Voltaic Cells Lab

Name: Crystal Bell

Title of Lesson: Voltaic Line Up

Date of Lesson: TBA

Length of Lesson: 3 -3.5 days

Description of Class: H. S. Chemistry I

References for the Lesson:

Whtitten, Davis, and Peck, General Chemistry, 6^th ed. Pennsylvania, USA: Harcourt, Inc.; 2000

How Stuff Works - Batteries < http://science.howstuffworks.com/battery.htm>

(accessed Oct. 2004)

TEKS addressed:

(1) Scientific processes. The student, for at least 40% of instructional time, conducts field and laboratory investigations using safe, environmentally appropriate, and ethical practices. The student is expected to:

(A) demonstrate safe practices during field and laboratory investigations

(2) Scientific processes. The student uses scientific methods during field and laboratory investigations. The student is expected to:

(B) collect data and make measurements with precision;

(D) organize, analyze, evaluate, make inferences, and predict trends from data; and

(E) communicate valid conclusions.

(5) Science concepts. The student knows that energy transformations occur during physical or chemical changes in matter. The student is expected to:

(A) identify changes in matter, determine the nature of the change, and examine the forms of energy involved;

(B) identify and measure energy transformations and exchanges involved in chemical reactions

(10) Science concepts. The student knows common oxidation-reduction reactions. The student is expected to:

(A) identify oxidation-reduction processes;

(11) Science concepts. The student knows that balanced chemical equations are used to interpret and describe the interactions of matter. The student is expected to:

The Lesson:

I. Overview

This lesson allows students to learn how to set up wet voltaic cells and explore the electrochemistry that is involved. Students will practice writing half reactions, learn to designate the cathode and anode, and make a reduction table.

II. Performance or learner outcomes

Students will be able to:

§ Setup, conduct, and explain voltaic cell reactions

§ Write equations for half-reactions and balanced full redox reactions

§ Identify the anode and cathode

§ Identify the type of reactions that occur at the anode and cathode

III. Resources, materials and supplies needed

For each group (3-4 students):

§ 1 Voltammeter (able to read voltage below 1V)

§ 20 mL Brine Solution

§ 10 - ¼" x 4" strips of filter paper

§ 6 - 100 mL beakers

§ 50 mL of each of the following 1M solutions: ZnSO₄, FeSO₄, NiSO₄, CuSO₄, Ag SO₄, Al SO₄

§ 1 - 2" long wire of each of the following wires: Zn, Fe, Ni, Cu, Ag, Al

IV. Supplementary materials, handouts

"Voltaic Line Up" Lab Sheet (make several copies of second sheet)

"Voltaic Line Up" Discussion Questions

Five-E Organization

Teacher Does

Student Does

Engage: Demo - How to set up a half-cell

Learning Experience:

The instructor will walk through the process of constructing and taking readings from a wet voltaic cell.

1. The half cell consists of the oxidized and reduced form of an element or species. In our case a solid metal and a solution of its ion. (i.e Mg and Mg²⁺) The half cell will be constructed in a 100 mL beaker with 50 mL of solution and a strip of the metal. Two half cells are used to make a voltaic cell.

2. The voltaic cell needs a salt bridge connecting the two half cells. Our salt bridge will consist of strips of paper soaked in a brine (saturated salt) solution.

3. Two half cells can be connected via a wire. This experiment will simply use the wire that connects the cells to the voltammeter.

Hoped for student response:

Students will remember oxidation and reduction from previous lessons. They will be excited to see how redox reactions work in the experimental sense. Students will be attentive to instructions and remember basic lab safety.

4. The voltammeter measurers the potential as electrons move through the cell. It will measure negative or positive flow depending on which way the wires are connected. Be sure to take positive measurements only.

Each group will set up several voltaic cells and take readings. Based on the readings you should be able to rank the half reactions in order of strongest oxidizing agent to strongest reducing agent.

Questions:

What is the difference between oxidation and reduction?

What is the difference between an oxidizing agent and a reducing agent.

What are the units of potential?

What does a negative reading on the voltammeter mean?

What are the lab safety rules?

Expected Student Response:

Students may mix up oxidation and reduction. They may also miss some of the instructions, but it will be on their lab handout as well. The students will know the lab safety rules.

Evaluate

Decision Point Assessment (DPA) – Students will be assigned a minute paper on how to set up a cell and how to take a potential reading.

Explore: Voltaic Lab

Learning Experience(s):

Students will be divided into groups of 3-4 and given a worksheet that outlines the procedure. For each voltaic cell students will report the half reaction equations, the balanced full equation, the anode and the cathode, the sites of oxidation and reduction, the positive voltage.

Of the twenty combinations each group conducts as many as they need to be able to order the half cells from strongest to weakest oxidizers and strongest to weakest reducers.

Questions:

See lab discussion questions.

Hoped for student response:

Groups will be on task and share the work of setting up the cells and taking readings and determining the half reactions.

Students will notice that the lists of strongest to weakest oxidizers and reducers are opposite of each other.

They will also see that all the reductions are at the cathode and all the oxidations are at the anode.

Expected student response:

The groups will have one or two people that will try to do everything while the rest of the group watches or does minimal work. The teacher will have to ensure that all members are involve.

Some students will see the pattern of how oxidizers and reducers line up. Some will notice that oxidation and reduction always occur at the same terminal. For others these things will have to be explicitly asked during the discussion.

Evaluate

DPA - All lab reports have been completed and turned in.

Explain: Discussion Questions

After the labs have been reviewed. the teacher will highlight key points and clarify any questions that were missed by more than one third of the class.

Questions:

What was the order of oxidizing and reducing agents? What is significant about these orders?

What other area of chemistry has this type of pattern?

Why is was a salt bridge needed?

Does oxidation occur at the anode or cathode? Reduction?

Which process (oxidation or reduction) causes mass of the metal strip to increase? Why? To decrease? Why?

What is the voltage across two identical half cells? Why?

Other questions depend on problem spots in the lab.

Hoped for student response:

Students will have thoughtfully answered the discussion questions and be able to thoroughly answer the questions.

Some students will also be able to explain the problem spots in the lab to their classmates.

Expected Student Response:

Students will give short answers and/or muddle through explanations. It may take calling on a few people to get a complete answer. After each question has been fully answered the teacher may need to concisely state the answer for clarification.

Evaluate

DPA – The class discussion has involved the majority of the class and students are able to clearly answer the key points of the lab.

Extend / Elaborate: Battery Redox Rxns

Using the internet, textbook, or other resource to find the materials used in a battery. Note what type of battery it is and write out the half reaction equation and full balanced equations that are involved.

Questions:

What type of battery did the group find?

What materials are used?

What are the half-reaction equations?

What is the full balanced equation?

Where is the cathode and anode?

What acts as a salt bridge?

How much voltage does this reaction supply?

Hoped for response:

It will be a little more difficult to determine the full half reactions, but students will be able to work it out.

Students will be excited to take the lab a little further. They will also be able to give a short presentation to the rest of the class on what they found.

Expected Student Response

Students will get the information for the half reactions from the source that tells them about the battery materials. If the source does not tell them they may not be able to work out the half or full reactions.

Evaluate

DPA – Students are able to translate the lab experience to a real world situation and identify the main areas of a voltaic cell on the battery.

Voltaic Cell Line Up

Name:

Date:

Lab Partners:

Class Period:

Voltaic Cell #______

Half - Reaction

Electrode

Reaction Type

Balanced Reaction

Voltage

Voltaic Cell #______

Half - Reaction

Electrode

Reaction Type

Balanced Reaction

Voltage

Name:______________________

Voltaic Cell #______

Half - Reaction

Electrode

Reaction Type

Balanced Reaction

Voltage

Voltaic Cell #______

Half - Reaction

Electrode

Reaction Type

Balanced Reaction

Voltage

Voltaic Cell #______

Half - Reaction

Electrode

Reaction Type

Balanced Reaction

Voltage

Name:______________

Voltaic Cell Discussion Questions

Why is was a salt bridge needed?

What does a negative voltage reading mean? How can it be corrected?

Does oxidation occur at the anode or cathode? Reduction?

List the half reactions by oxidizing strength starting with the strongest.

List the half reactions by reducing strength starting with the weakest.

What is significant about these orders?

Which process (oxidation or reduction) causes mass of the metal strip to increase? Why?

Which process causes the mass of the metal strip to decrease? Why?

What should be the voltage across two identical half cells? Why?