ELECTROCHEMICAL CELLS, BATTERIES, & PLATING INTRODUCTION Electrochemical Cells In this part of the experiment, four half cells are created by immersing metal strips of zinc, copper, aluminum, and magnesium in aqueous solutions containing cations of the same element (Zn 2+, Cu 2+, Ni 2+, and Mg 2+ ). An electrochemical cell is created when two of these half cells are connected by a KCl salt bridge and a wire (the leads from the voltage probe). Six different electrochemical cells can be created from the four half cells above. A positive cell potential is measured when the black lead is connected to the anode and the red lead is connected to the cathode. Cell notation will be used to describe the electrochemical cells. A reduction table will be created by designating copper as the standard electrode. Cu 2+ (aq) + 2 e! Cu(s) E = 0.00 V Lead Acid Battery A typical car battery contains lead (Pb) and lead oxide (PbO 2 ) plates in a sulfuric acid solution. In this part of the experiment a simplified form of such a battery is created by immersing lead plates into a sulfuric acid solution. The electrodes are charged by connecting to a power supply (a 9V battery). During the initial charging, hydrogen and oxygen gas is generated by the electrolysis of water and the lead electrodes become partially coated with PbO 2. The battery will be discharged by connecting the electrodes to a circuit containing a light emitting diode (LED). During the discharge process PbSO 4 (s) is created at both electrodes. When the battery is recharged, the discharge half-reactions are reversed and the electrolysis of water occurs (as it did during the initial charging process). The battery will be used to light LED semiconductors. The light color (energy) emitted by and voltage drop through the LEDs will be used to determine the band gap size trend for the following LEDs: GaP 1.00 As 0.00, GaP 0.85 As 0.15, GaP 0.65 As 0.35, GaP 0.40 As 0.60. 1
Before starting the experiment, the TA will asks you to do a quick demonstration or talk-through one of the following: 1) How to clean off the metal electrodes before using them 2) How to make a salt bridge 3) How to clean up the spot plates after you re done with the experiment SAFETY Safety goggles and aprons must be worn in lab at all times. Part A. Nickel can cause contact dermatitis and solutions containing nickel ions may be carcinogenic. Wear gloves when handling the metal or solution. Part B. Solutions containing NH 3 must be prepared in the hoods. Ammonia and sulfuric solutions are corrosive and can cause burns and respiratory problems; wash all affected areas thoroughly with cold water. Part C. 6 M H 2 SO 4 is corrosive, use gloves while handling. Ventilate the room if the odor is strong and inform the TA so s/he can find the source. PROCEDURES Work in pairs. Wear safety goggles and lab aprons. Part A: Creating Electrochemical Cells The stockroom will provide the following 0.1 M solutions: ZnSO 4, CuSO 4, NiSO 4, MgSO 4, and their metal electrodes (solid Zn, Cu, Ni, and Mg). (A key to help you identify the metal electrodes should be displayed somewhere in lab.) Obtain a spot plate from the plastic tub in the hood and clean it before use. Use sandpaper to remove any impurities from the metal electrodes; rinse with water and dry after sanding. Put about 25 drops of CuSO 4 and a Cu electrode in one of the wells to create a Cu 2+ /Cu half cell. Repeat the same procedure with the remaining solutions and electrodes, recording their location on the spot plate in your notebook. As shown below in Figure 1, place the wells adjacent to each other, forming a square so they can easily be connected by a salt bridge. 2
Figure 1. Electrochemical Cell Set Up Plug the voltage probe into CH1 of the LabQuest2. When the voltage probe leads are touched together, the voltage should display 0.00 V. When the two leads are not in contact with a cell (or each other), a meaningless default voltage may be displayed. Zeroing the voltage probe: Connect the two ends of the voltage probe together, wait for the voltage reading to stabilize. In the the drop-down menu. window, click on the big red box and choose zero from Select any two cells and connect them by the salt bridge (e.g. place one end of the salt bridge in the Cu cell and the other end in the Zn cell). Determine the potential by touching the voltage probes to the electrodes in the cells. Do this by bringing the black lead of the probe in contact with one metal electrode and red lead in contact with the other electrode. If the voltage reads 0.00 V, then reverse the leads until you have a positive voltage. Wait about 5 seconds to take a voltage reading and record the value in your notebook. If the potential fluctuates considerably, sandpaper the electrode gently to remove oxides and impurities. Determine which cell was the anode and which was the cathode: If the measured voltage is positive, the cell connected to the black lead is the anode and the cell connected to the red lead is the cathode. Once you have recorded this information, measure the potentials for the remaining cells, making as many combinations of two cells possible with the solutions provided. Be sure to note the anode and cathode for each combination and use a new salt bridge for each set of cells. 3
Part B: Measure Concentration Effect on an Electrochemical Cell Measure the voltage again for the Cu/Zn cell. Add 1 drop of 6 M NH 3 solution to the Cu well (stir with toothpick) and record the voltage. Place a piece of white paper under the spot plate to observe the color of the Cu(NH 3 ) 2+ 4 complex ion that is formed. (This color test is one that is frequently used to determine the presence of copper (II) ion in a solution.) Add one more drop of NH 3 and measure the voltage again. (Did you see any voltage change?) Use a disposable pipet and carefully transfer each solution from its well into the collection bottle in the hood. Place the empty spot plate into the large plastic tub in the hood. Do this carefully as a dilute bleach solution is in the tub which can spot clothing. Part C: Sulfuric Acid / Lead Battery Wear gloves for this part! Obtain two lead strips (these will serve as electrodes). Sand the strips to remove any oxides. Then bend and place the strips in a 50 ml beaker as shown in Figure 2. Add 30 ml of 6 M sulfuric acid to the beaker. Handle this strong acid with care! Figure 2. Charging the Lead Storage Battery. Figure 3. Measuring the cell voltage. Charge the cell using a 9 V battery: Use alligator clips to attach the leads from a battery snap to the Pb electrodes. Note: The electrode attached to the red lead will become the cathode; the electrode attached to the black lead will become the anode. 4
Charge the cell for 1 minute and then disconnect the battery. Measure the cell voltage: Attach the red and black clips of the voltage probe to the electrodes to obtain a positive voltage (Figure 3). Record the voltage after it stabilizes. Disconnect the Voltage Probe. Connect LEDs to the cell as shown in Figure 4. Figure 4. Circuit Board Insert the wire legs of the LED into two different numbered rows on a circuit board. Insert a wire with a connected alligator clip into one of the numbered rows containing a wire leg of a LED. Insert another wire connected to an alligator clip into the other numbered row containing a LED leg. Connect the alligator clips to the lead electrodes. If the LED does not light, reverse the alligator clips. Attach the voltage probe to the two legs of the LED. Record the LED color. The voltage should be positive, if not reverse the probe connections. Record the voltage when the LED is no longer brightly lit. Recharge the battery and repeat this step for each of the four available LEDs: GaP 1.00 As 0.00, GaP 0.85 As 0.15, GaP 0.65 As 0.35, and GaP 0.40 As 0.60. When finished, remove the Pb electrodes from the beaker and rinse thoroughly. Return electrodes and LEDs to the TA. Pour the solution in the beaker into a 1000 ml beaker and add NaHCO 3 with stirring until the solution is neutral (ph 7). When the solution is neutralized, pour it down the drain. Make sure to clear your email address and password of the LabQuest2 so others can t access your email account. Shutdown the LabQuest2 and not simply put it to sleep. To shutdown the LabQuest2: press the home key, select System! Shut Down! OK. 5
DATA & DISCUSSION Part A (1) Summarize Part A with a data table containing the headings: Anode, Cathode, Overall Cell Reaction, and Cell Potential. For each of cell combination, write the anode half reaction, the cathode half-reaction, the overall reaction in cell notation (the concentration of all solutions is 0.1 M), and the potential in volts. (2) Create a Reduction Potential Table with the cells containing copper from Part A. Assume the reduction potential for Cu 2+ + 2e! Cu is 0.00 V and calculate the remaining reduction potentials relative to Cu 2+ / Cu. List the reactions in order of decreasing reduction potential. Which of the four electrodes must always be the anode? the cathode? Part B (3) Using the K eq values below and your observations from Part B provide a qualitative explanation for the voltage changes that occurred as a result of reagent addition. (No calculations are required.) Cu 2+ (aq) + 4 NH 3 (aq)! [Cu(NH 3 ) 4 ] 2+ (aq) K eq = 1.2 x 10 12 Part C (4) In Part A and B of this experiment the half reactions were isolated in different wells. In Part C the oxidation and reduction half reactions occurred in the same well. Why was this possible? (5) Depending on the process occurring, the electrodes for the battery are composed of Pb(s), PbO 2 (s), and/or PbSO 4. What is (are) the half reaction(s) occurring at each electrode? Include the electrolysis of water. Process Anode Cathode Initial Charging Discharging Recharging 6
(6) Effect of Composition on Band Gap: How does voltage and color change going from LEDs with the highest phosphorous percentage to the lowest? Explain your data in terms of band gap energy. As part of your explanation, rank the LEDs in order of increasing band gap energy (lowest to highest). 7