# Electrolytic Cell: Plating Copper on Copper Demonstration

Electrolytic Cell: A postage balance is used to measure the mass of two copper electrodes prior to the demonstration.  The electrodes are placed in 1.0 M CuSO4(aq) solution. The top of one Cu electrode is connected to the negative terminal of a D.C. Power Supply.  The other Cu electrode is connected to the positive terminal of the D.C. Power Supply.  When current is applied to the electrolysis cell copper(II) ions in solution are reduced to copper atoms at the cathode.   Copper atoms on the anode are oxidized to copper(II) ions.  The cathode gains mass, the anode looses mass.  When the experiment ends, the electrodes are dried and the mass of each electrode weighed on the mini-balance.  The time to plate the metal and the the current (amps) applied is used to calculate the theoretical amount of mass that should plate on the cathode.  The actual mass obtained is compared to the theoretical amount calculated.

During the operation of the electrolysis cell, students would work to calculate the theoretical mass of copper plated on the cathode.  Students also can predict the moles of electrons passed, the moles of copper plated, and the mass of copper plated when the current is applied for half of the time.

Table 17.7  Sample Data and Results of Calculations of a Copper-Copper Electrolysis Cell

 Reduction Half-reaction Current (A) Time (s) Moles e- Moles Metal Mass of Metal (g) Cu2+ + 2e- -> Cu 8.00 900.0 0.0746 0.0746 Cu2+ + 2e- -> Cu 8.00 450.0 0.0373 0.0373

The effectiveness of this Electrolysis Cell demonstration can be enhanced when it is accompanied by an electrolysis cell computer simulation and computer animation at the particulate level (atom level).

Curriculum Notes

This demonstration, accompanied with the Class Activity and Electrolysis Computer Simulation, serves well as an introduction to the basic principles of electrolysis.  Students easily see and understand 1) the function of the DC Power Supply (forces electrons into the cathode and pulls electrons out of the anode) and what half-reactions occur at each electrode.  Explain to students this activity serves as an exercise to the principles of electrolysis because most students do not see any logical reason why one would want to plate copper metal onto a copper electrode.  Mentioning refining copper metal and showing a photo of refining copper metal by electrolysis helps to set the context for this activity.  We have found students understanding of electrolysis increasing when the details of electrolysis are introduced by first exploring a copper-copper electrolysis cell.

With respect to the active learning Class Activity, provide students with the "Electrolysis Model", physical constants, and an empty diagram of an electrolysis cell.  Have students complete the electrolysis cell diagram as the instructor sets-up the demonstration.  Have students identify the flow of electrons into and out of the DC Power supply, the direction of ion migration in the cell, half-reactions occurring at each electrode, the cathode, the anode, and which electrode gains gains mass.

This demonstration, when paired with the electrolysis computer simulation, provides a great opportunity for students to experience the three levels of representation: microscopic, macroscopic, and symbolic (Johnstone's Triangle).

Student Difficulties (Misconceptions) with Electrolysis Associated with this Demonstration

1. When identical electrodes are used in electrolysis, the same reactions occurs at both electrodes and the products are the same at both electrodes.

2. The power source used in an electrolytic cell pulls electrons in at the negative terminal and pushes electrons out at the positive terminal.

3. Electrons move through electrolytes by being attracted to positive ions in the solution.

4. No reaction will occur if inert electrodes are used.

5. In electrolytic cells, oxidation occurs at the cathode and reduction occurs at the anode.

6. In electrolytic cells, water is unreactive toward oxidation and reduction.

7. When predicting an electrolytic reaction, the half-cell reactions are reversed prior to combining them.

8.The calculated cell potentials in electrolytic cells can be positive.

9.  In electrolytic cells, the direction of the applied voltage has no effect on the reaction or the site of the anode and cathode.

Learning Objectives

1.  Given a diagram of an electrolytic cell, identify the anode, cathode, direction of which electrons and ions move, the location of the oxidation half-reaction, the location of the reduction half reaction.

2.  Given a description or a diagram of an electrolytic cell, write the oxidation half-reaction and the reduction half-reaction.

3.  Relate the amount of product(s) generated in an electrolytic cell to the stoichiometry of the reduction half-reaction and to the amount of electrical charge passed in the cell.

4.  At the particle level of representation (atom level), show how the number of electrons involved in a single reduction half-reaction, i.e. Cu2+ + 2e- -> Cu, scales up to the mole level: i.e. 1 mole Cu2+ 2 mole e-> one mole Cu.

5. Calculate the mass of product produced during electrolysis given the stoichiometry, the amount of electrical current passed in a specific time in the cell..

6. Calculate the quantity of of charged passed in an electrolytic cell, given the stoichiometry, and the amount of electrical current passed in a specific time in the cell..

7.  Determine the relationship among coulombs, faradays, time, and reduction-half reaction for an electrolysis cell.

Materials

two clean copper electrodes, 1.0 M CuSO4(aq), electrolysis cell, red and black wire leads, DC Power supply, ampmeter, stop watch, mini electronic balance to measure mas

Footnotes

References

Sia, D., Treagust, D., Chandrasegaran, A. (2012).  “High school students’ proficiency and confidence levels in displaying their understanding of basic electrolytic concepts."  International Journal of Science And Mathematics Education, Dec, Vol.10(6), pp.1325-1345.

Sanger, M.J. and Greenbowe, T.J. (2000). “Addressing Student Misconceptions Concerning Electron Flow in Electrolyte Solutions with Instruction Including Computer Animations and Conceptual Change Strategies.” International Journal of Science Education22, 521-537.

Sanger, M.J. and Greenbowe, T.J. (1997).  “Student Misconceptions in Electrochemistry: Current Flow in Electrolyte Solutions and the Salt Bridge.” Journal of Chemical Education74(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 Teaching34(4), 377-398.

Garnett, P. J., & Treagust, D. F. (1992). Conceptual difficulties experienced by senior high school students of electrochemistry: Electrochemical (galvanic) and electrolytic cells. Journal of Research in Science Teaching, 29(10), 1079-1099.

Prep. Notes

two clean copper electrodes, 1.0 M CuSO4(aq), electrolysis cell, red and black wire leads, DC Power supply, ampmeter, stop watch, mini electronic balance to measure mass

### Topics:

© Copyright 2012 Email: Randy Sullivan, University of Oregon Chemistry Department and UO Libraries Interactive Media Group