2. Dry CellsDry Cells  A compact, portable electrical energy source  A voltaic cell in which the electrolyte is a paste  Example: a flashlight battery/alkaline battery  Starts out at a certain voltage but decreases steadily over time

3. Alkaline BatteryAlkaline Battery

4. Alkaline Battery ReactionsAlkaline Battery Reactions  Oxidation: Zn(s)  Zn +2 (aq) + 2e -  Reduction: 2MnO 2 (s) + 2NH 4 + + 2e -  Mn 2 O 3 (s) + 2NH 3 (aq) + H 2 O(l)

5. Lead Storage BatteryLead Storage Battery  Example: car battery  A group of voltaic cells connected together  Recharging a battery is a reverse reaction to the charging of one, and is not a spontaneous reaction  Each recharge causes small amounts of lead sulfate fall from the electrodes and collect on the bottom of the cell  Eventually, there is not enough lead sulfate to recharge the battery and must be replaced

6. Lead Storage BatteryLead Storage Battery

7. Lead Storage ReactionsLead Storage Reactions  Oxidation: Pb(s) + SO 4 2- (aq)  PbSO 4 (s) + 2e -  Reduction: PbO 2 (s) + 4H + (aq) + SO 4 2- (aq) + 2e -  PbSO 4 (s) + 2H 2 O(l)

8. Fuel CellsFuel Cells  Cells with renewable electrodes  Contains a fuel substance that undergoes oxidation and continuously obtains electrical energy  Emit no air pollutants and are quieter and more cost-effective than traditional generators

9. Example of a Fuel CellExample of a Fuel Cell  Hydrogen-oxygen fuel cell  Three compartments separated by two electrodes of carbon  Oxygen (the oxidizer) is fed into the cathode  Hydrogen (the fuel) is fed into the anode  Central compartment contains potassium hydroxide

10. Fuel CellFuel Cell

11. Fuel Cell ReactionsFuel Cell Reactions  Oxidation: 2H 2 (g) + 4OH - (aq)  4H 2 O(l) + 4e -  Reduction: O 2 (g) + 2H 2 O(l) + 4e -  4OH - (aq)

12. Uses of Fuel CellsUses of Fuel Cells  Energy on spacecrafts  Continuous energy source  No pollutants released  Can drink the water produced by the fuel cells  Submarines  Future for cars?