Evaluating Reaction Rate Data using Stoichiometry

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Objective

Students will be able to use stoichiometry to evaluate initial data they obtained from a reaction rate experiment they conducted.

Big Idea

Performing stoichiometry is the only way to determine the theoretical yield from a chemical reaction.

Introduction

In a previous lesson, students conducted a single trial of their reaction rate experiment to see how the reaction works, and to see what revisions they may need to make to their experimental procedure. The reaction rate experiment involves measuring how fast carbon dioxide is produced by mixing calcium carbonate and hydrochloric acid. Different students investigate different variables, including temperature, concentration, and surface area.

In the last lesson they became reacquainted with the concept of the mole, and with calculating how many moles were in a sample. In this lesson, students combine ideas from these previous two lessons by using stoichiometry to evaluate the initial data they obtained from the reaction rate experiment they conducted. Performing stoichiometry is the only way to determine the theoretical yield from a chemical reaction.

This lesson aligns to the NGSS Disciplinary Core Idea of HS-PS1-5: Apply scientific principles and evidence to provide an explanation about the effects of changing the temperature or concentration of the reacting particles on the rate at which a reaction occurs because students are analyzing initial data from the reaction they are using in their reaction rate experiment. Due to its reliance on stoichiometry it also aligns to HS-PS1-7: Use mathematical representations to support the claim that atoms, and therefore mass, are conserved during a chemical reaction.

It aligns to the NGSS Practice of the Scientist of Planning and carrying out investigations because the lesson is designed to give students a sense of what their yields should look like. It aligns to the NGSS Practice of Using mathematics and computational thinking by giving students the chance to mathematically derive an answer to the question from a previous class: is the amount of carbon dioxide you produced a reasonable amount, or was there a measurement error?

It aligns to the NGSS Crosscutting Concept of Mechanism and Prediction: Cause and effect relationships can be suggested and predicted for complex natural and human designed systems by examining what is known about smaller scale mechanisms within the system because students are studying the effect of manipulating temperature, concentration, or surface area on the reaction rate. These variables all are influenced by kinetic-molecular theory.

In terms of prior knowledge or skills, students will benefit most from this lesson if it follows the sequence of events outlined above.

There are no special materials needed for this lesson, although I find it helpful to have a classroom set of calculators in case students do not have their own.

Do Now/Activator

10 minutes

Do Now: I ask students to begin class by comparing the answers they got from the previous day’s work, which was a set of practice problems. In those problems students had to calculate molar mass, and then calculate the number of moles from either the molarity formula or by converting from grams.

I reason that this is a good way to start class because it will give me a chance to walk around the room to give homework credit for students who have completed the assignment, and it will give students the chance to check in with one another about difficulties they had with the assignment.

Activator: Once I have taken the pulse of the class, I ask students to give me an example of a problem they struggled with. What  I learn from this is that students who did the work understand it, while students who did not do the work do not know whether they understand it. I decide that I am going to let the students who do what I asked them to do set the pace of the class, and it will be up to the other students to catch up, with my support during independent work if necessary.

I have chosen this approach because I want students to feel comfortable with calculating moles, as this skill is a prerequisite for stoichiometry, which is the focus of today’s class. I believe that if students did the homework from the previous class they should be ready for today.

Mini-lesson and Guided Practice

15 minutes

Mini-lesson: I begin by reviewing stoichiometry. I do this by discussing each of the steps in the notes at the top of the page called Stoichiometry Notes and Practice Problems.

First, I note that you must have a balanced chemical equation because this will show the ratio of one reactant to another; you use the coefficients in mole ratios.

Second, I explain that the reason we focused last class on converting grams of the limiting reactant into moles is because the mole is the unit in the balanced chemical equation, and so we must know how many mole we have in order to use the equation to figure out our product. I also note that the way the experiment is set up, the chemicals should run out at the same time, so I have chosen CaCO3 as the limiting reactant.

Third, I explain that once we know how many moles we have, we multiply this answer from step 2 by a mole ratio with the “asked” on top. This will allow us to find the product in moles. I note that the product in moles that we care about is CO2.

Fourth, I note that because our balance does not measure in moles, we must convert our answer from step 3 into grams. Mathematically this looks the same as converting grams into moles, except our gram-mole conversion factor is flipped.

Finally, in order to know how close the reaction yield was to what was possible, a percent yield is conducted. To derive this figure, divide the mass from your experiment by the answer from step 4 and multiply this number by 100 to get a percent yield.

I then model a practice problem in which I began with 12 g of CaCO3.

Guided Practice: Once I have shown students how to do a sample problem, I turn the class over to them by asking them to do the first practice problem.

I chose this particular focus so that students could go from passively watching me to being active participants. They will only learn the material by doing it, repeatedly, and so the sooner I can get them working the better. However, it is in the class’s best interest if most students know how to do the problems before I turn the class over to independent work time because then I will be able to work with the few students who need additional support.

Application

25 minutes

Student Activity: During this time students practice calculating the various percent yields from the scenarios in the practice problems. Some students help or get help from other students. I walk around and inspect student work, answer questions, and think about whether there is anything I need to tell the whole class in terms of how to do these problems.

I want students doing this work because I believe that they need to practice and spend time grappling with the material if they are going to learn it. This work will also flow into their lab report. In the hypothesis section students need to predict their yield, and in their conclusion they need to analyze their yield. They can only do this if they are comfortable with the stoichiometry.

I emphasize to students that they will only get credit for this assignment if they have set up their work the way I have shown them. I want them to have a record of how they got the answers they got so that if they get an answer incorrect we can identify why. I also want to encourage students to do the work and not just rely on others to fill in the table.

Catch and Release Opportunities: At one point I stop class today because I notice that students have gotten away from practicing the steps of stoichiometry because they figured out a quicker way to do the problem. I remind them that it is as important to know the process as it is to get the answer, and I encourage them to practice the problems.

Stopping class to discuss this is important because the learning target for today is that they can use stoichiometry, and my concern is that without practicing they will not move forward on this learning objective. This fear was probably not well-founded, because they are able to evaluate their own data using stoichiometry, as we can see in the debrief. Still, as this stoichiometry catch and release video shows, I do want to emphasize that the process is important.

Debrief

10 minutes

To wrap this lesson up I have a student show their work.

Ending class this way allows me diversify the leadership in the classroom, and it allows me to hear from a student about their process. For homework, I ask students to finish these problems, and to analyze their own data from their initial trial of their experiment. As shown in this stoichiometry debrief video, one student has already done this work because, as this student work shows, she already completed the stoichiometry practice problems.

The results of student analysis is interesting because this is where they will see whether the data they generate at the lab bench is reasonable or not, and it is with sharing this analysis that we will start next class. For this student's data, it is clear that there is a measurment error of some kind because while the stoichiometry suggests a theoretical yield of about 0.6 grams, the student obtained around twice that amount. 

I love how this lesson turns out. It clearly shows students the importance of stoichiometry's role in evaluating reaction rate data.