Lesson 3 of 7
Objective: Students will be able to calculate the percent yield of carbon dioxide in the chemical reaction between sodium bicarbonate and acetic acid using a calculated theoretical yield and actual yield from performing an experiment.
In this lesson students review for their Unit 5 exam on Stoichiometry while doing a lab activity.
- This activity alligns with HS-PS1-7: Use mathematical representations to support the claim that atoms, and therefore mass, are conserved during a chemical reaction.
- This lesson aligns with NGSS Science and Engineering Practice 3: Planning and Carrying out Investigations because students are performing a lab.
For this lab activity there are several materials needed per group including:
- 1- 125 mL Erlenmeyer flask
- 1 container with sodium bicarbonate
- 1 container with acetic acid
- 1 balloon
- 1 balance
- 1 weighing boat
- 1 scoopula
- 1 100mL graduated cylinder
To begin this lesson I spend time with students reviewing their Unit 5 Exam Review.
- This was homework to complete from the previous lesson so I first stamp their review for completion. Because it is two sides I stamp each for completion.
- I then go over the answers with students and answer their questions.
- The common mistakes that students make are the conversions in part G of the review (mole to mole and 3-step). This movie is an example of my going over the answers to the review with students.
- Here is a copy of one student's work for the review.
In this part of the lesson I introduce the lab to my students. I do this in several steps:
- I begin by passing out the Balloon Lab paper to students.
- I have students look over the lab paper for 2-3 minutes.
- When students are done looking over the lab I let them know that I am going to go over a couple of the important parts of the lab.
- I tell them that #4 is tricky and is a 3-step conversion so they are going to want their periodic tables and mole road maps.
- I show them how when they weigh out the baking soda in the weighing boat how they can scoop the boat to allow it to easily go into the balloon.
- I let them know that once they are starting the reaction that they need to be wearing their safety glasses.
- I tell them to make sure that in step #6 and #7 that they attach the balloon with the baking soda to the ER flask without letting the baking soda go into the vinegar. This video shows how the balloon should be attached to the ER flask and then can be lifted to allow the baking soda to react with the vinegar.
- Finally I tell them to make sure to let the reaction go to completion as they are making observations in step #8.
This section of the lab is where students are performing the lab.
- When students get up to their lab stations all of the supplies are out for them to use.
- As students work on the lab I walk around to make sure that all students are on task by filling in their lab papers and doing the activity. I also make sure to ask clarifying questions to students to make sure that they understand the lab. Some of the examples of these questions include:
- How did you determine your theoretical yield?
- How can you tell by looking at the reaction which reactant is limiting?
- Why does the balloon blow up?
See my reflection on checking for student understanding for more details.
- Some of the major areas where students need help include:
- Calculating the theoretical yield (some still have a hard time with the 3 step conversions)
- Knowing why the balloon blows up (I give them the hint to look at the products but some need even more help)
- Calculating the percent yield (I need to remind them to look for the equation and how to determine the actual and theoretical yields. This video demonstrates how I do this.)
- Determining the Limiting and excess reactants (I give them the hint to look at their observations and note if any sodium bicarbonate was left in the reaction and whether or not the solution in the ER flask still smelled like vinegar).
- As students are done with the lab I have them turn it into my basket and have them work on studying the Unit 5 Exam Review and making sure that their binders are organized for the binder check.
I grade this lab using a rubric.
- The area where most students lose points are on the analysis questions #1 and #4.
- For analysis #1 (Why does the balloon blow up) some students write "because of the chemical reaction" or "there is a gas" but I am looking for them to realize that one of the products in the reaction is carbon dioxide which is a gas. As students work on the lab I walk around to each group asking questions to help lead them to this answer, but some still have the incorrect answer.
- For analysis #4 (Which reactant is the limiting reactant? Which reactant is the excess reactant? How ere you able to figure this out?) Some students write down "the vinegar is excess because there is more of it - 75mL ", other students write down one or the other but don't explain why. The way that I set up the lab the baking soda should be limiting (this is why I have them use it to determine theoretical yield) so there should not be any baking soda left in the reaction at completion. The vinegar should be the excess reactant and the solution should still have a vinegar smell. As students work on the lab I walk around to help them with these questions, but some students still have incorrect or incomplete answers.
- For the conclusion of the lab there are several areas where students lose points.
- Many students come up with good sources of error such as incorrect weighing of the sodium bicarbonate, incorrect measurement of the vinegar in the graduated cylinder, some of the sodium bicarbonate getting stuck in the balloon, and not allowing the reaction to go to completion. However, some students use miscalculations or "lab partner messing up" which are unacceptable.
- Similarly with "what you would do differently next time" some students write "follow instructions better" or "have a different lab partner" which are not what I am looking for. Rather I expect answers such as "make sure that the reaction goes to completion" or "make sure that all of the sodium bicarbonate is released from the balloon".