Electrochemical Cell Zinc Copper Demonstration and AACT Simulation Galvanic (Voltaic) Cells

Category: 
Demonstration
Topics: 
Electrochemistry

In this interactive activity, the instructor demonstrates several aspects of galvanic cells using real electrochemical cells and a computer simulation. Students view an operating zinc-copper galvanic cell with a voltmeter measuring a voltage of +1.10 Volts. The salt bridge is taken out of the cell, the voltage drops to zero. When placed back into the cell, the voltage returns to +1.10 V.  Students view and or interact with a Voltaic cell simulation (AACT, 2020) including animations of the dynamic processes occurring simultaneously at the zinc anode, copper cathode, in the aqueous Zn2+ and Cu2+ solutions, the NaNO3(aq, saturated) solution in the salt bridge.

A standard cell comprising of two half-cells: zinc metal electrode in 1.0 M ZnSO​4 solution, a copper metal electrode in a 1.0 M CuSO4 solution, and a connecting salt bridge.  

The cell reaction is  Zn(s) +  Cu2+(aq)  --> Zn2+(aq) + Cu(s)   E°cell = +1.10 Volts.

The standard cell potential, E°cell, is the standard reduction potential of the cathode reaction, minus the standard reduction potential of the anode reaction:

E°cell = E°cathode (as a reduction potential) - E°Anode (as a reduction potential)  = +0.34 V - (-0.767 V)=+ 1.10 V

 
  

ACS AACT voltaic cell computer simulation.    Greenbowe, T.J.; Gelder, J.I., Boyd, A, Wixon, M. (2020). Galvanic/Voltaic Cells 1. American Chemical Society, American Association of Chemistry Teachers, Washington, D.C. https://teachchemistry.org/classroom-resources/voltaic-cells (accessed April, 2023).

If you use the simulation, cite the simulation.  This simulation may not be used to support a for profit instructional lesson.

Web page Author: T. Greenbowe, University of Oregon.   This page is under construction.

 

Materials 
  • 2 ea. 400 mL beakers, one containing a strip of zinc metal partially immersed in a 1.0 M ZnSO4 solution, the other containing a strip of copper metal partially immersed in a 1.0 M CuSO4 solution.
  • a salt bridge comprising a U-tube filled with 2 M KNO3 solution or 2 M K2SO4(aq, sat'd.).  The ends of the U-Tube should be tightly plugged with cotton that has been saturated in the KNO3 solution or the U-tube has sintered frits at each end.
  • a Vernier voltage probe.
  • a Vernier "Go Link" interface.
  • an empty 250 mL beaker to hold the salt bridge upright when not in use.
  • a computer with the Vernier "Logger Lite" program that is capable of projecting the screen image.

Optional Materials (available)

  • Blank Electrochemical Cell Diagram
  • POGIL Electrochemical Cell Activity or POGIL-ish Electrochemical Cell Activity Sheet
  • Power Point Presentation
  • Clicker Questions
Procedure 
  • Place the two beakers side by side.
  • Connect the probe leads to the metal strips where they stick up above the surface of the solutions.
  • Place the salt bridge upside down so that one arm is in each of the solutions.
  • Students should note that no voltage registers until the salt bridge is inserted into the solutions.
Safety Precautions 
  • Always wear goggles when performing chemistry demonstrations.
  • Gloves should be worn to protect your hands from the solutions.
 

Electrochemical cell ZnCu Equipment SetUp Diagram

This presentation/lesson is consistent with the principles of Universal Design for learning in that multiple representations are incorporated in the presentation. This presentation also provides an opportunity for students to make connections among the macroscopic, particle level, and symbolic levels of representation (Johnstone, 1982, 1991, 1993) associated with electrochemical cell processes. The AACT simulation has particle level animations of what occurs at the surface of the electrodes, migration of ions in the aqueous solution, oxidation and reduction half-reactions, and migration of ions in the salt bridge.

Students view a computer simulation of the ZnCu cell showing the dynamic oxidation half-reaction process occurring at the anode and then the dynamic reduction half-reaction process occuring at the cathode.

ACS AACT voltaic cell Galvanic/Voltaic Cells 1 computer simulation. Greenbowe, T.J.; Gelder, J.I., Boyd, A, Wixon, M. (2020). Galvanic/Voltaic Cells 1.  American Association of Chemistry Teachers, American Chemical Society: Washington, D.C.   

https://teachchemistry.org/classroom-resources/voltaic-cells

If you use the simulation, cite the simulation.  The simulation may not be used in any lesson sold for profit.

Next, students view a particle level animation of what occurs in the salt-bridge containing aqueous saturated potassium nitrate.  The cations and anions migrate in opposite direction. The cations migrate toward the cathode and the anions migrate toward the anode.  No electrons are observed in the solution in the salt bridge or in the aqueous solutions.  In the aqueous solutions (and the saturated solution in the salt bridge) migration of cations and anions in opposite direction constitutes a current.

ACS AACT voltaic cell Galvanic/Voltaic Cells 1 computer simulation. Greenbowe, T.J.; Gelder, J.I., Boyd, A, Wixon, M. (2020). Galvanic/Voltaic Cells 1.  American Association of Chemistry Teachers, American Chemical Society: Washington, D.C.

Students record the cell potential from the voltmeter and use their data to determine the reduction potential of each half reaction. Students will also identify anodes and cathodes, write half reaction equations and full chemical equations, and draw what is happening in each half cell and the salt bridge on a molecular scale on a provided blank cell diagram.

Electrochemical cell Blank Cell Diagram

There is an activity sheet to accompany this presentation.

Example of student work

Electrochemical cell Diagram ZnCu cell student work

The cell reaction is  Zn(s) +  Cu2+(aq)  --> Zn2+(aq) + Cu(s)   E°cell = +1.10 Volts.

E°cell = E°cathode (as a reduction potential) - E°Anode(as a reduction potential)  = +0.34 V - (-0.767 V)=+ 1.10 V

The simulation has a limited number of metals and solutions to construct galvanic/voltaic cells with different electrodes.

Note
In the ACS AACT voltaic cell simulation, the molecular view only shows the metal atoms and ions, as those are the ones that have the potential to change. The anion is the same – nitrate – for all solutions in the simulation (except for the animation of the standard hydrogen electrode which uses hydrochloric acid, in which case chloride ions are shown) and doesn’t change in the single replacement reactions, so it is excluded for clarity. Similarly, water molecules are not shown as they do not change either and would far outnumber the ions in the solution. Without these spectator species shown, it is easier for students to see what changes are occurring, but you could have a discussion with students about what else is present in the beakers.

Curriculum Notes 

 Prior to doing these electrochemical cells demonstrations, it is recommended that the demonstration showing the reaction of zinc with CuSO4(aq) and the no reaction of copper with ZnSO4(aq) be shown and discussed. This is a great demo to introduce the concept of electrochemical cells.   Pair this demonstration with computer animations showing a representation of the oxidation half-reaction occurring at the zinc anode and the reduction half-reaction occurring at the copper cathode.  

Overview of the Chemistry

Due to a differences in electromotive force between zinc and copper, zinc is a more active metal compared to copper, a spontaneous oxidation-reduction process occurs when the zinc electrode is connected to the copper electrode and a salt bridge inserted between the two half-cells. The spontaneous flow of electrons from anode to cathode and the migration of cations and anions in the solutions and salt bridge generates a current with a voltage near the theoretical Eo cell = 1.10 V at room temperature.

                 Eoreduction (Volts)

 Cathode  Cu2+ + 2 e- → Cu    + 0.34

  Anode   Zn2+ + 2 e- →  Zn    - 0.76

To determine the cell emf, we take the difference between reduction potentials. The cell potential is the potential difference between two half-cell reactions in a voltaic cell. In order to obtain this difference we need to "compare apples to apples".

The cell reaction is  Zn(s) +  Cu2+(aq)  --> Zn2+(aq) + Cu(s)   E°cell = +1.10 Volts.

E°cell = E°cathode (as a reduction potential) - E°Anode(as a reduction potential)  = +0.34 V - (-0.767 V)=+ 1.10 V

Reduction potential number line for ZnCu Ecell
 
 

    Additional Video Links leading to electrochemical cells

    http://www.youtube.com/watch?v=0oSqPDD2rMA

    Computer animations for the  Zn/Cu Voltaic cell   beta versions  (drafts) University of Oregon

    Zn|Zn2+   oxidation half-reaction at the zinc electrode   https://vimeo.com/220550690            

    Cu2+|Cu  reduction half-reaction at the copper electrode   https://vimeo.com/220550267

    animation of the migration of ions in the salt-bridge  https://vimeo.com/220548484

    animation of the movement of electrons in a wire   https://vimeo.com/220550589

    An alternative to the AACT Voltaic Cells simulation is a zinc-copper electrochemical cell animation with narration created by Chris Singh.

    electrochemical cell zinc anode particle animation image1831

    Some student Difficulties with Galvanic Cells or Electrochemical Cells

    1. Cell potentials are obtained by adding individual reduction potentials

    2.  Anodes, like anions, are always negatively charged and release electrons, and cathodes, like cations, are always positively charged and attract electrons.

    3. The anode is positively charged because it has lost electrons. The cathode is negatively charged because it has gained electrons.

    4.  Electrons flow through the salt bridge and the electrolyte solutions to complete the circuit,

    Learning Objectives
     
    1. Given a diagram of a simple electrochemical cell involving two metal electrodes and the corresponding solution of the metal ions identify: the site of oxidation reduction, the anode, the cathode, movement of electrons, migration of ions, the chemical equation representing the cell reaction.
     
    2. Calculate the emf of a cell, given a table of standard reduction potentials. 
     
    3. Draw a particle diagram representing the dynamic events occurring at each electrode and in the salt-bridge.
     
    4. Describe how the electrons flow through the electrode and wire of voltaic cell and how cations and anions migrate in opposite direction to create an electric current.
     
    AP Chemistry
    ENE-6: Electrical energy can be generated by chemical reactions.
     
    TRA-1.B: Represent changes in matter with a balanced chemical or net ionic equation: a. For physical changes. b. For given information about the identity of the reactants and/or product. c. For ions in a given chemical reaction.
     
    ENE-6.A: Explain the relationship between the physical components of an electrochemical cell and the overall operational principles of the cell.

    References

    Cole, M. H., Fuller, D. K., & Sanger, M. J. (2021). Does the way charges and transferred electrons are depicted in an oxidation–reduction animation affect students’ explanations? Chemistry Education Research and Practice, 22(1), 77–92.

    Greenbowe, T.J. (1994).   "An interactive multimedia software program for exploring electrochemical celIs."  Journal of Chemical Education, 71(7), 555.

    Greenbowe, T.J.; Gelder, J.I., Boyd, A, Wixon, M. (2020). Galvanic/Voltaic Cells 1. American Chemical Society, American Association of Chemistry Teachers, Washington, D.C. https://teachchemistry.org/classroom-resources/voltaic-cells

     Sanger, M.J. and Greenbowe, T.J. (1997).  “Student Misconceptions in Electrochemistry: Current Flow in Electrolyte Solutions and the Salt Bridge.” Journal of Chemical Education, 74(7), 819-823.

     Sanger, M. J. and Greenbowe, T.J.  (1997).   “Common Student Misconceptions in Electrochemistry: Galvanic, Electrolytic, and Concentration Cells.” Journal of Research in Science Teaching, 34(4), 377-398.

    Sanger, M.J. and Greenbowe, T.J. (1999).  “An Analysis of College of Chemistry Textbooks as Sources of Misconception and Errors in Electrochemistry.”  Journal of Chemical Education, 76(6), 853-860.

    Sanger, M. J.; Greenbowe, T. J; Addressing student misconceptions concerning electron flow in aqueous solutions with instruction including computer animations and conceptual change strategies. International journal of science education, 2000, Vol.22 (5), p.521-537.

    Shakhashiri, B. Z. In Chemical Demonstrations: A Handbook for Teachers of Chemistry; The University of Wisconsin Press: 1992; Vol. 4, p 101-106.

    de Jong O. and Treagust D. F., (2002), The teaching and learning of electrochemistry, in Gilbert J. G., de Jong O., Justi R., Treagust D. F. and van Driel J. H. (eds.), Chemical education: towards research based practicale, Dordrecht: Kluwer, pp. 317-338. 

    Abraham, M.; Gelder, J.; Greenbowe, T. (2007).  During Class Inventions and Computer Lab Activities for First and Second Semester General Chemistry. Hayden-McNeil: Plymouth, MI.

    Johnstone, A.H.  (1993). "The development of chemistry teaching: A changing response to changing demand. " Journal of Chemical Education, 70(9), 701-705.