1. Chapter 19: Electrochemistry • Lecture Topics • 1. Balancing Redox/Oxidation-Reduction Reactions. • 2. Voltaic Cells • 3. Electrolysis • 4. Corrosion

2. Chapter 19: Oxidation Reduction Reactions • (1) Oxidation: Loss e- • Increase in Oxidation Number • Zn(s)Zn2+ + 2e- • (2) Reduction: Acceptance of e- • Decrease in Oxidation Number • Cl2(g) + 2e- 2Cl-

3. Balancing Oxidation-Reduction Equations: Use Half-Reaction Method • The half-reactions for • Sn2+(aq) + 2Fe3+(aq)  Sn4+(aq) + 2Fe3+(aq) • are • Sn2+(aq)  Sn4+(aq) +2e- • 2Fe3+(aq) + 2e- 2Fe2+(aq) • Oxidation Half-Reaction: electrons are products. • Reduction Half-Reaction: electrons are reactants.

4. Half-Reaction Method for Balancing Oxidation-Reduction Equations • 1. Separate the equation into the two half-reactions. Write down the two half reactions. • 2. Balance each half reaction: a. First balance all elements other than H and O. b. Then balance O by adding water. c. Then balance H by adding H+ if have acidic solution. d. Finish by balancing charge by adding electrons.

5. Half-Reaction Method for Balancing Oxidation-Reduction Equations • 3. Multiply each half reaction to make the number of electrons equal. 4. Add the two half-reactions and simplify. To simplify, remove components common to both reactant and product sides. 5. Check!

6. Example • Balance: (acidic) • MnO4- (aq) + C2O42- (aq)  Mn2+ (aq) + 2CO2 (g) • The two incomplete half reactions are • MnO4-(aq)  Mn2+(aq) • C2O42-(aq)  2CO2(g)

7. Problem • Complete and balance the following equations, and identify the oxidizing and reducing agents • (a) Cr2O72- (aq) + I-(aq) Cr3+(aq) + IO3-(aq) • (acidic solution)

8. Balancing Oxidation-Reduction Reactions: Basic Medium • When the reaction occurs in an alkaline solution, follow the rule listed below to balance H. • 1. Balance O with H2O. Then balance H with H+. • 2. Must neutralize H+. Use coefficient in front or H+, and add equivalent/same number of OH- to both sides of the half-reaction. • 3. For the side with both H+ and OH-, write H2O. • 4. Resume regular steps for balancing the half-reaction.

9. Problem • Complete and balance the following half reaction(s) In each case, indicate whether oxidation or reduction occurs. • (d) OH- O2(g) • (basic solution) • (e) SO32-(aq) SO42-(aq) • (basic solution)

10. Problem • Complete and balance the following equations, and identify the oxidizing and reducing agents. Pb(OH)42- (aq) + ClO- (aq) PbO2(s) + Cl-(aq) • (basic solution)

11. Problem 19.2-B: Use the Half Reaction Method • (in a basic solution) Bi(OH)3 + SnO2 2- SnO32- + Bi