Simulated Lead Storage Battery
When two lead electrodes dipped in a sulfuric acid solution are connected to an electric motor, the motor does not run. The electrodes are then connected to a power supply, generating hydrogen gas at one electrode and oxygen gas (as well as a coating of PbO2) at the other. When the electrodes are reconnected to the motor, the motor runs until the lead(IV) oxide has been consumed. The half-reactions are the same as those that provide the energy in a lead storage battery.
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Keywords
applications - practical/real-life, electrochemical (electrolysis) cells, electrochemical (galvanic) cells, redox reaction, sulfur/sulfur oxides/sulfuric acid
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(QuickTime 3.0 Sorenson, duration 77 seconds, size 5.3 MB)
Two lead electrodes are placed in a beaker of sulfuric acid solution. When the two electrodes are connected to the leads of a motor, the motor does not run, since no chemical reaction is occurring in the beaker to provide electrical energy.
When the two electrodes are connected to a power supply, the electricity causes chemical reactions to occur at the electrodes. At one electrode, hydrogen gas is generated, while at the other electrode, oxygen gas is produced, and a lead(IV) oxide coating forms on the surface of the metal.
Now when the electrodes are connected to a motor, the motor runs, because a chemical reaction occurs within the beaker. Lead(IV) oxide is reduced to lead(II) sulfate at one electrode, and lead metal is oxidized to lead(II) sulfate at the other. These are the same chemical reactions that provide electrical energy in a lead storage battery.
 Charging. |
 Hydrogen gas at one electrode, oxygen gas at the other. |
 Discharging. |
Additional still images for this movie
Discussion
The half-reactions that provide the energy to drive the motor are the same as those that take place during the discharge of a lead storage battery.
Cathode: PbO2 (s) + 2e- + 4 H+ (aq ) + SO42- (aq ) --> PbSO4 (s) + 2 H2O (l )
Anode: Pb (s) + SO42- (aq ) --> PbSO4 (s) + 2e-
When a lead storage battery discharges, lead(II) sulfate forms on both electrodes. When the battery is recharged, the above half-reactions are reversed. However, a new lead storage battery has lead(II) oxide on one electrode and pure lead for the other, which is quite different from starting with two lead electrodes as in this demonstration.
The half-reactions used to generate PbO2 in this demonstration are quite different from those in a lead storage battery. Indeed, the generation of hydrogen and oxygen from sulfuric acid rather than lead and lead(IV) oxide from lead(II) sulfate is highly undesirable in a real battery. If a lead storage battery is overcharged, hydrogen and oxygen can be generated. They can form an explosive mixture in or near the battery, and that mixture can be ignited by a spark, cigaret, or match. When you recharge a battery or jump-start a car, the last connection in the circuit should be made far away from the battery. This lessens the probability of an explosion.
Demonstration Notes, Warnings, Safety Information, etc.
Exam and Quiz Questions
1. What gas is forming at the anode when the battery is charged? At the cathode? At which electrode is PbO2 also formed?
2. As the battery discharges, lead is converted to insoluble lead(II) sulfate at one electrode, while lead(IV) oxide is converted to lead(II) sulfate at the other. Write balanced equations for the two half-reactions.
3. The motor stopped running when one of the reactants was used up. What was the limiting reactant? What would you need to do to generate more of that reactant and get the motor to run again?