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Summary

In this activity, students will use a simulation to create a variety of non-standard condition galvanic/voltaic cells. This simulation allows students to choose the metal and solution for each half cell, as well as the concentration of those solutions. Students will build concentration cells and other non-standard cells and record the cell potential from the voltmeter. They will compare the results of different data sets, write net ionic equations, and describe electron flow through a galvanic/voltaic cell from anode to cathode as well as the direction of migration of ions, anions towards the anode and cations towards the cathode.

Grade Level

High School (AP Chemistry)

AP Chemistry Curriculum Framework

This activity supports the following units, topics, and learning objectives:

  • Unit 4: Chemical Reactions
    • Topic 4.2: Net Ionic Equations
      • 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.
    • Topic 4.9: Oxidation-Reduction (Redox) Reactions
      • TRA-2.C: Represent a balanced redox reaction equation using half-reactions.
  • Unit 9: Applications of Thermodynamics
    • Topic 9.7: Galvanic (Voltaic) and Electrolytic Cells
      • ENE-6.A: Explain the relationship between the physical components of an electrochemical cell and the overall operational principles of the cell.
    • Topic 9.9: Cell Potential Under Nonstandard Conditions
      • ENE-6.C: Explain the relationship between deviations from standard cell conditions and changes in the cell potential.

NGSS Alignment

This activity will help prepare your students to meet the following scientific and engineering practices:

  • Scientific and Engineering Practices:
    • Using Mathematics and Computational Thinking
    • Developing and Using Models
    • Analyzing and Interpreting Data

Objectives

By the end of this activity, students should be able to:

  • Identify the standard conditions of galvanic/voltaic cells.
  • Describe the effect of changing concentration on cell potentials in galvanic/voltaic cells.
  • Write net ionic equations that occur in galvanic/voltaic cells.
  • Describe how electrons flow and ions move through a galvanic/voltaic cell to create an electric current.

Chemistry Topics

This lesson supports students’ understanding of:

  • Galvanic (Voltaic) cells
  • Redox reaction
  • Reduction potentials
  • Nernst equation
  • Half reactions
  • Cathode and anode
  • Net ionic equations

Time

Teacher Preparation: 10 minutes

Lesson: 45-60 minutes

Materials

Safety

  • No specific safety precautions need to be observed for this activity.

Teacher Notes

  • Many thanks to Tom Greenbowe and John Gelder for their input on this simulation, which was inspired by their Flash-based simulation. Since Flash is no longer supported, they provided valuable insight as we designed this new simulation and resource based on their originals.
  • This simulation is closely related to other AACT simulations. It could be helpful to start with the Metals in Aqueous Solutions simulation, which explores the activity series; it is highly recommended that students explore the Galvanic/Voltaic Cells simulation and accompanying activity, which allows students to study galvanic cells at standard conditions, before using this one. This simulation is very similar to the Galvanic/Voltaic Cell simulation, but in addition to being able to choose metals and solutions for each beaker (half cell), this simulation allows students to manipulate the concentration of the solutions as well.
  • Be sure that students are aware that the terms “galvanic cell” and “voltaic cell” are equivalent.
  • Students should have been taught the Nernst equation before completing this activity.
  • In the pre-activity questions, students are asked about standard conditions for galvanic cells. This simulation allows them to adjust the concentration of the solution, but not the temperature or pressure. You could ask them to use a different form of the Nernst equation to think about how pressure and temperature changes would affect . Temperature, T, appears in the numerator of the coefficient of ln Q, and the pressure of a gas would appear somewhere in the reaction quotient, Q, its location depending on the reaction that involves a gas as a reactant or product.
  • The primary focus of this activity will be to examine concentration cells – galvanic cells where both half cells contain the same metals and solutions, but at different concentrations – but the simulation also allows for combinations of different electrodes at both standard and non-standard concentrations, one of which is used in this activity.
  • Be sure students understand that a particular half reaction does not always take place at the cathode or the anode – this depends on what other electrode it is paired with and whether it has a higher or lower reduction potential.
  • In this simulation, a positive voltage results when the left electrode is the cathode and the right electrode is the anode. This is due to how the voltmeter interprets the direction of the electron flow, so if the red and black leads were reversed, the sign of the voltage would be as well. (This is particularly relevant to Part I question 2.) A good activity from the AACT library is this one: Four-Way Galvanic Cell.
  • Students often have a hard time conceptualizing the salt bridge, particularly the ends being porous but not completely open. If you have a glass salt bridge in the lab, you could show them a galvanic cell set up and demonstrate how the liquid in the salt bridge doesn’t just pour out. You could also show them other types of salt bridges, such as a porous barrier or filter paper soaked in aqueous salt solution connecting the two half cells.
  • Students sometimes think electrons are moving through the salt bridge, rather than the wire. Be sure students understand that the electrons are moving along the wire of the external circuit from the anode to the cathode, and the salt bridge provides ions that move into the solutions to keep each half cell electrically neutral as electrons move from the anode to the cathode. (See analysis questions 4 and 5.)
  • In this 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 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.
  • If students get stuck on any of the questions in this activity, most of them can be figured out by returning to the Nernst equation. Be sure students truly understand the role of the different variables in the Nernst equation – it may be worth having a class discussion about pre-lab question 2 before students start the activity.
  • Students can easily access this simulation from the following link:

For the Student

Lesson

Background

Concentration cells are a type of galvanic cell in which the anode and cathode half cells use the same metals and solutions, but the concentration of the solutions differ. For example, one half cell could contain a silver electrode with a 1.0 M silver nitrate solution, and the other half cell could contain a silver electrode with a 0.01 M silver nitrate solution. In this activity, you will examine several concentration cells, as well as some other galvanic cells at non-standard conditions.

The table below lists the standard reduction potentials for the electrodes used in this simulation:

Electrode

Eored
Ag+ (aq) + e → Ag (s) 0.80 V
Cu2+ (aq) + 2 e → Cu (s) 0.34 V
Zn2+ (aq) + 2 e → Zn (s) –0.76 V
Mg2+ (aq) + 2 e → Mg (s) –2.38 V

Pre-Activity Questions

  1. What are the standard temperature, pressure, and concentration conditions for an electrochemical cell?
  2. You have learned about the Nernst equation (below) in class. Define each of the variables in the equation.


Ecell:

E°cell:

n:

Q:

Procedure

Collect the data and answer the questions below using the simulation that can be found at: https://teachchemistry.org/classroom-resources/galvanic-voltaic-cells-2

Part I – Silver Concentration Cell

Select “Silver (Ag) in AgNO3 (aq)” for both half cells. Input the concentrations for AgNO3 as specified in the table below and record the cell potential, Ecell, for each combination.

Trial [AgNO3] left half cell [AgNO3] right half cell Ecell
1 2.0 M 2.0 M
2 2.0 M 1.0 M
3 2.0 M 0.1 M
4 2.0 M 0.01 M
5 2.0 M 0.001 M
6 1.0 M 2.0 M
  1. What do you notice about the cell potential as you change the concentration of the AgNO3 in the right cell in trials 1-5? Why does this occur?
  2. Explain why the voltage in trial 6 is the same magnitude but opposite in sign as trial 2. (Hint: Click on “View Molecular Scale” buttons to see what is happening at the particle level.)
    1. Without changing the concentrations in trial 6, how could you adjust the system to get the same voltage as trial 2?
  3. Write the balanced half reactions and net ionic equation for the overall reaction of this galvanic cell, including the concentrations of Ag+. How many electrons are transferred in the cell reaction?
  4. Starting with equal concentrations of the metal ion in both the cathode and anode half cells, what would happen to the cell voltage when the concentration around the anode is increased?  Explain.

Part II – Copper Concentration Cell

Select “Copper (Cu) in Cu(NO3)2 (aq)” for both half cells. Input the concentrations for Cu(NO3)2 as specified in the table below and record the cell potential, Ecell, for each combination.

Trial [Cu(NO3)2] left half cell [Cu(NO3)2] right half cell Ecell
1 2.0 M 2.0 M

2

2.0 M

1.0 M

3

2.0 M

0.1 M

4

2.0 M

0.01 M

5

2.0 M

0.001 M

  1. What do you notice about the cell potential as you change the concentration of the Cu(NO3)2 in the right cell? How does this compare to your data from Part I?
  2. Write the balanced half reactions and net ionic equation for the overall reaction of this galvanic cell, including the concentrations of Cu2+. How many electrons are transferred in the cell reaction?
  3. Compare your Ecell values for Part II to those you obtained in Part I. Explain any differences you observe.

Part III – Non-standard Silver/Magnesium Cell

Select “Silver (Ag) in AgNO3 (aq)” for the left half cell and “Magnesium (Mg) in Mg(NO3)2 (aq)” in the right half cell. Input the concentrations for the two solutions as specified in the table below and record the cell potential, Ecell, for each combination.

Trial [AgNO3] left half cell [Mg(NO3)2] right half cell Ecell
1 2.0 M 2.0 M
2 2.0 M 1.0 M
3 2.0 M 0.1 M
4 2.0 M 0.01 M
5 2.0 M 0.001 M
  1. Write the balanced half reactions and net ionic equation for the overall reaction of this galvanic cell. How many electrons are transferred in the cell reaction?
  2. Compare your Ecell values for Part III to those you obtained in Parts I and II. Explain any differences you observe.

Analysis

  1. In a concentration cell, does the half cell with the higher concentration or the lower concentration act as the anode? What evidence from the simulation supports your conclusion? (Hint: click on “See Molecular Scale” to view the half reactions at the particulate level.)
  2.  What happens to the cell potential when both half cells use the same metals and solutions, at the same concentrations? Why does this occur?
  3. How would increasing the size of the metal electrode affect the cell potential of a galvanic cell if everything else in the cell remains the same? Explain.
  4. Explain how the ions in the salt bridge contribute to the function of the galvanic cell.
  5. Describe the path the electrons take in this simulation to get from the anode to the cathode.