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Redox potentials

Redox potentials. Electrochemistry. Redox Reactions. Oxidation loss of electrons Reduction gain of electrons oxidizing agent substance that cause oxidation by being reduced reducing agent substance that cause oxidation by being reduced. Electrochemistry.

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Redox potentials

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  1. Redox potentials Electrochemistry ICS

  2. Redox Reactions Oxidation • loss of electrons Reduction • gain of electrons oxidizing agent • substance that cause oxidation by being reduced reducing agent • substance that cause oxidation by being reduced ICS

  3. Electrochemistry In the broadest sense, electrochemistry is the study of chemical reactions that produce electrical effects and of the chemical phenomena that are caused by the action of currents or voltages. ICS

  4. ICS

  5. Voltaic Cells • harnessed chemical reaction which produces an electric current ICS

  6. Voltaic Cells Cells and Cell Reactions Daniel's Cell Zn(s) + Cu+2(aq) ---> Zn+2(aq) + Cu(s) oxidation half reaction anode Zn(s) ---> Zn+2(aq) + 2 e- reduction half reaction cathode Cu+2(aq) + 2 e- ---> Cu(s) ICS

  7. Voltaic Cells • copper electrode dipped into a solution of copper(II) sulfate • zinc electrode dipped into a solution of zinc sulfate ICS

  8. Voltaic Cells ICS

  9. Hydrogen Electrode • consists of a platinum electrode covered with a fine powder of platinum around which H2(g) is bubbled. Its potential is defined as zero volts. Hydrogen Half-Cell H2(g) = 2 H+(aq) + 2 e- reversible reaction ICS

  10. ICS

  11. ICS

  12. Standard Reduction Potentials • the potential under standard conditions (25oC with all ions at 1 M concentrations and all gases at 1 atm pressure) of a half-reaction in which reduction is occurring ICS

  13. Some Standard Reduction Potentials Table 18-1, pg 837 Li+ + e- ---> Li -3.045 v Zn+2 + 2 e- ---> Zn -0.763v Fe+2 + 2 e- ---> Fe -0.44v 2 H+(aq) + 2 e- ---> H2(g) 0.00v Cu+2 + 2 e- ---> Cu +0.337v O2(g) + 4 H+(aq) + 4 e- ---> 2 H2O(l) +1.229v F2 + 2e- ---> 2 F- +2.87v ICS

  14. If the reduction of mercury (I) in a voltaic cell is desired, the half reaction is: Which of the following reactions could be used as the anode (oxidation)? A, B ICS

  15. Cell Potential • the potential difference, in volts, between the electrodes of an electrochemical cell • Direction of Oxidation-Reduction Reactions • positive value indicates a spontaneous reaction ICS

  16. Standard Cell Potential • the potential difference, in volts, between the electrodes of an electrochemical cell when the all concentrations of all solutes is 1 molar, all the partial pressures of any gases are 1 atm, and the temperature at 25oC ICS

  17. Cell Diagram • the shorthand representation of an electrochemical cell showing the two half-cells connected by a salt bridge or porous barrier, such as: Zn(s)/ZnSO4(aq)//CuSO4(aq)/Cu(s) anode cathode ICS

  18. Metal Displacement Reactions • solid of more reactive metals will displace ions of a less reactive metal from solution • relative reactivity based on potentials of half reactions • metals with very different potentials react most vigorously ICS

  19. Ag+ + e- --->Ag E°= 0.80 V Cu2+ + 2e- ---> Cu E°= 0.34 V Will Ag react with Cu2+? yes, no Will Cu react with Ag+? yes, no ICS

  20. Gibbs Free Energyand Cell Potential DG = - nFE where n => number of electrons changed F => Faraday’s constant E => cell potential ICS

  21. Applications of Electrochemical Cells Batteries • device that converts chemical energy into electricity Primary Cells • non-reversible electrochemical cell • non-rechargeable cell Secondary Cells • reversible electrochemical cell • rechargeable cell ICS

  22. Applications of Electrochemical Cells Batteries Primary Cells "dry" cell & alkaline cell 1.5 v/cell mercury cell 1.34 v/cell fuel cell 1.23v/cell Secondary Cells lead-acid (automobile battery) 2 v/cell NiCad 1.25 v/cell ICS

  23. “Dry” Cell ICS

  24. “Dry” Cell ICS

  25. “Flash Light” Batteries "Dry" Cell Zn(s) + 2 MnO2(s) + 2 NH4+ -----> Zn+2(aq) + 2 MnO(OH)(s) + 2 NH3 Alkaline Cell Zn(s) + 2 MnO2(s) ---> ZnO(s) + Mn2O3(s) ICS

  26. “New” Super Iron Battery Mfe(VI)O4 + 3/2 Zn 1/2 Fe(III)2O3 + 1/2 ZnO + MZnO2 (M = K2 or Ba) Environmentally friendlier than MnO2 containing batteries. ICS

  27. ICS

  28. Lead-Acid(Automobile Battery) ICS

  29. Lead-Acid(Automobile Battery) Pb(s) + PbO2(s) + 2 H2SO4 = 2 PbSO4(s) + 2 H2O 2 v/cell ICS

  30. Nickel-Cadmium (Ni-Cad) Cd(s) + 2 Ni(OH)3(s) = Cd(OH)2(s) + 2 Ni(OH)2(s) NiCad 1.25 v/cell ICS

  31. ICS

  32. Automobile Oxygen Sensor ICS

  33. Automobile Oxygen Sensor • see Oxygen Sensor Movie from Solid-State Resources CD-ROM ICS

  34. pH Meter pH = (Eglass electrode - constant)/0.0592 ICS

  35. Effect of Concentration on Cell Voltage: The Nernst Equation Ecell = Eocell - (RT/nF)ln Q Ecell = Eocell - (0.0592/n)log Q where Q => reaction quotient Q = [products]/[reactants] ICS

  36. EXAMPLE: What is the cell potential for the Daniel's cell when the [Zn+2] = 10 [Cu+2] ?Q = ([Zn+2]/[Cu+2] = (10 [Cu+2])/[Cu+2] = 10Eo = (0.34 V)Cu couple + (-(-0.76 V)Zn couplen = 2, 2 electron change Ecell = Eocell - (0.0257/n)ln Q thus Ecell = (1.10 - (0.0257/2)ln 10) V Ecell = (1.10 - (0.0257/2)2.303) V Ecell = (1.10 - 0.0296) V = 1.07 V ICS

  37. + + + + [H ] ® [H ] [h ] ® [h ] n-type side acid side base side p-type side + + [H RT – 2.3  RT ] [h ] – 2.3  RT base side n-type side Q o E = E – ln = log E (in volts) = log + + nF F [H ] F [h ] acid side p-type side Nernst Equation ICS

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