During the last lesson students have been showing how redox reactions work using oxidation numbers. In this lesson students use the vocabulary associated with redox reactions and voltaic cells to create concept maps that show links between the different vocabulary.
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 focuses 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 study how a voltaic cell works and then synthesize that information with what they have already learned to create their concept maps.
It aligns to the NGSS Crosscutting Concept of Energy and Matter: Flows, Cycles, and Conservation because the concept maps show how changes in energy and matter occur within a voltaic cell system.
In terms of prior knowledge or skills, students should have a good understanding of oxidation and reduction, and what a redox reaction is.
The materials needed for this lesson include the following:
Note: I placed a nail in a beaker with CuSO4 and within one class period the zinc had turned a copper color. However, the results become more dramatic the longer the nail stays in, with copper accretions forming on the nail over time.
Do Now: Students enter class to find this prompt: “Find a classmate who has completed the oxidation/reduction assignments, and compare answers. Highlight any disagreements or confusion.”
I reason that this is a good way to start class because in the last lesson students were learning how to identify the loss and gain of electrons in redox reactions. I want students to look over their work and identify what they need help with so that I can efficiently review homework at the start of class.
Activator: Once students have done this, I answer their questions. They are very concerned about the confusing terms “oxidizing and reducing agents.” I ask them to forget those terms and I explain that they are needlessly confusing. As long as students can use oxidation numbers to identify what is being oxidized and what is being reduced, I am satisfied. This announcement calms students, and at this point the class is ready to move on.
I project the Analyzing a Redox Reaction with Answers so students can check their work. Most students are satisfied with their work, but a few are confused. I announce that I will run a clinic for these students during independent work time.
Mini-lesson: I begin the new material for today’s lesson by referring them to p. 496 of their text book. I note that over the past few days they have been showing how electrons go from one place to another using oxidation numbers. Today, we will describe how all this vocabulary relates to our final project, which is to make a voltaic cell. I note that a voltaic cell is what is pictured on p. 496. It is made of two half-cells. In one half cell oxidation occurs; in the other, reduction occurs. Electrons are lost and gained in a redox reaction, and I note that this is what happens in a voltaic cell.
I remind students that we already discussed electricity in a previous lesson, and that electricity is the flow of electrons. I note that when one metal is losing electrons and one metal is gaining, this implies that electrons are moving. If you put a light bulb in between the two metals and give the electrons a wire to travel on, then you can harness the electrons to do something useful like light the bulb.
I then discuss today’s work. I explain that I want students to do a couple of things. First, I want them to make a concept map that shows the relationship between the various vocabulary words we have been working with and how these relate to a voltaic cell. I pass out the Electrochemistry Concept Map directions and review them. The format is designed to insure that I can easily read the maps without feeling like I am playing Twister. Students have created concept maps in my class before, so they know the drill.
I also note that I would like them to link their vocabulary words to my nail in a beaker demonstration. I show them a clean zinc nail and some blue copper sulfate. I then show them the zinc nail that has been sitting in copper sulfate solution for several hours. I tell them that I will come by and chat with them about what has happened in the beaker, and that they are welcome to get up and inspect the demonstration at my lab bench.
Once I am sure that everyone has a plan of action for the day’s class, I release students to do their work.
Student Activity: The first thing I do is convene a small group to discuss how to tell if an element is being oxidized or reduced. I base this information on the last lesson that students had. I spend about ten minutes with this group, and at the end of it they have a better understanding of the material. I note that their next step is to practice the work. I give them copies of the answer key so they can check their work.
I then walk around the entire room and check in with everyone. I answer a few questions and offer a few words of encouragement. When I am satisfied that the class is running well and that every student is making progress I begin my discussions with students about the nail.
This conversation I have with a student in this explaining the copper covered nail video is instructive. At first the student expresses a superficial understanding of what is happening in the beaker. However, as this grappling with redox video shows, once I ask a few probing questions I am able to move the student’s thinking along.
While this is a somewhat time consuming way to approach this activity, I feel like it is time well spent because I get to meet each student or small group where they are and through questioning move their thinking forward. Some students overhear other group’s conversations, and this also helps to speed things up.
Catch and Release Opportunities: After about 15 minutes of students working I notice that they are having two challenges. First, we never discussed the idea of the watt, and it is not in their book. I spend a little time modeling what voltage and current are by showing them a graduated cylinder with water in it. I note that if I tip it a little, a little water comes out, and if I tip it a lot, the water flow increases. This, I note, is analogous to the idea of voltage—how much of a push is there on electrons. Amperage, on the other hand, has to do with how much water (or electrons) you have. The more electrons that are flowing, the more flow or amperage there will be.
Students are also confused by the idea of the salt bridge. I teach using the voltaic cell diagram in their book. I note that electrons are leaving the Mg, and this makes the Mg2+ ions soluble. They go into solution, making the solution more positive. On the other side, Cu2+ is leaving solution because they are being attracted to the Cu electrode, which is negative (it is gaining the electrons from the Mg). The salt bridge helps to keep the solutions neutral. The MgSO4 is getting Cl- from the salt bridge, and the CuSO4 solution is getting K+ from the salt bridge. Without these contributions, the flow of electrons would stop because the electrodes would get surrounded by positive and negative charges.
To wrap this lesson up I first identify someone who has made a lot of progress on their concept map. In this concept map explanation video a student explains to me how he links the various vocabulary words in his concept map. I then use this student work to show to the class using a document projector.
I then ask students whether making a concept map was or was not useful. Most students say that it was useful because they were able to make connections and get a better feel for how the vocabulary words relate to one another. It was also useful because students have a better understanding of how a voltaic cell works, which will be the subject of the next class. I note that this is a good skill for college--making sense of information and drawing connections, and I am glad they got something out of today's class.