Electrolysis Active Learning Instructional Sequence

Electrolysis Active Learning Instructional Sequence  using a) an instructional activity sheet, b) the electrolysis computer simulation, c) electrolysis lecture demonstrations, and d) PowerPoint Notes to guide the flow of instruction.  All of the individual components of this instructional sequence are available on this University of Oregon Chemdemos web site.

Overview of the Electrolysis Instructional Activity

There are several parts to this instructional activity.  Students observe that when copper metal is placed in 1.0 M zinc nitrate solution there is no reaction.  The goal is for students to design an electrolysis cell to plate zinc metal on a copper spoon.  First students are introduced to a simple electrolysis cell involving two copper electrodes placed in 1.0 M CuSO4(aq) by observing a demonstration of the cell and by working with a computer simulation of the cell.  Each electrode is connected to a direct current Power Supply or Battery.  Students explore this cell.  One concept is introduced at this point: it takes two electrons to reduce one copper(II) ion, it takes four electrons to reduce two copper(II) ions.  Students diagram the CuCu electrolysis cell.  Students calculate the number of electrons passed in the circuit based on the mass of copper deposited.

Next students are presented with the task of designing an electrolysis cell to plate zinc metal on a copper spoon.  Students diagram the ZnCu electrolysis cell, set-up an actual electrolysis cell, and work with a computer simulation to set-up and test an electrolysis cell.  Students decide which electrode is connected to the "+" terminal of the DC Power Supply and which electrode is connected to the "-" terminal of the DC Power Supply.  

Overview of the Sequence of Instruction

This sequence of instruction follows a three stage learning cycle: exploration, concept introduction, application - following the principles of instructional design in the guided-inquiry pedagogy.

1. Lecture demonstration. Place copper metal in 1.0 M zinc nitrate solution.  Show that the copper will not react with the zinc(II) ions and have students calculate E° for this theoretical reaction.

Cu(s) + Zn2+(aq) --> No reaction   

Cu(s) + Zn2+(aq) --> Zn(s) + Cu2+(aq)   E°rxn = -1.10 Volts

2. Electrolysis Activity Sheet.   Have students begin the Electrolysis Activity sheet.  The worksheet begins with a model and the model refers to the Electrolysis simulation.  There are three parts to this activity.  

With respect to the active learning Class Activity, provide students with the "Electrolysis Model", physical constants, and an empty diagram of an electrolysis cell. While observing the Electrolysis Cell with two copper electrodes (see below),  have students complete the Cu/Cu electrolysis cell diagram: 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.

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The Big Idea - Concept Introduction

[The questions and the tables on the Electrolysis Activity Sheet and the Electrolysis Computer Simulation will guide students to come to several conclusions.  It is important for the instructor to be available to facilitate students' learning but not to lecture.]

It takes two electrons to reduce one copper(II) ion to a copper atom    Cu2+ + 2e--> Cu   This half-reaction occurs at the cathode.  The two electrons are supplied by the DC Power Supply.  Simultaneously, one copper atom is oxidized to a copper(II) ion, and two electrons are released    Cu --> Cu2+ + 2e-   This half-reaction occurs at the anode.  These two electrons are being pulled into (eventually) the DC Power Supply along with other electrons.  When four electrons are forced into the cathode, two copper(II) ions are reduced to two copper atoms.

Compare the number of atoms of each element reduced at the cathode for three electrolysis cells given the number of electrons pushed into the cathode from the DC Power Supply.  For the silver electrolysis cell, there are two silver electrodes immersed in 1.0 M aqueous silver nitrate solution.   For the aluminum electrolysis cell, there are two aluminum electrodes immersed in 1.0 M aqueous aluminum nitrate solution. Complete the following Table. 

# of electronsAl atoms Cu atoms Ag atoms
     12  4  6  12
     30   
   120   
 1,200   
6.02 x 1023   

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3. Using the Electrolysis Computer Simulation, select two electrodes from the menu and select an appropriate aqueous solution.  For example set-up an experiment involving two copper electrodes immersed in 1.0 M CuSO4(aq) are connected to a Direct Current (DC) Power Supply.   Make note of the mass of both electrodes, 10.0 grams.  Set the current to 8.00 Amps and the time for 15.0 minutes. Record  the current (amps) and record the time.Turn the power supply "on". When current is applied to the electrolysis cell, students should notice that one electrode is gaining mass the other electrode is loosing mass. Note which copper electrode is connected to the "+" terminal of the DC Power Supply and which electrode is connected to the "-" terminal. 

Select the magnifying glass icon and have students view what occurs at the particle level of representation (microscopic view) on the simulation.  Students will observe copper(II) ions are reduced to copper atoms at the cathode    Cu2+ + 2e- --> Cu

copper atoms are oxidized to copper(II) ions at the anode.    

Cu --> Cu2+ + 2e

This corresponds to the observation that reduction occurs at the cathode. At the end of the experiment, student should record the mass of each electrode.  Calculate the mass of copper deposited two ways. 

Students 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

 
      
      

4. Lecture demonstration. While students are answering the copper-copper electrolysis cell questions on the worksheet, pause their work and have the students focus their attention on the copper-copper electrolysis cell lecture demonstration.  Have students record the mass of each electrode before the experiment and the mass of each electrode after the experiment.  Have students record the amount of time a current of 8.00 amps was passed (15 minutes). Show students the animations in the electrolysis simulation. Have students complete the copper-copper electrolysis cell diagram on their worksheet.

5.  Electrolysis Activity Sheet.  Students are given the task of plating zinc metal on a copper spoon.  Students should complete the diagram on the activity sheet.  Students need to decide which terminal of the DV Power Supply to connect to zinc metal and which terminal to connect to the copper spoon.  The students need to decide to have a 1.00 M aqueous zinc nitrate solution in the electrolysis cell.  The electrodes are connected to a battery capable of passing a current of 8.00 amperes for 15.0 minutes.  Students use the Experiment section of the Electrolysis Computer Simulation to set up an electrolysis cell designed to deposit zinc metal onto copper (see below).  The computer simulation helps students complete the diagram the components of the electrolytic cell and to help answer a series of questions on the activity sheet.

6. Lecture demonstration. While students are answering the zinc-copper electrolysis cell questions on the worksheet, pause their work and have the students focus their attention on the zinc-copper electrolysis cell lecture demonstration.  Have students record the mass of each electrode before the experiment and the mass of each electrode after the experiment.  Have students record the amount of time a current of 8.00 amps was passed (15 minutes). Have students complete the zinc-copper electrolysis cell diagram.

7.  Electrolysis Computer Simulation, set-up a zinc-copper electrolysis cell. Have students view what occurs at the particle level of representation (microscopic view) on the simulation.  Students will observe zinc(II) ions are reduced to zinc atoms at the cathode    Zn2+ + 2e--> Zn

zinc atoms are oxidized to zinc(II) ions at the anode.   

 Zn --> Zn2+ + 2e

This corresponds to the observation that reduction occurs at the cathode. At the end of the experiment, student should record the mass of each electrode.  Calculate the mass of zinc deposited two ways. 

The Class Activity and Electrolysis Computer Simulation serves well as an introduction to the basic principles of electrolysis.  Students easily see and understand 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.  

The Electrolysis computer simulation coded using in HTML5 is available at the following URL:

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

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

Curriculum Notes 

Student Difficulties (Misconceptions) with Electrolysis Addressed in this Activity

1. Not understanding the "+" and "-" signs on a battery or Direct Current Power Source.  At the "-" terminal electrons are pushed out of the battery. At the "+" terminal electrons are pulled into the battery.

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

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

4. When identical electrodes are used in an electrolysis experiment, the same reaction occurs at both electrodes and the products are the same at both electrodes.

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

6. Representing an electrolysis process using three levels of representation.  Relating what occurs at the macroscopic level to what occurs at the particle level, to the symbolic level (chemical equations).

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. Zn2+ + 2e-> Zn, scales up to the mole level: i.e. 1 mole Zn2+ 2 mole e-> one mole Zn.

5. 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.

6.  Calculate the mass of product produced during electrolysis given the stoichiometry, 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.

AP Chem Learning Objectives  The student can

LO 3.12:  make qualitative or quantitative predictions about electrolytic cells based on half-cell reactions and potentials and/or Faraday's laws.

LO 3.13:  analyze data regarding electrolytic cells to identify properties of the underlying REDOX reactions.

LO 5.15 is able to explain how the application of an external energy source (power source) can be used to cause processes that are not thermodynamically favorable to become more favorable (i.e. to occur).

Pre-Requisite Knowledge

1.  Oxidation-reduction reaction reactions involving a transfer of electrons.

2. Galvanic cells.

3.  Physics: voltage and current; the function of Direct Current (DC) power supplies; how batteries push electrons in a circuit

Discussion 

How does the mass (grams) of the metal plated depend on current and time?  Does the charge of the metal cation reduced to the metal atom have an influence on the amount of charge needed?

Current (Amp)

Time (Sec)

Grams of Zn

Grams of Fe

Grams of Ag

3.00 A

600

0.60

0.52

2.01

3.00 A

300

0.31

0.26

1.00

2.00 A

600

0.41

0.35

1.34

 

 

Footnotes 

References

Gelder, J.I., Abraham, M.R., Greenbowe, T.J.  (2015).  “Teaching electrolysis with guided-inquiry.”  In Sputnik to Smartphones: A Half-Century of Chemistry Education, M. Orna (ed.) ACS Symposium Series, Volume 1208, pp 141-154. American Chemical Society, Washington, D.C.

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.

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 practice, Dordrecht: Kluwer, pp. 317-338. 

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. Greenbowe, T. (1997). Common students misconceptions in electrochemistry: Galvanic, electrolytic and concentration cells. Journal of Research in Science Teaching, Volume 34, Issue 4, 377–398.

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.

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.

P. J. Moran and E. Gileadi, “Alleviating the Common Confusion Caused by Polarity in Electrochemistry,” J. Chem. Educ., Vol. 66, 1989, 912. 

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. 

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