3. Electrochemistry: Oxidation-Reduction Reactions Zn(s) + Cu +2 (aq)  Zn 2+ (aq) + Cu(s) loss of 2e - gaining to 2e - Zinc is oxidized - it goes up in charge and is the reducing agent reduced Copper is reduced because it goes down in charge and is the oxidizing agent

4. Electrochemistry Messing with electrons What is the charge on each atom KMnO 4 Cr(OH) 3 Fe(OH) 2 +1 -2 -8 +1 +1+7 +7 -2 +1 -3-3 +3 +3 -1 -2-2 = 1 +3 +3

5. Electrochemistry: Oxidation-Reduction Reactions By writing the oxidation number of each element under the reaction equation, we can easily see the oxidation state changes that occur Zn(s) + 2H + (aq)  Zn 2+ (aq) + H 2 (aq) 0 +1 +2 0 In any oxidation-reduction reaction (redox), both oxidation and reduction must occur.

6. Electrochemistry: Balancing Oxidation-Reduction Reactions Oxidation Number Method Al(l) + MnO 2  Al 2 O 3 + Mn 0 +4 -2 +3 -2 0 -3e’s +4e’s X 4 = -12 X 3 = +12 4 3 2 3

7. Electrochemistry: Balancing Oxidation-Reduction Reactions Sample problem: I 2 O 5 (s) + CO(g)  I 2 (s) + CO 2 (g) -2e’s +5e’s +2 +4 +5 0 X 2 = 10 X 5 = -10 5 5

8. Practice: Identify the oxidation state of each element in the following: MgBr 2 Fe 2 O 3 AlN SO 3 PO 4 3- Cr 2 O 7 2- HClO 2 CuSO 4 For each of the following reactants, identify: the oxidizing agent, the reducing agent, the substance oxidized and the substance reduced Cl 2 (s) + 2Na(s) → 2Na + (aq) + 2Cl - (aq) Pb + Fe 3+ → Fe 2+ + Pb 2+ Balance using oxidation number method: SO 2 (g) + HNO 2 (aq) --> H 2 SO 4 (aq) + NO(g)