StudentUnderstandingoftheElectrochemicalCellandtheSaltBridge’ · PDF...
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Transcript of StudentUnderstandingoftheElectrochemicalCellandtheSaltBridge’ · PDF...
Student Understanding of the Electrochemical Cell and the Salt Bridge Marta K. Maroń and Robert P. Parson
University of Colorado, Department of Chemistry and Biochemistry, Boulder, CO 80309
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Use the electrochemical cell shown below, which has a +1.08 volts for its emf, to answer quesIons 1 and 2.
1. Select all the diagrams which apply from below that depict on a micro level what is occurring at the salt bridge in the Cu|Cu2+ half-‐cell?
2. Select all the diagrams which apply from below that depict on a micro level what is occurring at the salt bridge in the Zn|Zn2+ half-‐cell?
only only
2009 -‐ 2010 31 % 62 % 18 %
2010 -‐ 2011 21 % 62 % 27 %
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2009 -‐ 2010 33 % 58 %
2010 -‐ 2011 22 % 65 %
2009 -‐ 2010
2010 -‐ 2011 0
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6 2009 -‐ 2010 2010 -‐ 2011
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2009 -‐ 2010 2010 -‐ 2011
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4. On a scale of 1 – 6 how confident do you feel in your understanding of electrochemistry, where 1 represents very liXle confidence and 6 represents a lot of confidence?
5. On a scale of 1 – 6 how confident do you feel in you understanding of what the purpose of the salt bridge is, where 1 represents very liXle confidence and 6 represents a lot of confidence?
Very Li:le Confidence A Lot of Confidence 1 2 3 4 5 6
Sample QuesEons and Student Responses from E-‐Cell QuesEonnaire
Lab 6 – Page 5 of 8
STUDENT ASSIGNMENTS I. Pre-laboratory Preparation Prior to attending the laboratory and recitation period, complete the following activities in an organized manner your laboratory notebook. This assignment is due at the beginning of the recitation period. A. Introductory Statement
In one paragraph, summarize in your own words (not the lab manual’s), the purpose, theory and procedure for this laboratory exercise.
B. Pre-laboratory Questions / Activities
1. Copy the general setup of a voltaic cell shown below into the Pre-lab section of the lab notebook. Label this general cell for the Cu(NO3)2 and ZnSO4 cell to be constructed in Part 2 of the lab. Label the key components of this cell; use your lecture notes, the figure in the introduction to this lab or Figure 21.5 in the Silberberg text as a guide. Be sure to indicate the direction of electron flow through the external circuit and the movement of ions in the salt bridge. Write a balanced reaction for the anode event and for the cathode event.
2. In Part 3 you will construct several cells where both compartments contain a copper electrode and a copper(II) solution. What cell potential (E°cell) would be expected for a cell comprised only of copper? In lab you’ll find that the cell actually runs! Explain how a “concentration cell” such as this runs. What drives the movement of electrons from anode to cathode? Under what conditions will the cell stop?
3. In Part 4 inactive electrodes will be used. Of what material are these made? Why is such an electrode labeled as “inactive”? What other material could be used for an inactive electrode?
Lab 6 – Page 3 of 8
Part 2. Measuring Standard Cells Potentials Construct a voltaic (galvanic) cell using cell compartments I and II (Cu(NO3)3 and ZnSO4). Place a copper metal strip into the beaker containing copper (II) nitrate and a zinc strip into the beaker containing zinc (II) sulfate. Bridge the two beakers with the salt bridge; ensure that the ends of the salt bridge are in the solution within each compartment adding more solution if necessary. Attach the voltmeter leads to each metal strip with alligator clips so that a positive cell potential is read; if the potential is negative, switch the leads. For this cell, the copper electrode is the cathode (positive) and the zinc electrode is the anode (negative). Record the color of the lead (red or black) going to copper and going to zinc. Based on this association between lead color and electrode type, you will be able in subsequent cell measurements to identify the cathode and the anode for the cell. Now remove the salt bridge and note observations about the cell performance in the Data Section. Replace and remove the salt bridge several times; note the cell potentials. When you’ve completed work with this cell, remove the salt bridge, rinse the ends with distilled water and carefully shake off the excess water. Do this after each measurement. If you notice a precipitate in a half cell, it is caused by contamination from an inadequately rinsed salt bridge. Now measure the cell potential of the other combinations of the three compartments: I/III and II/III. Connect the leads to the electrodes so that the cell potential reading is positive indicating a spontaneous redox event. If the display reads a negative potential, switch the leads to get a positive potential. Knowing the association made earlier regarding the color of the lead and the electrode type, you can determine which electrode is the cathode and which is the anode. Identify each metal as either the cathode (+) or the anode (-). When all combinations of cells have been measured, discard the contents of cells II and III in the appropriate waste container. Retain compartment I for use in Part 3. Rinse the electrodes with distilled water and dry them with a Kimwipe.
Before going on to Part 3, answer the following questions in the Data Section for Part 2.
1. Based on your observations of cell operation in the presence and in the absence of
the salt bridge, make a statement regarding its necessity in a voltaic cell. 2. What exactly is the role of the salt bridge in the cell operation? Be sure to address the
movement of ions through the salt bridge and into each cell compartment. 3. What is the role of the filter paper in the salt bridge? Do you think the bridge would
work as well without the filter paper?
Lab 6 – Page 9 of 8
IV. Post-Laboratory Assignment 1. Using your work in Pre-Lab Question 1 as a guide, draw the cell based on
Compartments I and VII from Part 4 of the lab. Label all key components and illustrate the flow of electrons as well as ions through the salt bridge.
a. If Compartment I was replaced by Compartment VIII, what type of cell
would this become? b. Which compartment would be the anode and which would be the cathode? c. What material could be used for the electrode in Compartment VIII?
2. Typically, in a cell drawing the anode is shown on the left and the cathode on the
right. Indicate the direction of flow for each of the following in a cell illustration where the anode was the placed on the right and the cathode on the left.
a. The flow of electrons b. The flow of cations through the salt bridge c. The flow of anions through the salt bridge
What change in the potential shown on the voltmeter would be expected in a cell with the anode on the right and the cathode on the left?
V. Results and Conclusion
Write a paragraph summarizing your results and what you determined during the exercises you participated in. Make sure to discuss your results. Include errors that occurred (human error will not be accepted) as well as interesting or significant implications of the information you obtained. Also note that several of the cells investigated in this exercise were the same cells qualitatively studied in last week’s lab. You’ll want to review the work from that exercise and comment on its connection to this week’s work.
Introduc)on …. The goal of the work presented here is to determine the degree to which a laboratory experiment contributes to students’ understanding of an electrochemical cell. Our iniIal results showed that while most of our students could answer quanItaIve quesIons about the operaIon of the cell, their conceptual understanding of the microscopic processes that occur within the cell was inconsistent. In parIcular, we noIced that while many students were able to correctly describe the events that take place at the surface of the anode and cathode, their understanding of the events that take place at the salt bridge was lacking. One prominent misconcepIon was a belief that electrons flow through the salt bridge from the anode to the cathode to complete the circuit. These observaIons are consistent with results reported in the Chemistry EducaIon literature. An analysis of the laboratory manual for this experiment revealed that relaIvely liXle aXenIon was devoted to qualitaIve issues in general and to the role of the salt bridge in parIcular. This led us to revise the experimental procedure by incorporaIng quesIons and operaIons designed to sImulate criIcal thinking within the laboratory environment. Experimental Design… The subjects involved in this study were enrolled in a General Chemistry II class at the University of Colorado, Boulder. The experiment was carried out over the course of several terms, and for the final results we compared two classes taught by the same instructor using the same course materials, homework assignments, etc. The invesIgaIon was conducted in the General Chemistry Laboratory in three phases, as shown below. (1) IdenIficaIon of misconcepIons. -‐ Students were given two ten minute quesIonnaires, each at the beginning of a laboratory period.
* The first quesIonnaire was given immediately before carrying out the laboratory experiment.
* The second quesIonnaire was given one week later, ager the students completed their post-‐laboratory assignment. (2) IdenIficaIon of possible sources or causes of the misunderstanding.
-‐ Based on student responses to the two quesIonnaires, the laboratory procedure was analyzed for possible sources that might contribute to the observed misconcepIons. -‐ A new quesIonnaire was developed that focused on the common misunderstandings of students. (3) Changes were made to the laboratory procedure so as to explicitly target the observed misconcepIons. These changes included new conceptual pre-‐
and post laboratory quesIons, instrucIons to students to explore the role of the salt bridge, and criIcal thinking quesIons interpolated within the laboratory procedure.
Summary … In this invesIgaIon, we were able to confirm the misconcepIons reported and idenIfied in previous studies. Our results suggest that a relaIvely modest, incremental revision of the experiment resulted in a reducIon in the prevalence of these misconcepIons, and helped the students to develop a consisted molecular-‐scale picture of the processes that occur within an electrochemical cell. Acknowledgment … Acknowledgment from R. P. and M. K. M. is made to the SEI for support of this work. R. P. and M. K. M. would also like to thank everyone who cooperated in this study, the General Chemistry II students, the course instructors (Dr. Margaret Asirvatham, Dr. Veronica Bierbaum, Dr. Susan Hendrickson, Dr. David Jonas, Dr. ChrisIne Kelly and Dr. MaXhew Wise), lab staff (Laurel Boni-‐Hyde, Alan Foster, and Hannah Robus), and the graduate TA’s. References …
1. Sanger, M. J. and Greenbowe, T. J. “Students’ MisconcepIons in Electrochemistry: Current Flow in Electrolyte SoluIons and the Salt Bridge” Journal of Chemical Educa2on, 1997, 74(7), 819 – 823.
2. Ogude, A. N. and Bradley, J. D. “Ionic ConducIon and Electrical Neutrality in OperaIng Electrochemical Cells” Journal of Chemical Educa2on, 1994, 71(1), 29 – 34.
3. Teichert, M. A., Tien, L. T., Anthony, A., and Rickey, D. “Effects of Context on Students’ Molecular-‐Level Ideas” Interna2onal Journal of Science Educa2on, 2008, 30(8), 1095 – 1114.
Highlights of Changes to Laboratory Exercise 6 – Voltaic Electrochemical Cells