Electrolytic Cell: Plating Zinc on Copper Demonstration

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A zinc electrode and a copper electrode are placed in a beaker containing an aqueous solution of zinc sulfate, ammonium citrate, and ammonium chloride.  The electrodes are connected to a DC power supply and a voltage is applied - the negative lead on the copper electrode and the positive lead on the zinc electrode.  A white coating appears on the copper electrode almost immediately, and after a few minutes, the copper electrode has a definite zinc plating on it. This demonstration is an application of Faraday's Law.

Table 17.8  Sample Data and Results of Calculations of a Copper-Zinc Electrolysis Cell 

Reduction Half-reactionCurrent (A)Time (s)Moles e-Moles MetalMass of Metal (g)
Zn2+ + 2e- -> Zn8.00900.00.0746 0.0746 
Zn2+ + 2e- -> Zn8.00450.00.03730.0373 
      
      

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).

The Electrolysis Computer Simulation provides an opportunity to construct and experiment with several metal-metal electrolytic cells.  Choose metal electrodes, electrolyte, current and time.  The simulation shows the mass deposited at the cathode (and the mass lost at the anode) and animations at the particle level of what occurs at each electrode.

http://media.pearsoncmg.com/bc/bc_0media_chem/chem_sim/html5/Electro/Electro.php

Greenbowe, Abraham, Gelder Chemistry Education Instructional Resources ©2016.  University of Oregon, University of Oklahoma, Oklahoma State University, Pearson.

It may be helpful to show students a short animation of a representation of the movement of electrons in a circuit with a battery

https://www.lightrocket.com/russellkightley/galleries/go/12734/electricity-animations 

Curriculum Notes 

This demo can be used to illustrate a non-spontaneous electrochemical process when teaching a unit on electrochemistry: electrolysis.  Have students predict which terminal of the DC Power supply the zinc metal electrode should be connected and which terminal to the DC Power Supply the copper electrode should be connected. The zinc electrode looses mass and the copper electrode gain mass. The mass lost at the zinc electrode is equal to the mass gained at the copper electrode, however drying and weighing the mass of each electrode is not feasible during a lecture presentation.

Recommendations: Sequence of activities.  Ask student to predict if the following reactions will occur or not and have the students offer an explanation: Cu(s) + ZnSO4(aq) --> ?  and Zn(s) + CuZnSO4(aq) --> ?    Calculate E°rxn for  each possible reaction. After students predict and explain, show students a piece of copper metal placed in aqueous zinc sulfate results in "no reaction":  Cu(s) + ZnSO4(aq) --> No Reaction.  Show students the basic set-up for an electrolysis cell, but do not connect the metal electrodes.  Have students decide which electrodes connect to which terminal of the D.C. Power Supply.   A Class Activity or a POGIL Activity will have a diagram  of the electrolysis cell with space for students to write in the parts of the cell and to show movement of electrons, ion migration, and half-reactions can accompany this demonstration.  Use a clicker question to have students  indicate the parts of the electrolysis cell and to identify the anode and cathode. It is recommended that this electrolysis demonstration be paired with the electrolysis simulation.  The simulations will show a computer animation of a representation of the reduction half-reaction reaction occurring at the cathode copper electrode (Zn2+ + 2e- -> Zn)  and the oxidation occurring at the anode  zinc electrode (Zn -> Zn2+ + 2e-).   The next lecture, Quiz questions can be used to assess students' understanding of basic electrolysis concepts and calculations. Two sample test questions assess students' conceptual understanding of electrolysis are posted.

LEARNING OBJECTIVES   After viewing the electrolysis demonstration and the accompanying computer simulation students should be able to:

1.  Given an empty block diagram of a metal metal electrolysis cell, identify and or label the parts of an electrolysis cell: type of metal at each electrode, the movement of electrons going out of and into the D.C. Power Supply (or battery), the half-reactions occurring at each electrode, the anode, the cathode.

2. Select the terminal of a D.C. Power Supply or battery ("+" or "-") the electrode serving as the cathode should be connected.  The metal will be deposited on which electrode? 

3.  Given the number of electrons "passed through" an electrolysis cell, determine and compare the number of metal ions reduced to metal atoms at the cathode.  Students will compare M+, M2+, and M3+ cations receiving the same "charge" (same number of electrons).

4. Describe qualitatively how the time and current "passed through" an electrolysis cell influences the amount of metal deposited on the cathode (moles and mass), taking into account the charge of the cation.

5. Calculate the moles and mass of product deposited on a cathode of an electrolytic cell, given the half-reactions, time and current of the cell.

Lead Time 
One day of lead time is required for this project.
Discussion 

Connecting the negative lead to the copper electrode supplies electrons to the copper electrode.  The Zn​2+ ions in solution near the copper electrode gain electrons to form zinc atoms.  The zinc atoms are platted out as metallic zinc, Zn2+ (aq) + 2e- -> Zn (s), on the copper electrode.

Connecting the positive lead to the zinc electrode removes electrons form the zinc electrode.  The Zn atoms form Zn​2+ ions   Zn (s) ->  Zn2+ (aq) + 2e- .  The Zn​2+ ions generated at the anode replace the Zn​2+ ions in solution being reduced to zinc atoms at the cathode.

The ammonium chloride improves conductivity, which allows us to plate rapidly at a low voltage.  The ammonium citrate acts as a buffer to maintain acidic conditions.

Materials 
  • 12 g ammonium citrate, (NH4)2HC6H5O7
  • 7.5 g ammonium chloride, NH4Cl
  • 30.0 g zinc sulfate heptahydrate, ZnSO4·7H​2O
  • ~300mL DI water
  • zinc electrode
  • copper electrode
  • 400 mL beaker
  • steel wool
  • DC voltage source with leads and alligator clips
  • voltmeter with leads
Procedure 
  • Place the electrodes in the beaker containing the plating bath.  Be sure that the electrodes do not touch each other.
  • Connect the DC power source to the electrodes and the voltmeter to the DC power source.
  • Turn on the power source, adjust it to about 1 V, and let it run for 2-10 minutes.
  • Turn off the DC power source.
  • Remove the copper electrode, dry it off with a paper towel, and buff it lightly with steel wool.
  • Exhibit the plated electrode to the class.
Safety Precautions 
  • Wear goggles when performing this demo.
  • Wear gloves when handling the wet electrode.
  • Dispose of all chemicals appropriately.
Footnotes 

This demo is a modification of "Galvanizing: Zinc Plating," from Bassam Z. Shakhashiri's Chemical Demonstrations, A Handbook for Teachers of Chemistry, v.4, (U. of Wisconsin, 1983) pp. 244-246.

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 Education, 22, 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 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.

Lee. R. Summerlin, Christie L. Borgford, and Julie B. Ealy, “Electroplating Copper,” Chemical Demonstrations, A Sourcebook for Teachers, Volume 2 (Washington: American Chemical Society, 1988), pp. 199−200. A stainless steel spoon is electroplated with copper in this demonstration. 

Prep. Notes 

The bath can be prepared by dissolving each of the solutes in about 300 mL of DI water in the 400 mL beaker.

The electrodes should be polished with steel wool before being placed in the bath.

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