Voltaic cell lab

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Objective

Students will be be able to build and characterize important parameters of a voltaic cell.

Big Idea

Voltaic cells consist of two half-cells. At the anode, oxidation occurs, and at the cathode reduction occurs. Electrons flowing from the anode half-cell to the cathode half-cell can be harnessed to do work.

Introduction

In the last lesson students studied how voltaic cells work and they created concept maps to show how redox vocabulary relates to the voltaic cell. In this lesson students actually build a voltaic cell and then they draw a diagram to show how charges move within the system.

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 Planning and carrying out investigations because students will build and examine a voltaic cell. It also aligns to the Practice of Constructing explanations because after they build the voltaic cell they will 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 studied redox reactions and voltaic cells.

The materials needed for this lesson include:

  • multimeter
  • patch cables (2)
  • plastic cup with 50 ml of 1M MgSO4
  • plastic cup with 50 ml of 1M CuSO4
  • 3 ml 1M NaCl for salt bridge
  • pieces of a cotton ball for salt bridge
  • pipette to fill salt bridge
  • 1 piece of tubing for salt bridge
  • 1 pair of electrodes
  • paper towels
  • safety goggles

Do Now/Activator

10 minutes

Do Now:  Students begin class by reading the handout Building a Voltaic Cell from Two Half Cells. I ask them to underline anything that seems confusing to them. This is a good way to start class because it relates to what they will be doing today; students will already have some traction around the learning activity when I start teaching.

Activator: I ask students at the start of class if there are questions from the reading. I have anticipated that we will need to talk about redox reactions and the salt bridge, but I am curious to hear what else I should discuss from their perspective. The term “patch cables” is unknown to one student, and so I show him what these are; I usually refer to them as alligator clips. “Electrodes” is another word that I can easily show students—these are pieces of metal that will participate in the redox reaction.

Mini-lesson and Guided Practice

15 minutes

Mini-lesson: The first thing I planned to discuss before I head from students is the idea of relating today’s lab to the material that came before in this unit. In this video I relate redox reactions to voltaic cells to help students understand how the redox reaction relates to the voltaic cell. This is review; we discussed this in the previous lesson as well, but because it is the main point of the unit, I feel that the material merits reiteration.

I then ask that they review the voltaic cell on page 496 of their text book. I note that the set up is similar. However, we will use NaCl in our salt bridge, not KCl. I also note that our wires and salt bridge will look different than the ones in the book. However, I note that our electrodes will be the same, and the overall setup, with two half-cells (two beakers) will be identical.

Form past experience I know that there are typically two problems that students encounter from doing this lab. First, they do not always make a salt bridge void of air bubbles. Second, sometimes the electrodes are corroded and require sanding. This circuit video shows how I address these challenges.

Before letting students go, I discuss safety. I note that I do not want stuents disposing of solutions because we can reuse them. I note that the MgSO4 is the material that Epsom salts are made of, but CuSO4 is a mild irritant and is also poisonous to fish, which is why we do not dispose of it in the drains. I explain that if students’ skin comes in contact with the solutions they should rinse off the exposed area with cold water. I also mandate safety goggles for the lab.

Application

25 minutes

Student Activity: During this time students have two tasks. First, they must build a voltaic cell. With the diagram in the book, and the instruction I provide in the Mini-lesson, students do not have a difficult time with this for the most part. A few students end up with a bulb that does not work, and I show them how to use a multimeter to determine if in fact it is the bulb that is the culprit or if there is a problem with their voltaic cell. Some students also find that after they sand their electrode that their bulb lights brighter.

The second task students must complete is to draw a diagram that describes their voltaic cell. While most students are able to replicate the voltaic cell in a drawing, many have trouble applying their understanding of redox reactions to the voltaic cell. In this making sense of the voltaic cell video, I am essentially re-teaching these ideas to a group. Some groups need less support in this task, which allows me to work with students who need the extra support.

Debrief

10 minutes

To wrap this lesson up I ask two students to show their work to the class. In student work sample 1 I see confusion about flow of charge in salt bridge. I mention this to the class and ask them to fix this. I then ask a second student to explain how flow of charge relates to the salt bridge. In this student work sample 2 I see improvement, but I still do not see ions coming out of the salt bridge.

I plan to have students conduct a formal presentation of their diagrams next class, and so I know what I need to help them with before that happens from these practice presentations.