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Lesson Write-Up for Spring Quarter 2012 Your name: David Luong, Albert Medina, Lynn Rice Title of Lesson: Electrochemical Cells Grade Level: 11th-12th Subject(s): Physics, Chemistry Summary: This lesson is intended to introduce students to the fundamentals of electrochemical cells governing the production of electrical potential or voltage. Electrochemical cells convert chemical energy into electrical energy. Students are given a physical description analogous to the production of voltage and current, the transport of charged particles. This production process is covered in greater depth chemically in identifying the basic components of an electrochemical cell: the anode, cathode, and electrolyte. Students are tasked with creating various electrochemical cells with the metals and electrolytes supplied to develop some insight into how the chemical composition of a battery determines performance. Voltage is measured with the use of a multimeter. Finally, limited by their single electrochemical cell voltage, students work as a class to create an array of cells, or battery, electrically connected in series in an effort to increase voltage output. Time Required: 55 minutes Group Size: 3-4 students per group Cost to implement: ~$45 (for 8 periods of 30+ students) 10lb bag of potatoes ($5) 2 cups of lemon juice ($5) 0.5 gallon of vinegar ($5) Electrode materials ($25) o Copper o Zinc o Aluminum o Steel o Magnesium o Tin (Reusable) condiment cups ($5) Learning Goals: After this lesson, students should be able to: 1. Cite components of an electrochemical cell 2. Cite functions of components of electrochemical cell 3. Discuss oxidation and reduction reactions: redox 4. Discuss electrochemical cell design considerations such as electrode combinations 5. Connect multiple cells in series to produce greater electrical potential Level of Inquiry: After students are comfortable with the mechanics of an electrochemical cell, they are left to their own device to indirectly reconstruct the redox potential table. First they are asked to use Copper as the mandatory cathode and test the remaining metals as the anode in the electrolytes provided. Secondly they are asked to use Zinc as the mandatory anode while the remaining metals act as the cathode. In this manner, students are able to observe the electrical potential created from the various electrode combinations and discuss chemically why certain combinations outperform others. Additionally, students will encounter both positive and negative voltage measurements from their multimeter given the cathode/anode specified above. They are to determine what these measurements imply about the role each electrode assumes in the cell and the direction of current within the circuit. Introduction / Motivation: Describe current and voltage with analogy to elevating a bucket of water (potential energy) and pouring it to the ground (flow of current). Describe the induction of voltage and current with two dissimilar electrodes connected with a wire and placed in an electrolyte. Explain the responsible chemical reactions (oxidation and reduction) with the acronym “Oil Rig” (oxidation is loss, reduction is gain) or “Leo says Ger” (lose electrons oxidation, gain electrons reduction). Explain the procedure by modeling the steps. Provide a challenge to the class that each group’s best batteries will be placed in series to light up the LED ladder. Procedure: 1. Before the lab, the instructor should build the Bar Graph Meter on page 3 (typical application) of the LM3914 datasheet. Choose R2 and R1 so that R2/R1 = 9. This will ensure that students will need to collectively provide a 12V battery source using their batteries to light up every LED. 2. Gather the electrolytes: Electrolyte 1: place vinegar in the container provided Electrolyte 2: cut a piece of potato 3. Attach the designated electrode to the multimeter terminal using an alligator clip Case 1: connect the copper cathode to the positive terminal of the multimeter Case 2: connect the zinc anode to the negative terminal of the multimeter 4. Attach the remaining multimeter terminal to one of the specified electrodes using an alligator clip: Case 1: lead, zinc, tin, magnesium Case 2: zinc, copper, steel, tin 5. Insert both electrodes in the electrolytes (first vinegar, then the potato), measure the voltage produced and record results in the table below. Be sure the alligator clips are not immersed in the vinegar or potato.r 6. Repeat steps 1-4 for case 2 7. Using alligator clips, have students connect their best batteries in series to the Bar Graph Meter one at a time. Anode of one battery is connected to the cathode of the next battery, so on and so forth. As more batteries are serialized, the more LEDs will light up. Materials List: LED demo by instructor: 1. LED 2. Multimeter 3. Lemon juice 4. Strips of metal: Copper and Magnesium Each group of students will need: 1. Multimeter 2. Alligator clips 3. Small cup or container 4. Potato 5. Vinegar 6. Labeled strips of metals: Copper, Zinc, Lead, Tin, Magnesium, Steel, Aluminum LED ladder for class activity: 1. LEDs 2. Breadboard 3. Electrical wire 4. LM3914 Dot/Bar Display Chip 5. 9V battery Safety Issues: N/A Lesson Closure: In closing, students reflect on their cell configurations and reconstruct their most promising electrochemical cell based on the greatest voltage achieved. It should be made clear that although they were able to produce a finite amount of voltage, the cell still remains largely impractical for useful applications. However, there is a way to boost the voltage output using the very cells they just created. This is achievable by connecting multiple cells in series, effectively creating a battery. Now as a class, students are asked to add their cell one at a time to the series circuit to contribute to the voltage output. The increase in voltage may be monitored by a multimeter with a large display or better yet a light-emitting diode (LED) ladder. The LED ladder seed a line of LEDs with the number of LEDs lighting up depending on the amount of voltage applied to the circuit. As the class adds additional cells to the circuit additional LEDs light up accordingly. Assessment: Pre-Activity Assessment: Ask students what they know about basic electrical terms such as voltage and current. Make an analogy to potential energy by elevating a bucket of water and its gradual energy loss as it gets poured. Activity Embedded Assessment: Students are given a worksheet where they are asked to fill in the blanks with answers given from the pre-activity assessment. Post-Activity Assessment: Bring students back together and accept a battery from each group. Connect the batteries in series one at a time, emphasizing the anode of one battery is connected to the cathode of the next. While connecting, ask the class about their lab results. Which electrolyte worked best? Was it clear? What were signs that oxidation and reduction occurred? Why do you think batteries cannot last forever? Were there signs in the lab that indicated this? Is this lesson based upon or modified from existing materials? If yes, please specify source(s) and explain how related: References: Attachments: Battery worksheet (Word) LM3914 Datasheet List CA Science Standards addressed: 1. Electric Phenomena 2. Acids