Conducting the Experiment
Lesson 7 of 8
Objective: Students will be able to calculate the mass of carbon dioxide produced from a reaction by subtracting the mass of the products from the mass of the reactants.
This lesson is part of a three-day lab. In the first day students design their lab, which includes solving a stoichiometry problem. On the second day they conduct the lab, and on the third day they write and critique their lab report.
In this lesson students will conduct a lab that they planned in the previous lesson. In their experimental design, students used stoichiometry to predict how much carbon dioxide would be produced from a set amount of vinegar and baking soda. After their experimental design has been approved students will conduct the lab.
This lesson aligns to the NGSS Disciplinary Core Idea of HS-PS1-7. Use mathematical representations to support the claim that atoms, and therefore mass, are conserved during a chemical reaction by using the baking soda-vinegar reaction and stoichiometry to account for the results of the reaction.
It aligns to the NGSS Practice of the Scientist of Planning and carrying out investigations because in this lesson students are in the middle of a three-day investigation about stoichiometry. The lab handout they received in the last lesson was called the Stoichiometry Lab Report. Each student was assigned a unique amount of vinegar as detailed in this vinegar volume assignment spreadsheet.
It aligns to the NGSS Crosscutting Concept of Energy and Matter because students must account for the change in mass that occurs as the loss of carbon dioxide from their reaction beaker.
The materials needed for this lesson include the following:
- 10 g of baking soda per trial
- About 30 ml of vinegar per trial
- 2 cups or beakers
- A stirring rod
- An electronic balance capable of measuring to the 0.01 g
Do Now: I start class by asking students to take out their experimental designs. I track students who have been approved in a spreadsheet, so I am aware of who still needs help with their experimental design and who is ready to start the experiment. While students are accessing their file on the computer, I take attendance and check in with students who were absent last class. They will need time with me to ask questions after they have reviewed the handouts from last class.
I reason that this is a good way to start class because pretty quickly students are aware of what they will be doing today—either taking care of their experimental design or conducting the experiment.
Mini-lesson: At this point, many students have a fairly strong idea of what they need to accomplish in the lab; this objective partially explains my rationale for having students write their experimental design as a prerequisite for doing the lab.
At this point, because many students are ready to conduct the lab, it is time to discuss provisions, clean-up, and lab safety. I explain the following:
First, I discuss safety: Our rule is that goggles are worn anytime we use heat, chemicals, or glass. Goggles must be worn because we are using glass beakers. I remind students to walk slowly and to watch where they are going.
Second, I discuss provisions. I have a balances set up around the room, and I have baking soda and vinegar at each station. I remind students to zero the balances after putting a beaker on them so that only the mass of each chemical is measured.
Third, I discuss clean-up. I assign different students to different balances, and I remind students that all spills should be cleaned up immediately. I explain that the chemicals can be poured in the sink, and that all glassware should be cleaned and put on the drying racks when not being used.
This instructional choice reflects my desire to help keep the project moving forward. I want the students who have worked hard to get their experimental design completed and revised to feel good about their progress. Students who are still working on the experimental design get motivated when they see students moving forward with the next phase of the project. I encourage the few students who get demoralized by being behind.
Student Activity: Once I have discussed safety, provisions, and clean-up, I release students who have completed their experimental design to conduct the experiment. During this time, I am busy. I help students who need assistance finishing their experimental design, but I also keep one eye on the class. I want to make sure students are not wasting materials, that there is access to the balances, and that the procedures are being followed. I also want to watch for errors in the procedures. One of the biggest challenge students have is remembering to zero the balance and to record all of their data. The other challenge is to keep an eye on the reason for the lab amid all of the details of conducting the lab. This stoichiometry lab video shows how I remind students of the connection between their stoichiometry and the lab they are conducting.
Catch and Release Opportunities: I stop class if there are safety issues or if the lab is getting messy, but typically I speak only to individual students about transgressions as long as the overall climate of the lab is productive and safe.
I also keep a tight eye on the clock. I want to make sure that I give students plenty of time to clean the lab before the next class or before the end of the day—because I do not want to be stuck doing it!
To wrap this lesson up I first inspect the lab. I stop all reactions several minutes before my debrief to ensure that students have time to clean. I ask students to evaluate the safety and respect of the lab.
I then turn to helping students think about their results by reviewing with them the directions for the lab report sections called Analysis/Results and Discussion and Conclusion.
I first draw their attention to the equation for percent yield, which they will need to calculate once they have their three trials.
I then ask them to review the questions in the Discussion and Conclusion section and ask if they have any questions. When there are no questions I ask the class to explain how they will answer each question and let someone volunteer their answer. This helps me to insure that students understand what they need to discuss.