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)  Zn 2+ + 2e - (2) Reduction: Acceptance of e - Decrease in Oxidation Number Cl 2(g) + 2e -  2Cl -

3. Balancing Oxidation-Reduction Equations: Use Half-Reaction Method The half-reactions for Sn 2+ (aq) + 2Fe 3+ (aq)  Sn 4+ (aq) + 2Fe 3+ (aq) are Sn 2+ (aq)  Sn 4+ (aq) +2e - 2Fe 3+ (aq) + 2e -  2Fe 2+ (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) MnO 4 - (aq) + C 2 O 4 2- (aq)  Mn 2+ (aq) + 2CO 2 (g) 1.The two incomplete half reactions are MnO 4 - (aq)  Mn 2+ (aq) C 2 O 4 2- (aq)  2CO 2 (g)

7. Problem Complete and balance the following equations, and identify the oxidizing and reducing agents (a) Cr 2 O 7 2- (aq) + I - (aq)  Cr 3+ (aq) + IO 3 - (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 H 2 O. 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 H 2 O. –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 -  O 2(g) –(basic solution) (e) SO 3 2- (aq)  SO 4 2- (aq) –(basic solution)

10. Problem Complete and balance the following equations, and identify the oxidizing and reducing agents. Pb(OH) 4 2- (aq) + ClO - (aq)  PbO 2(s) + Cl - (aq) –(basic solution)

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