1. Lecture 41 - Electrochemistry V

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

3. Electrolytic Cells l Reaction is not spontaneous l E o cell < 0 l Used for producing a chemical product

4. Electrolysis of Water We want: 2 H 2 O (l)  2 H 2(g) + O 2(g) 2 H 2 O (l)  O 2(g) + 4 H + (aq) + 4 e - E o = -1.23 V 2 H 2 O (l) + 2 e -  H 2(g) + 2 OH - (aq) E o = -0.83 V E o cell = -2.06 V i.e. non-spontaneous!

5. Electrolysis of Water H 2 O (l) + salt

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

7. - + anode cathode CuCl 2(aq) Electrowinning of Metals

8. at the cathode: Cu +2  Cu (s) ?? or H 2 O  H 2(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) E o = +0.15 V 2 H 2 O (l) + 2 e - Ž H 2(g) + 2 OH - E o = - 0.83 V

11. at the anode: Cl -  Cl 2(g) ?? or H 2 O  O 2(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 Cl 2(g) + 2 e - E o = -1.36 V 2 H 2 O (l) Ž O 2(g) + 4 H + + 4 e - E o = - 1.23 V Thus, we predict water gets oxidized at the anode.

14. Overpotential E required > E o cell for electrolysis In this case, Cl - is oxidized

15. - + anode cathode CuSO 4(aq) 2 Cl -  Cl 2(g) + 2 e - Cu +2 (aq) + 2 e -  Cu (s) Electrowinning of Metals

16. Overall Reaction: Cu +2 (aq) + 2 Cl - (aq) Ž Cu (s) + Cl 2(g) E o cell = -1.21 V

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

18. Making Fluorine, F 2(g) Electrolysis of HF  2 KF (l) oxidation: 2 F -  F 2(g) + 2 e - reduction: 2 H + + 2 e -  H 2(g)

19. Making Chlorine, Cl 2(g) Electrolysis of brine (NaCl (aq) ) oxidation: 2 Cl - (aq)  Cl 2(g) + 2 e - reduction: 2 H 2 O (l) + 2 e -  H 2(g) + 2 OH - (aq)

20. Overall, 2 NaCl (aq) + 2 H 2 O (l)  Cl 2(g) + H 2(g) + 2 NaOH (aq) disinfection, plastics (PVC) cleaners, paper, etc

21. Manufacture of Aluminum Why not just electrowin Al +3 (aq) ? Al +3 (aq) + 3 e -  Al (s) E o = -1.66 V 2 H 2 O (l) + 2 e -  H 2(g) + 2 OH - (aq) E o = -0.83 V (reduction of water is favored)

22. Manufacture of Aluminum Bauxite = impure Al 2 O 3(s) m.p. = 2045 o C Solution? 1.Lower the melting point 2.Perform electrolysis

23. Manufacture of Aluminum Al 2 O 3(s) + AlF 3(s) + 3 NaF (s) Na 3 AlF 6, cryolite m.p.  900 o C

24. Manufacture of Aluminum reduction: AlF x O y 3-x-2y + n e -  Al (l) + ? oxidation: AlF x O y 3-x-2y  O 2(g) + n e - + ? sinks!oxidizes C electrodes!