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  %  

only  

2009  -­‐  2010   33  %   58  %  

2010  -­‐  2011   22  %   65  %  

2009  -­‐  2010  

2010  -­‐  2011  0  

5  

10  

15  

20  

25  

30  

35  

40  

45  

1   2  3  

4  5  

6  2009  -­‐  2010  2010  -­‐  2011  

0  

10  

20  

30  

40  

50  

1   2   3   4   5   6  

2009  -­‐  2010  2010  -­‐  2011  

0  

10  

20  

30  

40  

1   2   3   4   5   6  

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