Stoichiometry Experimental Design

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Objective

Students will be able to design an experiment in which they can evaluate how closely an experiment’s actual yield corresponds to the theoretical yield.

Big Idea

Percent yield is derived by dividing the actual yield from the experiment by the theoretical yield predicted by stoichiometry.

Introduction

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 learn how to design an experiment in which they can evaluate how closely an experiment’s actual yield corresponds to the theoretical yield. For the hypothesis, students use stoichiometry to predict how much carbon dioxide is produced when mixing a known amount of vinegar and baking soda.

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 because students use proportional relationships between masses of atoms in the reactants and the products, and the translation of these relationships to the macroscopic scale using the mole as the conversion from the atomic to the macroscopic scale. Using stoichiometry, students can predict the amount of product they produce and then compare that prediction to their actual yield.

It aligns to the NGSS Practice of the Scientist of Planning and carrying out investigations—students must plan an investigation into how closely theoretical yield and actual yield align.

It aligns to the NGSS Crosscutting Concept of Stability and Change because the experimental design is about quantifying changes that reactants undergo as they become products.

In terms of prior knowledge or skills, students should have a solid understanding of how to conduct gram-to-gram conversions using stoichiometry as described in this lesson.

The materials needed for this lesson are simply a word processor, but in order to conduct the experiment in a subsequent lesson they will need the following:

  • Baking soda
  • Vinegar
  • A vessel to mix the two reactants
  • An electronic balance that measures at an accuracy of 0.01 g

 

 

Do Now/Activator

10 minutes

Do Now: I begin class by instructing students to read the Stoichiometry Lab Report directions and record any questions they have about the assignment.

I reason that this is a good way to start class because students can get some traction on today’s work while I am taking attendance and tending to any administrative tasks.

Activator: Once I have taken attendance I ask students what questions they have. If they do not ask questions then I ask them questions using cold call to check for understanding.

I have chosen this approach because sometimes students are afraid to ask questions but they have questions that they need to ask. Sometimes I mix up my approach. First I take questions, and then I ask questions that get at the key points. These may include:

What are the reactants in this experiment? (baking soda, vinegar)

What is the product we care about? (carbon dioxide)

What will you be measuring? (the mass of the reactants and the  products?

Why are you doing this?  (to see how close product mass comes to what stoichiometry predicts it should) 

 

Mini-lesson

15 minutes

Mini-lesson: This is the first formal lab report I am asking students to write this year. I start by showing what a lab report looks like by using this Sample Student Lab Report. I want them to see that it is typed, that contains headings and data tables that have labels on them, and that it contains certain sections. This lab report video shows how I discuss the first part of the lab report.

I note then that the first thing students need to do is to write their experimental design. This serves two purposes. First, it forces students to articulate what it is they will do at the lab bench before they begin using chemicals and equipment. Second, it helps give momentum to their lab report. The experimental design is the first sections of the lab report, including the introduction, a hypothesis, the procedure and materials, and a blank data table. 

At this point in class I reveal that each student will have their own amount of vinegar that is unique to them. These vinegar volumes are posted in a few locations in the class. 

This instructional choice reflects my strategy of breaking down larger projects into smaller manageable ones. First students do the design, and on subsequent days they conduct their experiment and then analyze their results to writes a conclusion. 

 

Application

25 minutes

Student Activity: Once there are no more questions from me or the students, I ask them to begin work. Because each student brings different strengths to this work, during this time I circulate. I help some students get started, and I answer questions for others. I am looking to see that students are following the lab report format and that they understand the details for each part of the lab report.

Catch and Release Opportunities: Here are some pitfalls I am watching out for. Some students need help with:

  • how to start; catch them early so they do not waste a whole class staring at their screen.
  • forming the question, or need help seeing how the lab relates to stoichiometry
  • performing the stoichiometry; special care should be spent making sure students are using the acetic acid mass, not the mass of the vinegar. To save time I have made this Stoichiometry lab answer key so I can quickly check student work.
  • creating a step-by-step procedure
  • creating a data table

 

I can address each of these challenges individually, but if I find myself repeating myself I may stop students from working to teach, or I may ask a student to project his or her work.

 

Debrief

10 minutes

To wrap this lesson up I hand out the High School Lab Report Rubric that we will use for the rest of the year. I ask them to look over the first page. All of the criteria I am looking for is listed in the row labeled “4”. I ask students to read it and see if they have any questions. I ask them to use it to assess their work for homework. I note that an approved experimental design is a prerequisite for conducting the lab.

I then ask a student or two, depending on time, to show the class a section they have completed. I do this because sometimes it helps students to get a picture of what the work looks like, and it also helps them to realize that other students are meeting success.

I am pleased with how this lesson came out. Based on student work, everyone was able to complete the experimental design. A few students needed more support with the stoichiometry, and I was glad to give these students the incentive of a lab to help them ask for help, which they should have done anyways.

In student work sample 1, I do not notice any errors. In student work sample 2, I notice that more attention needs to be paid to scientific conventions--in this case, writing chemical formulas with subscripts where appropriate.