PART 2: Avogadro’s Constant from a Copper Electrolytic Cell First Electrode Second Electrode Mass of electrode, initial: Mass of electrode, final: Mass change, absolute value: Moles Cu, absolute value: For the following calculations, refer to the example on page 12:4. Current Time Charge ______________________ ______________________ ______________________ How many electrons were transferred? Show the calculation below. Charge on an electron is required. Step 3 How many copper atoms were etched or plated? Show the calculation below. Need Cu:eˉ mole ratio. Step 4 Calculate Avogadro’s constant twice, once using the moles of copper etched and once using the mole of copper plated. Show the calculation below. Step 5 and Step 6 Pick the value that is closest to the accepted value of Avogadro’s number, NA, and calculate the percent error below.PART 2 Avogadro’s Constant from a Copper Electrolytic Cell In this experiment electric current will be used to cause copper to be etched off of one electrode and deposited onto another. From the change in mass, the number of coulombs transferred and the molar mass of copper, Avogadro’s constant will be determined. The half-cell reactions are Anode: Cu(s) -> Cu2+(aq) + 2e‾ Cathode: Cu2+(aq) + 2e‾ -> Cu(s) You will notice that there is no net reaction; everything cancels! However, mass will be transferred from the anode to the cathode. In fact, this is how electroplating is performed. Supply List Netbook computer with LoggerPro running LabQuest Mini interface and the constant current system Two copper strips (anode and cathode); copper plating solution Magnetic stirrer and stir bar; 150-mL beaker As the name suggests, the constant current system (CCS) will attempt to deliver a constant flow of electric current by automatically adjusting the voltage. The dial on the CCS controls the current and the computer software merely monitors the actual current delivered. The computer does not control the device. The crocodile clips on the CCS are difficult to use. So instead attach double ended alligator clips to them and use these to make connections to the copper strips. Work in groups of four students for Part 2. PROCEDURE 1. Obtain two strips of copper. Clean the surface gently with an abrasive pad only if they look badly corroded or are flaking. Make sure the electrodes are dry and weigh each of them, recording their masses on the Data and Analysis sheets. 2. Pour around 100 mL of Part 2 copper sulfate solution into a 150-mL beaker. Place the copper electrodes into the solution and secure them to the beaker with tape or clothes pins so that they will not touch each other. Both electrodes must be well submerged into the solution. Add a magnetic stirring bar and gently stir the solution. 3. Make sure that the LabQuest Mini is connected the computer and that the constant current system is connected to the Mini and plugged into an outlet. LoggerPro should recognize that the constant current system is connected. General Chemistry II Laboratory, Experiment 12 Electrochemistry: Voltaic and Electrolytic Cells 12:8 4. Connect ONLY ONE of the alligator clips to one of the copper strips. Turn the constant current knob to the two o’clock position. See the diagram below. 5. Click Data Collection (clock icon) in LoggerPro. Make sure that you are in Time Mode and that Duration is set for 1800 seconds. Click the Triggering tab, check triggering and have it set to turn on at 0.25 amperes (increasing). Click Done. 6. Click the green Collect button in LoggerPro (erase any previous files, if prompted), then make the final electrical connection to the other copper strip. The timer will then automatically start and the current should flow at around 0.4 amperes. 7. Watch the time closely near the end. At 1800 seconds disconnect the clips and remove the copper electrodes from the beaker. The current will not automatically stop flowing at 1800 seconds – you MUST disconnect a wire in order to stop the current. Gently rinse the copper electrodes under some running tap water and then swish them around in the bottle of acetone (front of the classroom) to remove surface water. Let them air dry for a couple of minutes. Do not wipe them dry. 8. Weigh the electrodes and record their masses. 9. Return the Part 2 copper sulfate solution to the original bottle and the copper electrodes to their container when you are done. Clean up your work space and unplug the constant current system. 10. Complete the calculations on the Data and Analysis sheets in order to come up with the calculated value for Avogadro’s constant. Figure 3 The Electrolytic Cell Setup wire with alligator clips dial set to two o’cloc