Reaction Rate Experiment: Putting the Pieces Together

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Objective

Students will be able to explain how kinetic molecular theory and collision theory relate to a reaction rate experiment.

Big Idea

Chemical reactions result from collisions between moving molecules. Higher temperatures, greater surface area, and larger concentrations lead to higher reaction rates.

Introduction

In previous lessons in this unit students have participated in a number of learning activities, including reading about reaction rates, collision theory, and the factors that influence them. They have written a draft of an experimental design, and they have tried their procedure to see how well it works. More recently, they relearned how to conduct stoichiometry in order to evaluate the early data that the trial run of the experiment generated.

In this lesson students are asked to put all of this together by writing the introduction, question, hypothesis, and procedure for the reaction rate experiment that they have been working towards.

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 must be able to explain these effects in their introduction and design an experiment that tests these principles.

It aligns to the NGSS Practice of the Scientist of Planning and conducting an investigation because students must have a complete and replicable procedure before conducting their experiment.

It aligns to the NGSS Crosscutting Concept of Cause and effect because it makes students explain the relationship between what they see at the macroscale and what is known about smaller scale mechanisms within the system.

In terms of prior knowledge or skills, students should have an understanding of kinetic molecular theory and collision theory, and they should also have an understanding of how reaction rates are effected.

There are no special materials needed for this lesson.

 

 

 

Do Now/Activator

10 minutes

Do Now: I begin class by asking students to compare the answers they found in their stoichiometry homework from the previous lesson. I tell them that I would like them to identify where they are with this skill, so that I know whether I should spend time re-teaching the whole class or small groups. Or, in the event that students feel comfortable with this skill, I can simply move on to the next topic in our unit.

I reason that this is a good way to start class because students will be tasked with interpreting the amount of product they get (carbon dioxide) from their reaction rate experiment. Comparing the theoretical yield derived from stoichiometry to their actual yield will be one of the ways they will be able to assess whether the data they get is reasonable.

Activator: I start by showing a cute short video about kinetic molecular theory, which states that molecules in liquids are always in motion, and the more thermal energy they have the more kinetic energy they have.

I have chosen this approach because it is one of the underlying principles responsible for why reactions happen in solutions.

Mini-lesson

15 minutes

Mini-lesson: I then ask students to do a thought experiment. I ask them to suppose that the entire junior class is in our classroom. I ask them to think about the likelihood of students getting into a collision if they were moving slowly around the room, compared to if they were moving very fast around the room. They make the obvious assumption that collisions increase with speed, and I remind them that molecules move faster as temperature increases.

I then ask them to think about the likelihood of students getting into a collision if there were only 2 people moving around the room, compared to if the entire junior class was in the room. They make the obvious assumption that collisions increase as the room gets more crowded, and I remind them that increased concentrations lead to increased collisions.

Finally, I ask them to think about the likelihood of me being to hit a lot of students with a dodgeball if the students are all grouped in a tight group, with most students protected in the middle of the group. I ask them to compare this to having students spread throughout the room. I ask them to tell me under which scenario will I be most successful in hitting the students and they conclude that I will hit the group if the sample has more surface area—by now they see where I am going with my analogy!

I then review the Reaction Rates Experiment Graphic Organizer. I note that what we have been discussing is a review of what they already read and discussed, and that this will be a part of their introduction.

I briefly note that while I had originally planned on having all students type the lab report from scratch, I decided instead to provide a graphic organizer because our school computers have been commandeered for PARCC testing. I also note that the organizer is a reflection of the Reaction Rates Experiment directions that they have already been working with.

I note that students should now have enough information to complete all of the information up to the data table. I note that each student should conduct 3 trials of their assigned variable, with three variations of each.

The instructional choice I made regarding the graphic organizer reflects my desire to give students the opportunity to plan and carry out an investigation, within parameters that guarantee that students will be safe and that I will have enough materials for everyone in the class to participate. 

Application

25 minutes

Student Activity: I now release students to work on the first part of their lab report. Students have a number of concerns that I address individually due to the fact that they are so different. For example, one student wonders what the question is that we are working on for the experiment. I have to redirect him to the original reaction rates experiment directions, and when he reads the big picture section he remembers. This student just needed help getting started.

Some students have a hard time with the introductory questions, and this requires that I have conversations. I ask one, for example, what concentration is. If they cannot tell me, then I ask them to go back to the text book and review this material, because I expect them to know it at this point in the unit. The student checks back in later and shows that she understands the material.

Some students do not ask for any help. For some, this is because they truly do not need any. However, there are other students who still have difficulty asking for help even this late in the school year. Their failure to ask questions does not deter me, however, because I know if I reach out to them that we will have interesting conversation that helps move them forward.

I want students doing this work because I want them to synthesize what we have been studying—the underlying science behind reaction rates, and stoichiometry. By doing this heavy thinking before they go to the lab bench, students are more appreciative of what they experience when they get there.

During this time I do two things. I conference with students as shown in this video of a student conference, and I when I am not doing that I walk around the room to answer quick questions and help students who are less motivated by taking an interest in them and their work.

Debrief

10 minutes

To wrap this lesson up I ask students to huddle with me so that we can review some of the key elements of the lab report. I ask students to explain how kinetic molecular theory and collision theory relates to their experiment.

I then note that students should try to complete this part of the lab report in the next day. I note that when they have the work complete they need to turn it in for approval or revision suggestions. In this way, I can be sure that students have a basic understanding of what they are about to do at the lab bench, and what is happening at both the nanoscale and the macroscale.

I feel like the lesson went well. I gave students a mental picture of collision theory, and supported students who needed more clarity in their experimental introduction and design. This student work sample is typical of what students produced, and this completed grading sheet is typical of what they got back when they were cleared for conducting their experiment.