Electrochemistry Revised 04/29/15

Transcription

1 INTRODUCTION TO ELECTROCHEMISTRY: CURRENT, VOLTAGE, BATTERIES, & THE NERNST EQUATION Experiment partially adapted from J. Chem. Educ., 2008, 85 (8), p 1116 Introduction Electrochemical cell In this experiment, a Cu/CuSO 4 reference electrode will be used in conjunction with a silver wire working electrode. The oxidation of copper takes place at the anode while the reduction of silver takes place at the cathode. See Figure 1 for a diagram of an electrochemical cell. e Cu 2+ Cl Figure 1. Scheme of an electrochemical set-up, containing a Cu/CuSO 4 reference electrode and an Ag wire working electrode. (See Figure 2.) The arrows represent the electron and ion flow; the textboxes highlighted in blue indicate the half-cell reactions. The balanced electrochemical reaction is as follows: Cu 0 (s) + 2Ag + (aq) 2Ag 0 (s) + Cu 2+ (aq) (1) The line notation for the cell is: Cu 0 Cu 2+ (0.100 M)ǁ Ag +, AgCl(s)ǁ Ag 0 (2) The overall standard electrochemical potential, E o cell is: E o cell = E o red (Ag + /Ag) E o red(cu 2+ /Cu) (3) Page 1 of 8

2 The Nernst Equation An electrode potential measures the magnitude of the difference between the concentrations of the species in the half-cell and their equilibrium values. The Nernst equation provides a quantitative relationship between the concentration of species and the electrode potential. A reversible half-reaction can be defined as follows: aa + bb + + ne cc + dd + (4) where the capital letters represent atoms, molecules, or ions, e represents electrons, and the lower-case italic letters represent the number of moles of each species. The electrode potential corresponding to equation (8) is given by the following equation: E = E!!" ln!!!! (5)!"!!!! where, E! = standard electrode potential R = ideal gas constant, J/Kmol F = Faraday s constant, 96,485 C/mol e - T = temperature, K n = number of moles of electrons ln = natural logarithm The equation can be rewritten by substituting numerical values for the constants, converting to base 10 logarithms, and defining the temperature as 25 C (roughly room temperature): E = E!!.!"#$ log!!!! (6)!!!!! The letters in brackets technically represent activities, but usually molar concentrations are substituted into the equation. For example, if species A is a solute, [A] is the concentration of A. If A is a pure liquid, pure solid, or solvent, the activity is one. In this experiment, Ag + gets reduced to Ag in a one-electron process: Ag + + e - à Ag (s) (7) The Nernst equation describing the electrochemical potential in this experiment is:! E!"## = E!"##!.!"#$ log!! [!"! ] (8) In this portion, students will be monitoring the electrochemical potential at various concentrations of Ag +, and gain a better understanding of the Nernst Equation by plotting E cell versus log!, where the intercept is the E!!"!!"## and the slope is equal to!.!"#$.! 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 Page 2 of 8

3 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 2). 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 2. Scheme of the electrochemical set-up. The following reaction is observed at the cathode (copper): ZnSO 4 (aq) + 2 e à 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): q T = I t (9) 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 mzn =, (10) 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 portion of the 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. Page 3 of 8

4 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, qt M N A = n mzn q e, (11) Safety where q e is the charge of one electron, and the other variables are as defined in equation (10). Safety goggles and aprons must be worn in lab at all times. Part A. Silver nitrate solutions are irritating to skin and eyes. It may create black stains on clothes or discoloration of the skin, and is harmful if swallowed. Store all the waste in the correct labeled waste container. Part B. 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 4 of 8

5 Procedures Part A. Understanding the Nernst Equation The stockroom has provided a reference electrode that includes the following: A glass reference electrode with the bottom submerged in 0.1 M CuSO 4. A 0.1 M CuSO 4 solution. Silver and copper wires, both connected to electrical wires at one end. An NMR cap to seal the chamber. Figure 3. Image of the Cu and Ag electrode set-ups. The inner (reference) electrode contains a copper wire dipped into 0.1 M CuSO 4 solution. The bottom of this reference electrode contains a Vycor frit, which must be in solution at all times. A Vycor frit is a porous piece of glass and is used to allow for ionic conductivity between the filling solution of the reference electrode and the solution in which the electrode is submerged. After putting the reference electrode together, place it back in CuSO 4 solution until ready for the experiment. The silver wire is placed outside as the working electrode. One electrical wire is connected to the silver wire while the other electrical wire is connected to the copper wire. 1. 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. Page 5 of 8

6 2. 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". 3. Use a multimeter to measure the potential of a 9V battery. Record the potential. Connect the voltage probe leads to the battery. Make sure the voltage is a positive value. When the voltage reading in the calibration window stabilizes enter the potential you measured with the multimeter in the second field beneath "Enter Value". Save this calibration for the rest of the voltage measurements. 4. Measure out exactly 100 ml of nanopure water and pour into a clean, dry 250 ml beaker that contains a small magnetic stir bar. Place the beaker on a stir plate. 5. Rinse the reference electrode thoroughly with DI water and position it in the beaker so the stir bar will not hit the electrode. Make sure the silver wire is submerged in solution. Connect the black lead of the voltage probe to the reference electrode and the red lead of the voltage probe to the silver wire. Begin collecting data in Logger Pro by clicking Start. 6. Obtain approximately 25 ml of a 0.10 M stock AgNO 3 solution. Add exactly 2 ml of AgNO 3 solution to the reaction beaker and wait for the potential to stabilize. Once stabilized, click Keep to record the data point. 7. Repeat this procedure, adding 2 ml aliquots of AgNO 3 solution at a time to the beaker until you ve added a total of 20 ml of AgNO 3. Record the stabilized potential after each 2 ml aliquot added. Click Stop once finished and save your data. 8. In the end, the total volume in the beaker should be 120 ml. Calculate the concentration of Ag + ion after each step. Part A Cleanup 1. DO NOT throw any solution in the sink. All waste should be placed in the appropriate container in the fume hood. 2. Rinse the reference electrode thoroughly with DI water. 3. Place the reference electrode back into its container making sure the Vycor frit is in solution. 4. Wash and dry the copper wire and silver wire. Return the entire package to the stockroom. Page 6 of 8

7 Part B. Electrochemical Plating of Zn on Copper 1. Mass a copper strip on a watch glass and record the mass. 2. 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 an aluminum strip (anode) and the positive pole of a 9.0 V battery. Finally attach the negative pole of the current probe and the 9.0 V battery with a third wire (Figure 2). 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. a) b) Figure 4. a) An aluminum electrode and b) the electrochemical plating setup 3. 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 flat-lined 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.) 4. 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. Page 7 of 8

8 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). 5. 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 8 of 8