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EXPERIMENT: 26 ELECTROCHEMISTRY: GALVANIC AND ELECTROLYTIC CELLS Materials Required : 50 mL beakers (2) test tubes sandpaper glass U-tube cotton voltmeter 9.0 V battery connecting wires with alligator clips (2) thermometer iron nail paper clips (2) copper wire magnesium ribbon magnesium sulfate (MgSO) copper sulfate pentahydrate (CuSO, 5H,O) sodium chloride (NaCI) saturated sodium chloride (NaCI)solution phenolphthalein indicator SAFETY PRECAUTIONS: EYE PROTECTION MUST BE WORN AT ALL TIMES IN LABORATORY. Purpose of Experiment: Oxidation-reduction reactions will be performed and galvanic and electrolytic cells will be operated in this experiment. Reactions involving metals and their ions will be studied in order to determine which is more easily oxidized and reduced. A galvanic cell will be constructed, and the voltage it produces will be measured. The measured voltage will be compared with that calculated by the Nernst Equation, and then used to calculate the free energy change for the reaction. An electrolytic cell will also be constructed and the chemical reaction will be examined. 26-1 Experimental Procedure: Weigh out 1.00 g of magnesium sulfate and place in a 50 mL beaker. 2. Weigh out 2.00 g of copper sulfate pentahydrate and place in another 50 mL 3. Add enough deionized water to each to make approximately 40 mL of 1. beaker solution and stir to dissolve. This will make approximately 0.200 M solutions of each. 4. Pour enough of each solution in separate test tubes to give a depth of 5. Sand an iron nail and a piece of copper wire with sandpaper to remove the 6. Cut approximately a one-centimeter piece of the copper wire and drop it into approximately one half centimeter. oxide layer until they are shiny. the magnesium sulfate solution in the test tube. 7. Drop the iron nail into the copper sulfate solution in the test tube. 8. Observe what happens to each over time as you continue the experiment. Part B. The galvanic cell 1. Loop the end of the rest of the shiny copper wire and place it in the copper 2. Sand a piece of magnesium ribbon until it is shiny. Place it in the magnesium 3. Connect the magnesium ribbon to one of the terminals of the voltmeter and 4. Fill a U-tube completely with saturated sodium chloride solution using a 5. Plug the ends of the tube with small pieces of cotton. Be sure there are no sulfate solution in the 50 mL beaker. sulfate solution in the other 50 mL beaker. connect the copper wire to the other terminal. Note any change. dropper. air bubbles in the tube 26-2 6. Place one end of the tube in each beaker to form a salt bridge. Note any change in the reading. 7 8. Record the voltage. 9. Read and record the room temperature with the thermometer. Assume this is the If the needle swings negative, reverse the positive and negative connections. temperature of the solutions. rt C. The electrolytic cell 1. Place approximately 40 mL of deionized water in a clean 50 mL beaker. 2. Bend two paper clips so that they can grip the rim of the beaker with one end 3. Place the clips on opposite sides of the beaker so they do not touch one another. 4. Briefly connect one clip to the positive terminal of a battery and the other to the 5. Add a small scoop of plain salt (NaCI) and one drop of phenolphthalein indicator 6. Again briefly connect the clips to the battery and note any change. immersed in the water. negative. Note which clip is positive and which is negative. Note any change. lo Changge to the water. Gently swirl the beaker and note any change. (The phenolphthalein does not participate in the electrochemical reactions) 7. Calculations and Questions: Part A. Oxidation-reduction reactions. 1. Which is the stronger reducing agent, Fe or Cu ? 2. Which is the stronger oxidizing agent, Cu" or Mg" Part B. The galvanic cell 1. Write the half-reactions for the two electrodes and the overall cell reaction. Caleulate the standard Emf, Eo, for the cell. (Be sure to properly identify the anode and cathode.) 2. Using the Nernst Equation, calculate the Emf, E, for the cell. (Do not be surprised if it does not agree with your measured value. There are several reasons for the disagreement.) How does the computed value compare with Ee, above? Explain. 3. Calculate the value of AGo for the cell reaction. Part C. The electrolytic cell 1. Write the half-reactions occurring at the positive and negative electrodes and the overall reaction. What reaction occurs when you swirl the beaker? 2. 3. If a current of 0.25 A flowed in the cell for 30 minutes, how many grams of Clh gas would be generated? 26-4 Part A. Oxidation-reduction reactions - .009 -a 009 Mass of magnesium sulfate Mass of copper sulfate pentahydrate Observations: Part B. The Galvanic cell Voltage Lp Room temperature Observations: Uguoe vurnced gruen Part C. The Electrolytic cel Observations:

Answer 1

ost A 21 34 V More So es more t 2) E megative the po erd oxidig