1. General Chemistry II CHEM 152 Unit 4 Week 13

2. Week 13 Reading Assignment Chapter 18 – Sections 18.1 (intro), 18.3 (voltaic cells), 18.4 (cell potential)

4. ELECTROCHEMISTRY Unit 4 Study of chemical reactions that produce electrical currents or voltages and of the chemical reactions that are caused by the action of currents or voltages.

6. What is different in the second reaction? Ag + + Cl -  AgCl(s) Ag + + Cu(s)  Cu 2+ + Ag(s) Cu Ag Ag + The charge on Ag and Cu change. That’s an Oxidation- reduction reaction (REDOX) Cu 2+

7. Ag + + Cu(s)  Cu 2+ + Ag(s) Copper loses two electrons Cu was OXIDIZED Ag was REDUCED Ag + was the OXIDIZING AGENT Cu was the REDUCING AGENT Charge balance vs. mass balance 2 2 Ag + gains electrons

9. In a neutral COMPOUND some elements often have a fixed OXIDATION NUMBER H almost always is +1 O almost always is -2 F always is -1 3) In a NEUTRAL COMPOUND the SUM of all the atomic oxidation numbers is ZERO CH 4 CH 3 COOH

10. 4) In a COMPLEX ION the SUM of all the atomic oxidation numbers is equal to the TOTAL CHARGE on the ion. ClO 3 − MnO 4 − PO 3 3-

11. What is the oxidation number of P in H 3 PO 4 ? What is the oxidation number of Cr in Cr 2 O 7 2- ? What is the oxidation number of C in (NH 4 ) 2 CO 3 ? What is the oxidation number of C in C 3 H 8 (propane)?

12. Another Way Use the structure – the oxidation number of an atom is equal to # of valence electrons − # of lone pair e - − # of bond e - IF the atom is the most electronegative in the bond C = 4 – 0 – 0 = +4 (oxygen is more electronegative) This method treats atoms like ions – it assumes all the bonded electrons go to the more electronegative atom in the bond

13. Propane again… Using the structure method, what is the oxidation number of each carbon in propane (C 3 H 8 )?

14. What is the oxidation number of the oxygen-bonded C in CH 3 COOH (acetic acid)?

15. OXIDATION IS THE LOSS OF ELECTRONS REDUCTION IS THE GAIN OF ELECTRONS (Oxidation Number is reduced) Oil Rig O I L R I G Oxidation and Reduction Reactions LEO the LION goes GER L E O G E R

16. Cu + AgNO 3  Ag + Cu(NO 3 ) 2

18. At the atomic level

19. 2Al + Fe 2 O 3  2Fe + Al 2 O 3 THERMITE REACTION

21. Identifying REDOX S 8 + 12O 2  8SO 3 Ionic or covalent? O.N.? REDOX? Ox. Agent; Red. Agent? 2AgNO 3 + Na 2 S  Ag 2 S + 2NaNO 3 REDOX? 5As 4 O 6 + 8MnO 4 - + 18H 2 O  20 AsO 4 3- + 8 Mn 2+ + 36H + REDOX?

22. S 8 + 12O 2  8SO 3 Is it redox? If so, What is oxidized? What is the reducing agent?

23. 2 AgNO 3 + Na 2 S  Ag 2 S + 2 NaNO 3 Is it redox? 5 As 4 O 6 + 8 MnO 4 - + 18 H 2 O  20 AsO 4 3- + 8 Mn 2+ + 36 H + What is oxidized and reduced? What is the oxidizing agent (OA) and reducing agent (RA)?

24. . Uncontrolled spontaneous reaction How can this be harnessed to do work?

25. . Controlled redox reaction Zn  Zn 2+ + 2e − Cu 2+ + 2e −  Cu Zn + Cu 2+  Cu + Zn 2+ + overall reaction but electrons are forced outside of the cells

26. Voltaic Cell Δ G = neg Galvanic Battery

27. Electrochemical Cells Devices to harness a chemical reaction which produces an electric current Daniel's Cell Zn is oxidized and is the reducing agent Zn(s)  Zn 2+ (aq) + 2e- ANODE (oxidation) Cu 2+ is reduced and is the oxidizing agent Cu 2+ (aq) + 2e-  Cu(s) CATHODE (reduction) Zinc anode Copper cathode Cu +2 Cu +2 Cu +2 Cu +2 1.10 v Zn +2 Zn +2 Zn +2 Zn +2

28. Simple Electrochemical Cell Also called voltaic or galvanic cells

29. Electrochemical Cell Explain what happens in this system. Follow the circulation of one negative charge in the cell.

34. How do we do it?

35. Overall reaction is reduction of H + by Zn metal Zn(s) + 2H + (aq)  Zn 2+ + H 2 (g) E o =+0.76 V E o for Zn  Zn 2+ (aq) + 2e - is +0.76 V E o for Zn 2+ + 2e -  Zn is –0.76 V Zn is a better reducing agent than H 2 .

37. TABLE OF STANDARD REDUCTION POTENTIALS 2 E o (V) Cu 2+ + 2e- Cu +0.34 2 H + + 2e- H 0.00 Zn 2+ + 2e- Zn -0.76 oxidizing ability of ion reducing ability of element

39. Predict the outcome of the following reaction. Classify each of the reactions as processes as product-favored or reactant-favored. Fe(s) + Sn 2+ (aq)  O 2 (g) + Au(s)  Na(s) + H 2 O(l)  Al 3+ (aq) + Hg(l) 

40. Summary Activity A lead acid battery uses the following two reactions to provide a voltage: Pb + SO 4 2-  PbSO 4 + 2e - +0.36V PbSO 4 + 2H 2 O  PbO 2 + 4H + + SO 4 2- + 2e - -1.70V What is the overall reaction? What is the overall standard cell potential? Which reaction is the oxidation and which reaction is the reduction?