Presenting the Voltaic Cell

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Objective

Students will be able to present and explain the chemical processes represented in their diagram of a voltaic cell.

Big Idea

Voltaic cells consist of two-half cells that undergo oxidation and reduction. A salt bridge helps to balance the charge in each half-cell.

Introduction

In the last lesson students made voltaic cells and they created diagrams of them to show how they relate to redox reactions. In this last class of the year, students refine their understanding of the voltaic cells, present their diagrams, and evaluate the chemistry course for the year.

This lesson aligns to the NGSS Disciplinary Core Idea of HS-PS1-2: Construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties because redox reactions focus on a chemical reaction in which the outermost electrons are transferred from one element to another.

This lesson aligns to the NGSS Practices of the Scientist of Obtaining, evaluating, and communicating information because students will explain how their voltaic cell works. It also aligns to the Practice of Constructing explanations because of this need to explain it.

It aligns to the NGSS Crosscutting Concept of Stability and Change because this lesson helps students to construct an explanation of the electrochemical changes that occur in the electrochemical cell.

In terms of prior knowledge or skills, students should have already built a voltaic cell, and drawn a rough draft of it, as shown in this lesson.

There are no special materials needed for this lesson.

Do Now/Activator

10 minutes

Do Now: Students enter class and are asked to explain their voltaic cell diagram with a partner. The partner who is listening should take note of what their partner needs to express more clearly or correctly, and shares this information with their partner at the conclusion of the explanation. The roles are then reversed, with the presenter becoming the person giving feedback and the person who gave feedback becoming the presenter.

I reason that this is a good way to begin class because it gets every student engaged in talking about their cell diagram, which will allow them to move toward being able to explain it to the whole class.

Mini-lesson

10 minutes

Mini-lesson: I pass out the Voltaic Cell Presentation directions and ask students what they think is the most challenging part of the diagram. As I was hoping, they note that explaining how the salt bridge works is the biggest challenge. I anticipated this based on the last class.

I use the How Electrons Travel in a Voltaic Cell diagram to re-teach this material. I note that in the diagram all the materials have changed but the roles that each material plays is analogous to the students’ voltaic cells. I explain the diagram.

I note that electrons are leaving the Al electrode, and Al3+ ions are going into solution, making it more positive. To balance this charge, NO3- ions are coming out of the salt bridge and into the Al(NO3)3 solution. On the other side, Pb2+ is coming out of solution, being attracted to the Pb electrode that is negatively charged as it is gaining electrons from the Al. K+ is going into the Pb(NO3)2 solution from the salt bridge to balance this change in charge.

This instructional choice reflects my desire to honor the need for re-teaching in a way that does not just give away the answers. During this time students take notes on the diagram.

Application

25 minutes

Student Activity: During this time students look over their diagrams from the previous class with an eye toward revising their work. Some students talk through their diagram with a partner. Some students ask me clarifying questions about their diagrams; their biggest challenge is making the connection between the diagram I taught from today and their own diagram.

After about ten minutes I ask for volunteers to present their diagrams. Most students are able to discuss electron flow, as evidenced by this Electron flow video. Not all students are able to explain the role of the salt bridge; however, this student is typical of the students who are able to, as shown in this How the Salt Bridge works video.

I feel like this was a good way to spend the last class of the school year. The work is social in nature, and allows students the chance to articulate what they know about voltaic cells. The very act of articulating increases their understanding of the subject. 

Debrief

10 minutes

Because this is the last lesson of the year, I ask students to take some time evaluating me and the Chemistry course using this 2015 end of year survey.

I explain to students that they can be anonymous in the form. I explain to them that I do not read my evaluations until the year is over. I note that they can write their names on them if they want, or they can write in a way that I will know who wrote the evaluation by including things like private jokes or stories that are clearly about the evaluator. I explain to the students that I will use what they tell me to make my course better next year.

These evaluations and this other set of evaluations are a random sample of the feedback I receive. Overall I am pleased with the feedback from the students.