Dr ADK MSc Electrochemistry Elective Unit II A .pptx

2020 – 2021/2023-2024 REGULATION �MCH 34 A / DCH 34A/ 23PECH14 A
UNIT-I:
ELECTRODE – ELECTROLYTE INTERFACE
Electrochemistry is the study of the relations between
chemical reactions and electricity

- Electrochemical processes involve the transfer of electrons from one substance to another

1. ELECTRO CHEMISTRY Dr.A.DINESH KARTHIK ASSOCIATE PROFESSOR &HEAD, RESEARCH SUPERVISOR, P G & RESEARCH DEPT. OF CHEMISTRY SHANMUGA INDUSTRIES ARTS & SCIENCE COLLEGE, TIRUVANNAMALAI-606603 . CELL : 9486887461 [email protected] .

2. UNIT-I: ELECTRODE – ELECTROLYTE INTERFACE PART A 2020 – 2021/ 2023-2024 REGULATION MCH 34 A / DCH 34A/ 23PECH14 A Dr.A.DINESH KARTHIK ASSOCIATE PROFESSOR &HEAD, RESEARCH SUPERVISOR, P G & RESEARCH DEPT. OF CHEMISTRY SHANMUGA INDUSTRIES ARTS & SCIENCE COLLEGE, TIRUVANNAMALAI-606603 . CELL : 9486887461 [email protected] .

3. Dr.A.DINESH KARTHIK

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5. - Electrochemistry is the study of the relations between chemical reactions and electricity - Electrochemical processes involve the transfer of electrons from one substance to another ELECTROCHEMISTRY Dr.A.DINESH KARTHIK

6. Electrochemistry Electrochemistry - Field of Chemistry dealing with transfer of electrons from one species to another. E.g. Zn in CuSO 4 ( aq ). Electrochemical cell - combination of two half reactions to produce electricity from reaction. E.g. Danielle cell: Zn and Cu electrodes in salts of these ions. Dr.A.DINESH KARTHIK

7. - Deals with the relationship between electricity and chemistry - Deals with the measurement of electrical quantities (current, potential, charge) and their relationship to chemical parameters Applications Fabrication of flow detectors Quality control Environmental monitoring Electronics Electrochemical sensors ELECTROANALYTICAL TECHNIQUE Dr.A.DINESH KARTHIK

8. Reducing Agent Oxidizing Agent e - e - e - e - e - e - Anode Cathode Dr.A.DINESH KARTHIK

9. REACTIONS CONTROLLED BY RATE OF ELECTRON TRANSFER Tafel Plot (plot of log(i) against η ) η (mV) log(i/i o ) Anodic branch Cathodic branch -100 200 100 -200 -1 -2 1 2 Dr.A.DINESH KARTHIK

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11. Structure of electrified interface 1. The electrical double layer 2. The Gibbs adsorption isotherm 3. Electrocapillary equation 4. Electrosorption phenomena 5. Electrical model of the interface Dr.A.DINESH KARTHIK

12. Gibbs adsorption isotherm a s b Definitions G – total Gibbs function of the system G a, G b, G s - Gibbs functions of phases a,b,s Gibbs function of the surface phase s: G s = G – { G a + G b } Dr.A.DINESH KARTHIK

13. Gibbs Model of the interface Dr.A.DINESH KARTHIK

14. The amount of species j in the surface phase: n j s = n j – { n j a + n j b } Gibbs surface excess G j G j = n j s /A A – surface area Dr.A.DINESH KARTHIK

15. Gibbs adsorption isotherm Change in G brought about by changes in T,p, A and n j dG=-SdT + Vdp + g dA + Sm j dn j – surface energy – work needed to create a unit area by cleavage - chemical potential dG a =-S a dT + V a dp + + Sm j dn j a dG b =-S b dT + V b dp + + Sm j dn j b and dG s = dG – {dG a + dG b }= S s dT + g dA + + Sm j dn j s Dr.A.DINESH KARTHIK

16. cations Mobility anions V  cm -1 F : Faraday’s constant = eN A molar conductivity: Transference numbers Ion transport number : Dr.A.DINESH KARTHIK

17. Strong electrolytes: Kohlrausch’s law: limiting molar conductivity; at infinite dilution Law of the independent migration of ions: low concentration limiting molar conductivity number Dr.A.DINESH KARTHIK

18. so-called weak electrolytes (incomplete dissociation) a salt CA ( C cation , A anion) in a diluted electrolyte a : activity c : concentration dissociation equilibrium constant Total concentration of salt: c ; degree of dissociation:  . Ostwald’s dilution law Plot 1/  m vs c  m    and K C . Dr.A.DINESH KARTHIK

19. Strong electrolyte weak electrolyte A.J. Bard and L.R. Faulkner , “ Electrochemical Methods : Fundamentals and Applications ,” Wiley , New York, 1980, 64-67. Strong electrolyte: Weak electrolyte: C   slightly down Ion interaction  Ion velocity  C   down quickly Ionization degree  Dr.A.DINESH KARTHIK

20. Consider the electrolyte dissociation Equilibrium Mean ionic activity: dissociation equilibrium Dr.A.DINESH KARTHIK

21. Model for the activities of dilute electrolyte solutions Undissociated molecule is neutral: c i : the concentration of the i th ion How about local? local charge density. (kinetic theory of gases): Probability  exp (-Energy/ k B T ) Energy = ez i  ( r )  ( r ): electrical potential around one of the ions in the solution local charge density Positive ion at center   ( r ) > 0; negative ion z i < 0  Net excess negative charge  screening Dr.A.DINESH KARTHIK

22. + + + + + + + + + + + +            +   For = 0 I : ionic strength is a measure of the concentration of ions in that solution If unit of I : mol L -1 ; mol dm-3; mol kg -1 will be used, You have to time N A . In the above derivation, [ c i ] # L -1 ; # dm -3 ; # kg -1 Dr.A.DINESH KARTHIK

23. How much charge will be induced by having an ion (with charge Q ) at the origin ? Gauss’s law: the induced charge balance the charge at the origin; i.e., local electro-neutrality Polar coordinate: spherical symmetry Without screening , the potential is simply Coulomb’s law  : electric permittivity of the solvent With screening , the potential is account for screening  : Debye screening wavevector ;  D = 1/  : Debye screening length Next Dr.A.DINESH KARTHIK

24. Derivation of the Gibbs adsorption isotherm dG s = -S s dT + g dA + + Sm j dn j s Integrate this expression at costant T and p G s = A g + Sm j n j s Differentiate G s dG s = Ad g + g dA + S n j s d m j + Sm j dn j s The first and the last equations are valid if: Ad g + S n j s d m j = 0 or d g = - G j d m j Dr.A.DINESH KARTHIK

25. Gibbs model of the interface - Summary Dr.A.DINESH KARTHIK

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27. The electrocapillary equation Cu’ Ag AgCl KCl, H 2 O,L Hg Cu’’ Dr.A.DINESH KARTHIK

28. s M = F( G H g - G e ) + Dr.A.DINESH KARTHIK

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30. Lippmann equation Dr.A.DINESH KARTHIK

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34. Differential capacity of the interface Dr.A.DINESH KARTHIK

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37. Capacity of the diffuse layer Thickness of the diffuse layer Dr.A.DINESH KARTHIK

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40. 2.4 Electrosorption phenomena Dr.A.DINESH KARTHIK

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42. 2.5 Electrical properties of the interface In the most simple case – ideally polarizable electrode the electrochemical cell can be represented by a simple RC circuit Dr.A.DINESH KARTHIK

43. Implication – electrochemical cell has a time constant that imposes restriction on investigations of fast electrode process Time needed for the potential across the interface to reach The applied value : E c - potential across the interface E - potential applied from an external generator Dr.A.DINESH KARTHIK

44. Time constant of the cell t = R u C d Typical values R u =50 W; C=2 m F gives t =100 m s Dr.A.DINESH KARTHIK

45. Current flowing in the absence of a redox reaction – nonfaradaic current In the presence of a redox reaction – faradaic impedance is connected in parallel to the double layer capacitance. The scheme of the cell is: The overall current flowing through the cell is : i = i f + i nf Only the faradaic current –i f contains analytical or kinetic information Dr.A.DINESH KARTHIK

46. Dr.A.DINESH KARTHIK [email protected]