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Lecture 41 - Electrochemistry V

Lecture 41 - Electrochemistry V. Review. Galvanic Cells: Reaction is spontaneous E o cell > 0 The “product” is an electrical current Some can be reversed (recharged). Electrolytic Cells. Reaction is not spontaneous E o cell < 0 Used for producing a chemical product.

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Lecture 41 - Electrochemistry V

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  1. Lecture 41 - Electrochemistry V

  2. Review Galvanic Cells: Reaction is spontaneous Eocell > 0 The “product” is an electrical current Some can be reversed (recharged)

  3. Electrolytic Cells Reaction is not spontaneous Eocell < 0 Used for producing a chemical product

  4. Electrolysis of Water We want: 2 H2O(l) 2 H2(g) + O2(g) 2 H2O(l)  O2(g) + 4 H+(aq) + 4 e- Eo = -1.23 V 2 H2O(l) + 2 e-  H2(g) + 2 OH-(aq) Eo = -0.83 V Eocell = -2.06 V i.e. non-spontaneous!

  5. Electrolysis of Water H2O(l) + salt

  6. Electrolysis of Water Note reversal of polarity! battery - + H2O(l) + salt anode cathode 2 H2O + 2 e-® H2(g) + 2 OH- 2 H2O ® O2(g) + 4 H+ + 4 e-

  7. Electrowinning of Metals - + CuCl2(aq) anode cathode

  8. at the cathode: Cu+2® Cu(s) ?? or H2O® H2(g) ??

  9. Rule of thumb… The cathode reaction with the most positive reduction potential will occur preferentially

  10. Cu+2(aq) + 2 e-Ž Cu(s) Eo = +0.15 V 2 H2O(l) + 2 e-Ž H2(g) + 2 OH- Eo = - 0.83 V

  11. at the anode: Cl-® Cl2(g) ?? or H2O® O2(g) ??

  12. Same rule of thumb… The anode reaction with the most positive oxidation potential will occur preferentially

  13. Same rule of thumb… 2 Cl-(aq) Ž Cl2(g) + 2 e- Eo = -1.36 V 2 H2O(l)Ž O2(g) + 4 H++ 4 e- Eo = - 1.23 V Thus, we predict water gets oxidized at the anode.

  14. Overpotential Erequired > Eocell for electrolysis In this case, Cl- is oxidized

  15. Electrowinning of Metals - + CuSO4(aq) anode cathode 2 Cl- ® Cl2(g) + 2 e- Cu+2(aq) + 2 e-® Cu(s)

  16. Overall Reaction: Cu+2(aq) + 2 Cl-(aq) Ž Cu(s) + Cl2(g) Eocell = -1.21 V

  17. Electrorefining of Metals - + Impure Cu Pure Cu Cu

  18. Making Fluorine, F2(g) Electrolysis of HF · 2 KF(l) oxidation: 2 F-® F2(g) + 2 e- reduction: 2 H+ + 2 e-® H2(g)

  19. Making Chlorine, Cl2(g) Electrolysis of brine (NaCl(aq)) oxidation: 2 Cl-(aq)® Cl2(g) + 2 e- reduction: 2 H2O(l) + 2 e-® H2(g) + 2 OH-(aq)

  20. Overall, 2 NaCl(aq) + 2 H2O(l) ® Cl2(g) + H2(g) + 2 NaOH(aq) cleaners, paper, etc disinfection, plastics (PVC)

  21. Manufacture of Aluminum Why not just electrowin Al+3(aq)? Al+3(aq) + 3 e- ® Al(s) Eo = -1.66 V 2 H2O(l) + 2 e- ® H2(g) + 2 OH-(aq)Eo = -0.83 V (reduction of water is favored)

  22. Manufacture of Aluminum Bauxite = impure Al2O3(s) m.p. = 2045oC Solution? 1. Lower the melting point 2. Perform electrolysis

  23. Manufacture of Aluminum Al2O3(s) + AlF3(s) + 3 NaF(s) Na3AlF6, cryolite m.p. » 900oC

  24. Manufacture of Aluminum reduction: AlFxOy3-x-2y + n e- ® Al(l) + ? oxidation:AlFxOy3-x-2y® O2(g) + n e- + ? oxidizes C electrodes! sinks!

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