Summary

In this lesson students compare energy densities of lead acid and lithium ion batteries to understand the relationship between electrochemical cell potentials and utilization of stored chemical energy.

Grade Level

High school

NGSS Alignment

This lesson will help prepare your students to meet the performance expectations in the following standards:

• HS-PS1-7: Use mathematical representations to support the claim that atoms, and therefore mass, are conserved during a chemical reaction.
• HS-PS3-1: Create a computational model to calculate the change in the energy of one component in a system when the change in energy of the other component(s) and energy flows in and out of the system are known.
• HS-ETS1-3: Evaluate a solution to a complex real-world problem based on prioritized criteria and trade-offs that account for a range of constraints, including cost, safety, reliability, and aesthetics as well as possible social, cultural, and environmental impacts.

AP Chemistry Curriculum Framework

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

• Unit 4: Chemical Reactions
  • Topic 4.9: Oxidation-Reduction (Redox) Reactions
    • 4.9.A: Represent a balanced redox reaction equation using half-reactions.
• Unit 9: Thermodynamics and Electrochemistry
  • Topic 9.3: Gibbs Free Energy and Thermodynamic Favorability
    • 9.3.A: Explain whether a physical or chemical process is thermodynamically favored based on an evaluation of ∆G^o.
  • Topic 9.8: Galvanic (Voltaic) and Electrolytic Cells
    • 9.8.A: Explain the relationship between the physical components of an electrochemical cell and the overall operational principles of the cell.
  • Topic 9.11: Electrolysis and Faraday’s Law
    • 9.11.A: Calculate the amount of charge flow based on changes in the amounts of reactants and products in an electrochemical cell.

Objectives

By the end of this lesson, students should be able to

• Calculate the change in standard Gibbs free energy for redox reactions.
• Calculate electrical work that can be done by electrochemical reactions using only half-cell reactions, cell potentials and Faraday’s laws.
• Estimate energy densities for lead acid and lithium ion battery systems.

Chemistry Topics

This lesson supports students’ understanding of

• Electrochemistry
• Redox reactions
• Oxidation
• Reduction
• Thermodynamics
• Gibb’s Free Energy (DG)
• Spontaneity

Time

Teacher Preparation:

Lesson:

• Engage: 10 minutes
• Explore: 20–30 minutes
• Explain: 20–30 minutes
• Elaborate: 15–20 minutes
• Evaluate: 60 minutes

Materials

• 3 x 50 mL beaker
• 250 mL beaker
• Cotton wool
• Thermometer
• Voltmeter
• Connecting wire and crocodile clips (or similar)
• Filter paper
• 1 M copper sulfate solution, CuSO₄(aq)
• 1 M zinc sulfate solution, ZnSO₄(aq)
• 0.7 g of mossy zinc freshly cut into portions
• Strip or rod of zinc metal
• Strip or rod of copper metal
• Saturated potassium nitrate solution, KNO₃ (aq)

Safety

• Always wear safety goggles when handling chemicals in the lab.
• Students should wash their hands thoroughly before leaving the lab.
• Students should wear proper safety gear during chemistry demonstrations. Safety goggles and lab apron are required.

Teacher Notes

• This resource could be used as a post-AP Chemistry exam activity.
• This lesson can be included within units on thermodynamics and electrochemistry, and can be adapted to the skill and experience of the students.
• The Introductory Student Lesson is appropriate for General and Honors Chemistry students.
• The Advanced Student Lesson is appropriate for AP Chemistry students, and supports enduring understandings 3.C (Chemical and physical transformations may be observed in several ways and typically involve a change in energy.) and 5.B (Energy is neither created nor destroyed, but only transformed from one form to another) as well as essential knowledge 3.C.3 (electro­chemistry shows the interconversion between chemical and electrical energy in galvanic and electrolytic cells).
• Students tend to confuse energy (the ability to do work) with power (the rate at which energy is converted). Energy (in joules, kilojoules, kilowatt hours, or volt-coulombs) tells us how far the car can go. Power (in watts, kilowatts, or volt-amperes) can tell us how fast the car can go (or how fast we spend the energy in the battery).
  
  For example, we can calculate how much energy is needed to drive 100 miles using a lead-acid battery system:

The number of individual batteries is independent of energy, and is dependent on the serial or parallel arrangement of redox reactions, and the architecture of the motor. We could provide that 193 kg of reactants in one large 2-volt battery and high current connect many small batteries in series to have a low current at a high voltage, or connect many small batteries in parallel to have a high current at a low voltage. In all of these scenarios, we are using the same 193 kg of fuel to deliver 1.188 x 10⁸ V-C.

• The demonstrations and introductory activity are class activities. The “How much fuel is necessary to drive an electric car 100 miles?” portion can be completed in class or as homework.

• Engage: Students observe and discuss demos of redox reactions of copper sulfate and zinc metal: a one beaker displacement reaction of copper sulfate and zinc metal and a Daniell cell. Students should observe that if zinc is simply immersed in a CuSO₄ solution, zinc dissolves (forming ZnSO₄(aq)) and copper precipitates with the evolution of heat. If the two half reactions are separated from each other as in the Daniell cell, the electron transfer occurs with the production of electrical energy, rather than heat energy.

Teacher Preparation

• 1 M copper sulfate solution, CuSO₄(aq) can be prepared from 25 g CuSO₄ per 100 mL solution
• 1 M zinc sulfate solution, ZnSO₄(aq) can be prepared from 29 g ZnSO₄ per 100 mL solution
• 0.7 g of mossy zinc freshly cut into portions
• Saturated potassium nitrate solution, KNO₃ (aq) can be prepared from 33 g KNO₃ in 100 mL H₂O

For the displacement reaction, nest a 50 mL beaker inside the 250 mL beaker, and fill the space between the beakers with cotton wool. Add 10 mL of 1 M CuSO₄(aq) to the beaker and insert a thermometer in the solution. When students are present, add 0.7 g of mossy zinc to the solution and have students record the temperature periodically (e.g., every 30-60 seconds).

For the displacement reaction, nest a 50 mL beaker inside the 250 mL beaker, and fill the space between the beakers with cotton wool. Add 10 mL of 1 M CuSO₄(aq) to the beaker and insert a thermometer in the solution. When students are present, add 0.7 g of mossy zinc to the solution and have students record the temperature periodically (e.g., every 30-60 seconds).

The classic example of a galvanic cell is the Daniell cell based on the spontaneous reaction between copper sulfate and zinc metal:

CuSO₄(aq) + Zn(s) → ZnSO₄(aq) + Cu(s)

Half reactions:

Cu²⁺(aq) + 2 e^- → Cu(s)

Zn(s) → Zn²⁺(aq) +2 e^-

The Daniell cell consists of a zinc electrode in zinc sulfate solution and a copper electrode in copper sulfate solution connected by a salt bridge (a strip of filter paper soaked in saturated potassium nitrate aqueous solution (see figure below). The electrodes can be connected to a voltmeter or load such as a small 1-V fan.

Have the cell running when class starts and let students observe and record the cell potential periodically.

• Explore: Students analyze the reaction of copper sulfate in both contexts (Analysis section of Introductory Activity). Students practice identifying half-cell reactions and converting Gibbs free energy into measurements typical for describing electrical work. They should observe that different methods can be used to calculate the Gibbs free energy.
  
  
• Explain: After completing calculations for the simple redox reaction and galvanic cell observed in class, students use their understanding of energy and electrical work to estimate energy density for standard battery designs (How much energy is necessary to drive an electric car 100 miles?).
  
  
• Elaborate: Material properties of reagents in electrochemical systems affect potential uses. Students put the estimated energy densities in context by considering how much energy is needed per mile traveled, size of typical cars, and the size of battery relative to energy needed (Analysis of “How much energy” section).
  
  
• Evaluate: Student calculations should be checked to verify competency in dimensional analysis, and thermodynamic and electrochemical calculations. Students reflect on applying chemistry principles in context of a real world problem. Students may want to present their estimates of battery systems in commercial electric vehicles (e.g., Chevy Volt, Nissan Leaf).