INTRODUCTION TO ELECTROCHEMISTRY: CURRENT, VOLTAGE, & BATTERIES 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 salts of cations of the same element (i.e. 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 Cu 2+ (aq) + 4 NH 3 (aq) [Cu(NH 3 ) 4 ] 2+ (aq) K eq = 1.2 x 10 12 Electrochemical Plating (Electroplating) Electrochemical plating is a method used to apply a metallic coating on surfaces (e.g. chrome plating of fenders, grills, and toasters) to protect them against corrosion and passivation, by using an electric current. The first step in electroplating is to negatively charge the object (to be coated) by connecting it to an electric circuit and applying a bias voltage. The object is then dipped into a solution containing a metal salt. The metal cations, in the solution, partake in redox reactions and eventually are reduced to a neutral metal which is deposited onto the object s surface to become a protective coating. In this experiment, the object is a copper strip and the metal salt is ZnSO 4. The positive pole of a current probe will be connected to the copper strip and the negative pole to a battery. The positive pole of the battery will be connected to an aluminum strip (Figure 1). The aluminum strip is used as the anode since it is made out of metal and therefore has the ability to conduct electrons and transfer electrons to species in solution. In practice the anode can be any type of small object made out of metal (e.g. Zn, Cu, Fe). Current Probe _ + Battery _ + Figure 1. Scheme of the electrochemical set-up. Page 1 of 5
The following reaction is observed at the cathode (copper): ZnSO 4 (aq) + 2 e Zn(s) + SO 4 2- (aq) As seen in the reaction above the zinc ions in the solution are reduced at the cathode to Zn(s), which is deposited onto the copper. A current probe is used to measure the current (I) passing through the system as a function of time (t, in sec). The total electrical charge passed can then be calculated (q T ) using the following equation (Faraday s Law): (1) q T = I t If the electrical charge is known, the mass of zinc (m Zn ) plated on the copper can be calculated using the following equation: qt M (2) mzn, n F where M is the molar mass of Zn (65.39 g/mol), n is the stoichiometry for the number of electrons in the half reaction, and F is Faraday s constant (96,485 C/mol) In this experiment the students will determine the mass of zinc plated on the copper strip, theoretically, using the equations above, and experimentally, from the mass of the copper strip before and after plating. Students will also calculate Avogadro s number, N A, by using the experimentally determined values of the mass of Zn(s) plated onto the copper strip and the total amount of electrical charge passed, q T, q M (3) N T A n mzn q, e where q e is the charge of one electron, and the other variables are as defined in equation (2) 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 acid solutions are corrosive and can cause burns and respiratory problems. If a spill occurs, wash all affected areas immediately and thoroughly with cold water, and inform your TA. Part C. ZnSO 4 is harmful and can cause severe irritation upon contact with eyes. When handling ZnSO 4, wear gloves and if it contacts your eyes or skin, flush with water for at least 15 min. Page 2 of 5
Procedures Part A. Creating Electrochemical Cells 1. Obtain a spot plate from the plastic tub in the hood and clean it before use. The stockroom will provide the following 0.1 M solutions: ZnSO 4, CuSO 4, NiSO 4, and MgSO 4, as well as their metal strips (solid Zn, Cu, Ni, and Mg). (A key to help you identify the type of each metal strip is displayed at the front of the room.) Use sandpaper to remove any impurities from the metal strips, and then rinse with water and dry. In one of the wells, put ~25 drops of CuSO 4 and a Cu strip (partially immersed in the solution) to create a Cu 2+ /Cu half cell. Repeat the same procedure with the remaining solutions and strips, recording their location on the spot plate in your ELN. As shown below in Figure 2, place the wells adjacent to each other, forming a square so they can easily be connected by a salt bridge. To make the salt bridge, take a piece of filter paper and soak it in saturated KCl solution. Figure 2. Digital image of the electrochemical cell set-up. 2. Prepare the computer for data collection by opening "Exp 28" from the Chemistry with Vernier experiment files of Logger Pro. The computer is now set to monitor potential in volts. The potential will appear in the Meter window when the leads are connected to a cell. Verify that when the voltage probe leads are touched together, the voltage displays 0.00 V. When the two leads are not in contact with a cell (or each other), a meaningless voltage may be displayed. 3. Calibrate the voltage probe: Go to the "Experiment" menu and choose "Calibrate". In the window that appears make sure the "Calibration" tab is chosen. Click on "Calibrate Now". Connect the two ends of the voltage probe together. When the voltage reading in the calibration window stabilizes enter 0.00 in the field beneath "Enter Value". Connect the Mg and Cu half cells with a salt bridge (a small strip of filter paper saturated with 1 M KCl(aq)). Connect the red voltage probe lead to the Cu strip electrode, and the black voltage probe lead to the Mg strip electrode, each by simply touching the probe to the metal that is not submerged in the solution. When the voltage reading in the calibration window stabilizes enter 2.71 in the second field beneath "Enter Value". Save this calibration set-up for the rest of the voltage measurements. 4. 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 Page 3 of 5
voltage probes to the electrodes in the cells. Do this by bringing the black voltage probe lead in contact with one metal electrode and the red voltage probe 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, and then rinse and dry it again as explained above. 5. 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 as 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. Part B. Measuring the Effects of Concentration on an Electrochemical Cell 1. Measure the voltage again for the Cu/Zn cell. Add 1 drop of 6 M NH 3 solution to the Cu well (stir with a toothpick) and record the voltage. Place a piece of white paper under the spot plate to observe the color of the Cu(NH 3 ) 4 2+ 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?) 2. 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. Electrochemical Plating of Zn on Copper 1. Mass a copper strip on a watch glass and record the mass. 2. If electrical wires with alligator clips are not available, follow these instructions to create them: a. Use a wire cutter to cut off three ~10 cm pieces of electrical wire. b. With the cutter, strip off ~1 cm of the coating from each end of each wire. c. Attach each end of the wire to its own alligator clip (through the hole in the back, Figure 3). 3. Check to make sure the voltage of your 9.0 V battery is at least 7 V. Attach the copper strip (cathode) and the positive pole of the current probe to each end of one electrical wire. Using a second electrical wire, attach the aluminum strip used in Part A (anode) and the positive pole of a 9.0 V battery. Finally attach the negative poles of the current probe and the 9.0 V battery with a third wire (Figure 1). Connect the current probe to channel 1 of the Logger Pro interface. Go to the Experiment menu and choose Data Collection. Change the length to 60 sec and click Done. Page 4 of 5
a) b) Figure 3. a) An aluminum electrode and b) the electrochemical plating setup 4. Pour ~40 ml of 2.0 M ZnSO 4 solution into a 100 ml beaker. Click Collect and immerse the copper and aluminum electrodes (make sure the electrodes do not touch) into the solution at the same time. Using plastic tongs, remove the copper from the solution after (exactly) 60 sec. Click Done. The current probe cannot measure current higher than 0.6 A. (If your current is flatlined at 0.6 A, add a resistor (680 ohms or 470 ohms) in series in your circuit until you observe a peak current that is less than 0.6 A, followed by a decay in the current. Rerun the 60 second trial with this new copper electrode and the resistor in the circuit. An alternative is to dilute the concentration of ZnSO 4 from 2 M to 1 M, or more, and rerun the 60 second trial.) 5. Using the air jets in the hood, dry the copper carefully, place it on a watch glass, and record the final mass, (m final m initial = mass of zinc plated on the copper). After obtaining a satisfactory plating current line (i.e. one where the current peaks at first and is then followed by a decay), determine the charge passed by selecting only data under that curve. Go to the Analyze menu and choose Statistics and record the mean of the current. Another way to determine the charge passed is go to the Analyze menu and chose Integral. Use both of these methods in order to calculate the charge passed. Compare your results. Repeat the above plating experiments for 120 sec and 180 sec. (No need to change the ZnSO 4 solution but new copper and aluminum electrodes is needed). 6. Pour the zinc solution into the waste bottle in the hood. Do not flush it down the drain. The aluminum strip should be placed in the collection beaker in the hood. Do NOT, under any circumstances, throw the solid electrodes in the trash. Wash your hands thoroughly before leaving the lab. Page 5 of 5