized electrode showed a very large change in potential upon the passage of small current .; thus ideal polarizability is characterized by a horizontal region of an I versus E curve (fig. 10a).
- 13. • An ideal non polarizable electrode whose potential doesn’t change upon passage of current is an electrode of fixed potential. • The reason for this behavior is that the electrode reaction is extremely fast (has an almost infinite exchange current density). Non polarizability is characterized by vertical region on I vs E curve (fig 10b). An SCE would approach ideal non-polarizability at small current. The extent of polarization is measured by the over potential ,η,which is the deviation of potential from the equilibrium value: η= E-Eeq
- 15. • The key difference between polarizable and non polarizable electrode is that polarizable electrodes have a charge separation at the electrode- electrolyte boundary whereas non-polarizable electrodes have no charge separation at this electrode-electrolyte boundary. • E • EE • EC • CE • ECE • ECEC
- 17. Nature of electrode reaction The simplest electrode reaction is one which interconverts at an inert surface , two species , O and R, which are completely stable and soluble in the electrolysis medium containing an excess of an electrolyte which is electro- inactive. 1.Simple ET O + ne- ↔ R In an experimental condition where O is reduced to R , the electrode reaction must have three steps to maintain a current:
- 19. 2. i. Adsorption Adsorption is the formation of some type of bond between the adsorbate and electrode surface. The interaction may be electrostatic (e.g. the adsorption of cations or anions on a surface of opposite charge) or due to the formation of covalent bond. Electrode reactions are most strongly affected when the adsorbate is the electroactive species, a reaction intermediate , or the product, but the adsorption of species apparently not directly in- volved in the electrode process can also change the rate of electron transfer and the final product.
- 20. 2.ii. Phase formation The electrode reaction may involve the formation of new phase or the transformation of one solid phase to another e.g. , Co2+ + 2e ===== Co PbO2 + 4H++SO4 -2 +2e- → PbSO4 + 2H2O
- 21. • 3. EE Reaction (Multiple Electron Transfer) • Cu2+ + 2e ===== Cu • 4. EC Reaction • p iodonitrobenzen +e - radical - -I - nitrobenzene • 5. CE reaction CH3COOH = ==== CH3COO- + H+ 2H+ + 2e – H2
- 22. Variables affecting the rate of an electrode reaction i. Electrode variables (material, surface area, geometry, surface condition (roughness)). ii. Mass transfer variables. -Mode (diffusion, convection, migration) -Surface concentrations -adsorption iii. Solution variables Bulk concentration of electroactive species (CO, CR) pH, solvent
- 23. • Iv. External variables • -Temperature (T) • - Pressure (P) • - Time (t) v. Electrical variable - Potential (E) - Current (i) - Quantity of electricity (Q)
- 24. Modes of mass transport • Three modes of mass transport • Diffusion • - Spontaneous movement as a result of concentration gradient • - Movement from regions of high concentration to regions of low concentration
- 25. Convection • Transport to the electrode by gross physical movement • e.g external mechanical energy, solution stirring or flowing and electrode rotation or vibration. • • Migration • - Movement of charged particles along an electric field - Charge is carried through the solution as a result of movement of ions
- 26. The Flux (J) • - The number of molecules penetrating a unit area of an imaginary plane in a unit time • - The measure of the rate of mass transport at a fixed point • Units: mol/cm2∙s
- 27. • Relationship between Faradiac current “i’ and Electrolysis rate (dN/dt) • i (amperes) = dq/dt (coul.s-1) • N (moles electrolysed) = q(coulombs)/(nF(coul.mol-1) • Electrolysis rate = dN/dt = (1/nF).(dq/dt) • or dN/dt = i/nF • Since electrode reactions are heterogeneous, there rates are usually described in units of • mol.s-1.cm-2 = dN/dt = i/nFA = J/nF
- 28. CO(x=0) iL,C - iC --------- = ----------- (i) C*(x=0) iL,C iL,C C*(x=0) = ----------- (ii) nFAm0 iL,C - iC CO(x=0) = ----------- (iii) nFAm0