Skills Practice Voltaic Cells DATASHEET FOR IN-TEXT LAB In voltaic cells, oxidation and reduction half-reactions take place in separate halfcells, which can consist of a metal electrode immersed in a solution of its metal ions. The electrical potential, or voltage, that develops between the electrodes is a measure of the combined reducing strength of one reactant and oxidizing strength of the other reactant. OBJECTIVES Construct a Cu-Zn voltaic cell. Design and construct two other voltaic cells. Measure the potential of the voltaic cells. Evaluate cells by comparing the measured cell voltages with the voltages calculated from standard reduction potentials. MATERIALS 0.5 M Al 2 (SO 4 ) 3, 75 ml 0.5 M CuSO 4, 75 ml 0.5 M ZnSO 4, 75 ml Aluminum strip, 1 cm 8 cm Copper strip, 1 cm 8 cm Zinc strip, 1 cm 8 cm Distilled water 100 ml graduated cylinder Emery cloth 150 ml beakers, 3 Salt bridge Voltmeter Wires with alligator clips, 2 Always wear safety goggles and a lab apron to protect your eyes and clothing. If you get a chemical in your eyes, immediately flush the chemical out at the eyewash station while calling to your teacher. Know the locations of the emergency lab shower and the eyewash station and the procedures for using them. Do not touch any chemicals. If you get a chemical on your skin or clothing, wash the chemical off at the sink while calling to your teacher. Make sure you carefully read the labels and follow the precautions on all containers of chemicals that you use. If there are no precautions stated on the label, ask your teacher what precautions you should follow. Do not taste any chemicals or items used in the laboratory. Never return leftovers to their original container; take only small amounts to avoid wasting supplies. PREPARATION 1. Use the Data Table provided to record your data. Modern Chemistry 275 Electrochemistry
2. Remove any oxide coating from strips of aluminum, copper, and zinc by rubbing them with an emery cloth. Keep the metal strips dry until you are ready to use them. 3. Label three 150 ml beakers Al 2 (SO 4 ) 3, CuSO 4, and ZnSO 4. PROCEDURE 1. Pour 75 ml of 0.5 M ZnSO 4 into the ZnSO 4 beaker and 75 ml of 0.5 M CuSO 4 into the CuSO 4 beaker. 2. Place one end of the salt bridge into the CuSO 4 solution and the other end into the ZnSO 4 solution. 3. Place a zinc strip into the zinc solution and a copper strip into the copper solution. 4. Using the alligator clips, connect one wire to one end of the zinc strip and the second wire to the copper strip. Take the free end of the wire attached to the zinc strip, and connect it to one terminal on the voltmeter. Take the free end of the wire attached to the copper strip, and connect it to the other terminal on the voltmeter. The needle on the voltmeter should move to the right. If your voltmeter s needle points to the left, reverse the way the wires are connected to the terminals of the voltmeter. Immediately record the voltage reading in the Data Table, and disconnect the circuit. 5. Record the concentration of the solutions and sketch a diagram of your electrochemical cell. 6. Rinse the copper and zinc strips with a very small amount of distilled water. Collect the rinse from the copper strip in the CuSO 4 beaker and the rinse from the zinc strip in the ZnSO 4 beaker. Rinse each end of the salt bridge into the corresponding beaker. 7. Use the table of standard reduction potentials in the textbook to calculate the standard voltages for the other cells you can build using copper, zinc, or aluminum. Build these cells and measure their potentials following steps 1 6. DISPOSAL 8. Clean all apparatus and your lab station. Wash your hands. Place the pieces of metal in the containers designated by your teacher. Each solution should be poured in its own separate disposal container. Do not mix the contents of the beakers. Modern Chemistry 276 Electrochemistry
Data Table Cell Zn Zn 2 Cu 2 Cu Al Al 3 Zn 2 Zn Al Al 3 Cu 2 Cu Diagram Conc. Voltage (V) Analysis 1. Organizing Ideas For each cell that you constructed, write the equations for the two half-cell reactions. Obtain the standard half-cell potentials for the half-reactions from the table in the textbook, and write these E 0 values after the equations. 2. Organizing Ideas For each cell you tested, combine the two half-reactions to obtain the equation for the net reaction. Modern Chemistry 277 Electrochemistry
3. Organizing Ideas Use the E 0 values for the half-reactions to determine the E 0 for each cell. 4. Resolving Discrepancies Compare the actual cell voltages you measured with the standard cell voltages in item 3. Explain why you would expect a difference. Conclusions 1. Inferring Conclusions Based on the voltages that you measured, which cell produces the most energy? 2. Applying Ideas On the basis of your data, which metal is the strongest reducing agent? Which metal ion is the strongest oxidizing agent? 3. Applying Ideas Indicate the direction of electron flow in each of your cell diagrams. Modern Chemistry 278 Electrochemistry
EXTENSIONS 1. Predicting Outcomes Describe how and why the reactions would stop if the cells had been left connected. 2. Designing Experiments Design a method that could use several of the electrochemical cells you constructed to generate more voltage than any individual cell provided. (Hint: consider what would happen if you linked an Al-Zn cell and a Zn-Cu cell. If your teacher approves your plan, test your idea.) Modern Chemistry 279 Electrochemistry