1. Chapter 11 Electrochemistry

2. Electromotive Force ( 電動勢 ) 1 joule of work is produced or required when 1 coulomb of charge is transferred between two points in the circuit that differ by a potential of 1 volt.

3. Galvanic Cells (Voltaic Cells ) Galvanic cell - an electric cell that generates an electromotive force by an irreversible conversion of chemical to electrical energy; cannot be recharged. The electron flow from the anode to the cathode is what creates electricity. In a galvanic cell, the cathode is positive while the anode is negative, while in a electrolytic cell, the cathode is negative while the anode is positive.

4. Galvanic Cells

7. Standard Reduction Potentials

8. Standard Hydrogen Electrode 2H + (aq)+Zn(s) →Zn +2 (aq)+H 2 (g) Oxidation half-reaction Zn(s) →Zn +2 (aq)+2e - Standard hydrogen electrode 2H + (aq)+2e - →H 2 (g) The cathode consists of a platinum electrode in contact with 1 M H + ions and bath by hydrogen gas at 1 atm. We assign the reaction having a potential of exactly 0 volts.

9. Copper-Zinc Voltaic Cells

10. The Cell Potentials E 0 cell =E 0 (cathode)-E 0 (anode) The value of E 0 is not changed when a half reaction is multiplied by an integer. 2Fe +3 +2e - →2Fe +2 E 0 (cathode)=0.77 V Cu →Cu +2 +2e - - E 0 (anode)=-0.34 V Cu+ 2Fe +3 →2Fe +2 +Cu + E 0 cell =E 0 (cathode)-E 0 (anode)=0.43 V oxidationreduction

11. Cell Diagrams For a copper-zinc voltaic cells Cu’ Zn ZnSO 4 (aq) CuSO 4 (aq) Cu 1. A vertical line indicates a phase boundary. 2. A dashed vertical line indicates the phase boundary between two miscible liquid. Dashed line

12. Pt L | H 2 (g)|HCl(aq)|AgCl(s)|Ag| Pt R Anode: H 2 (g)=2H + +2e - (Pt L ) Cathode: [AgCl(s)+e - (Pt R )=Ag+Cl - ] ×2 Overall: 2AgCl(s)+ H 2 (g)=2Ag+ 2H + +2Cl - Cu L |Cd(s)|CdCl 2 (0.1M)|AgCl(s)|Ag(s) |Cu R Anode: Cd=Cd +2 +2e - Cathode: [AgCl+e - =Ag + +Cl - ] ×2 Overall: Cd+2AgCl=2Ag+Cd +2 +2Cl -

13. Nernst Equation E 0 : standard reduction potential n: moles of electrons F: Faraday constant 96485 C/mol

14. Thermodynamic-Free Energy The maximum cell potential is directly related to the free energy difference between the reactants and the products in the cell.

15. Calculation of Equilibrium Constants for Redox Reactions

16. Reaction Quotient ( Q ) The positive E (  R >  L ) means that Q<K 0. As Q increases toward K 0, the cell emf decreases, reaching zero when Q=K 0

17. The Equilibrium Constant of a Cu-Zn Cell Zn+Cu +2 (aq)=Zn +2 (aq)+Cu E 0 =0.34V-(-0.76V)=1.10V

18. Concentration Cells

19. Ag L Ag + (0.1M) Ag + (1M) Ag R Pt L Cl 2 (P L ) HCl(aq) Cl 2 (P R ) Pt R

20. Liquid Junction Potential Liquid junction: the interface between two miscible electrolyte solutions. Liquid-junction potential: A potential difference between two solutions of different compositions separated by a membrane type separator. The salt will diffuse from the higher concentration side to the lower concentration side.

21. HCl H+H+ Cl - a 2 < a 1 ＋＋＋＋＋＋＋＋＋＋＋＋＋＋＋＋＋＋ －－－－－－－－－－－－－－－－－－－－ H+H+ Ag + ＋＋＋＋＋＋＋＋＋＋＋＋＋＋＋＋＋＋ －－－－－－－－－－－－－－－－－－－－ AgNO 3 HNO 3 a 2 = a 1

22. Liquid Junction Potential The diffusion rate of the cation and the anion of the salt will very seldom be exactly the same. Assume the cations move faster; consequently, an excess positive charge will accumulate on the low concentration side, while an excess negative charge will accumulate on the high concentration side of the junction due to the slow moving anions. When the cell has a liquid junction, the observed cell emf includes the additional potential difference between the two electrolyte solutions.

23. How to Solve the Liquid Junction Potential Liquid junction potentials are generally small, but they certainly cannot be neglected in accurate work. By connecting the two electrolyte solutions with a salt bridge, the junction potential can be minimized. A salt bridge consist of a gel made by adding agar to a concentrated aqueous KCl solution.

24. A Cell Diagrams Containing a Salt Bridge For a copper-zinc voltaic cells Cu’ Zn ZnSO 4 (aq) CuSO 4 (aq) Cu A salt bridge is symbolized by two vertical dashed lines.

25. Estimate the Liquid Junction Potential from EMF Measurement Ag AgCl(s) LiCl(m) NaCl(m) AgCl(s) Ag m(LiCl)=m(NaCl), E 0 =0 Anode: Ag+Cl - (in LiCl(aq))=AgCl+e - Cathode: AgCl+e - =Ag+Cl - (in NaCl(aq))

26. Estimate the Liquid Junction Potential from EMF Measurement

27. Applications of Electrochemistry pH meter ATP Synthase Potential for a resting nerve cell

28. Determination of pH Pt H 2 (g) soln. X KCl(sat.) Hg 2 Cl 2 (s) Hg Pt’ 1/2H 2 (g)+1/2Hg 2 Cl 2 =Hg(l)+H + (aq,X)+Cl - (aq) The cell reaction and emf Ex: Junction potential between X and the saturated KCl solution

29. If a second cell is set up to except that solution X is replaced by solution S, the emf E s of this cell will be:

31. Reference electrode: saturated calomel electrode (SCE) Sensing electrode: Ion Selective Electrode (ISE) Pt Ag AgCl (s) HCl (aq) glass soln. X KCl (sat.) Hg 2 Cl 2(s) Hg Pt’

32. Determine the pH of a Solution by a pH Meter When the glass electrode is immersed in solution X, an equilibrium between H + ions in solution and H + ions in the glass surface is set up. This charge transfer between glass and solution produces a potential difference between the glass and solution.

33. Ion Selective Electrodes (ISE) for PH Meter An ion selective electrode contains a glass, crystalline, or liquid membrane whose nature is such that the potential difference between the membrane and an electrolyte solution it is in contact with is determined by the activity of one particular ion. It is dependent on the concentration of an ionic species in the test solution and is used for electro-analysis.

34. Marcus theory for Electron transfer reactions Rudolph A. Marcus was awarded the 1992 Nobel Prize in chemistry

36. Membrane Equilibrium In a closed electrochemical system, the phase equilibrium condition for two phases  and 

37. Free-energy change during proton movement across a concentration gradient The movement of protons from the cytoplasm into the matrix of the mitochondrion.

38. Proton Pumping Proton pumping maintains a pH gradient of 1.4 units, then  pH = + 1.4  G = -2.303RTΔpH =- 2.303 (8.315 × 10 -3 kJ/mol)(298K)(1.4) = - 7.99 kJ/mol Proton concentration gradient

39. Free-energy change during solute movement across a voltage gradient In mitochondria, electron transport drives proton pumping from the matrix into the intermembrane space. There is no compensating movement of other charged ions, so pumping creates both a concentration gradient and a voltage gradient. This voltage component makes the proton gradient an even more powerful energy source.

40. Membrane Potential  m =  in –  out =0.14 V  G =-nF  m =-(1)(96485)(0.14 ) = - 13.5 kJ/mol

41. Proton-motive force Proton-motive force (  P) is a  that combines the concentration and voltage effects of a proton gradient.  G=-nF  P = - 2.303 RT  pH + nF  m =(-7.99 kJ/mol)+( - 13.5 kJ/mol) = -21.5 kJ/mol

42. ATP synthesis Mitochondrial proton gradient as a source of energy for ATP synthesis Estimated consumption of the proton gradient by ATP synthesis is about 3 moles protons per mole ATP.  G = 50 kJ/mol for ATP synthesis  G = 50 + 3(- 21.5) = - 14.5 kJ/mol The synthesis of ATP is spontaneous under mitochondrial conditions.

43. Potential for a resting nerve cell Goldman-Hodgkin-Katz equation P: permeability ( 穿透率 ) D: diffusion coefficient ( 擴散係數 )  : thickness of membrane ( 薄膜厚度 )

44. Resting Nerve Cell of a Squid Concentrations cell △  (K+) =-95 mV △  (Na+) =+57 mV △  (Cl-) =-67 mV (mmol/dm 3 ) K+K+ Na + Cl - int4104940 ext10460540 P(K+) /P(Cl-)=2 P(K+)/P(Na+)=25 The observed potential for a resting squid nerve cell is about -70 mV at 25 o C.

45. Resting Nerve Cell of a Squid The observed potential for a resting squid nerve cell is about -70 mV at 25 o C. Hence Cl - is in electrochemical equilibrium, but K + and Na + are not. Na + continuously flows spontaneously into the cell and K + flows spontaneously out. Na + -K + pump

46. Batteries Secondary batteries: Voltaic cells whose electrochemical reactions can be reversed by a current of electrons running through the battery after the discharge of an electrical current. A secondary battery can be restored to nearly the same voltage after a power discharge.

48. Lead Storage battery Anode reaction Pb+HSO 4 - → PbSO 4 +H + +2e- Cathode reaction PbO 2 +HSO 4 - +3H + +2e - → PbSO 4 +2H 2 O Cell reaction Pb+PbO 2 + 2H + +2HSO 4 - → 2PbSO 4 +2H 2 O

50. Dry Cell Battery Anode reaction Zn→ Zn 2+ +2e - Cathode reaction 2NH 4+ +2MnO 2 +2e - → Mn 2 O 3 +2NH 3 +H 2 O Cell reaction 2MnO 2 +2NH 4 Cl+Zn→ Zn(NH 3 ) 2 Cl 2 + Mn 2 O 3 +H 2 O

51. Alkaline Dry Cell Anode reaction Zn+2OH - → ZnO+H 2 O+2e - Cathode reaction MnO 2 +2H 2 O+2e - → Mn 2 O 3 +2OH - Cell reaction MnO 2 +H 2 O+Zn→ Mn 2 O 3 +ZnO

52. 燃料電池工作原理 Anode reaction 2H 2 +4OH - →4H 2 O+4e- Cathode reaction 4e - +O 2 +2H 2 O →4OH - Cell reaction 2H 2 +O 2 → 2H 2 O

53. 質子交換膜燃料電池 Polymer Electrolyte Membrane Fuel Cell (PEMFC) Proton Exchange Membrane Fuel cell (PEFC)

55. Corrosion of Iron

56. Anodic Region Fe→Fe +2 +2e - Cathodic Region O 2 +2H 2 O+4e - →4OH - Overall Reaction 4Fe +2 (aq)+O 2 (g)+(4+2n) H 2 O(l) →2Fe 2 O 3 ‧ nH 2 O(s)+8H + (aq)

57. Electrolysis Electrolytic Cell: use electrical energy to produce chemical change The process of electrolysis involves forcing a current through a cell to produce a chemical change for which the cell potential is negative.

58. standard galvanic cell standard electrolytic cell Zn+Cu +2 →Zn +2 +Cu Zn +2 +Cu→Zn+Cu +2

59. Electrolysis of Water Anode reaction: 2H 2 O→O 2 +4H + +4e - Cathode reaction: 4H 2 O+4e - →2H 2 +4OH - 2H 2 O →2H 2 +O 2 E 0 =-2.06V

60. Electrolysis of Mixture of Ions A solution in an electrolytic cell contains the ions Cu +2, Ag + and Zn +2. The more positive the E 0 value, the more the reaction has a tendency to proceed in the direction indicated. Ag + > Cu +2 > Zn +2

61. Electrolysis of NaCl/H 2 O System Anode reaction: 2H 2 O→O 2 +4H + +4e - -E 0 =-1.23 V 2Cl - →Cl 2 +2e - -E 0 =-1.36 V The Cl - ion is first to be oxidized. A much higher potential than expected is required to oxidized water. The voltage required in excess of the excepted (overvoltage) is much greater for the production of O 2 than for Cl 2.

63. electroplating

64. Electrolysis of NaCl