Investigating Rates of Reactions, pt 1

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Objective

SWBAT record baseline data and design an investigation to speed up or slow down the rate of reaction of an alka seltzer tablet with water.

Big Idea

One can use their understanding of the collision theory of reactions to manipulate the rates of chemical reactions.

Background

This lesson resulted from work in both our district chemistry professional learning community, and our BetterLesson online PLC.  Eric Girard and I work in very different schools within the same district.  He did this lesson the previous school year when teaching kinetics, and we collaborated in the summer to enhance the project and potentially change it to a performance assessment.

When we were both selected for BetterLesson, there was an opportunity to collaborate on a lesson and each teach it our own way.  Eric and I chose this lesson to complete it and potentially kick it up a notch.  However, we both independently decided it would work far better as an inquiry investigation than as a performance assessment.

One difference I made from our common lesson is the structured baseline test and lab planning.  In the AP Chemistry curriculum, many inquiry labs begin with a structured part one, followed by a student designed investigation.  I chose to have the students focus on what is the same and different in their plans from the baseline so that they can better isolate a single variable to change.

I know my students this year have struggled with inquiry, even when heavily scaffolded.  Therefore I thought providing a baseline procedure that they could alter would be instructionally appropriate, as I want their focus on collision theory and the content, with a manageable change in the process. 

This lesson aligns with the following standards:

  • 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.
  • Science and Engineering Practice 1Asking questions and defining problems
  • Science and Engineering Practice 3Planning and carrying out investigations.
  • Cause and Effect Cross Cutting ConceptSystems can be designed to cause a desired effect.

Equipment per lab group is:

  • 1 100mL graduated cylinder
  • 1 400mL beaker
  • 2 tablets of alka seltzer (I bought generic to save a little money)
  • Thermometer
  • Timer/stopwatch/cellphone

Lab Introduction

5 minutes

When the period begins I pass out the Alka Seltzer Investigation and remind them that we had discussed this lab briefly yesterday.  I ask students to list the three parts of the collision theory on their paper while I move to the back of the room.

We review the three parts of the collision theory aloud as a class, ensuring everyone has:

  • Collisions
  • Minimum Kinetic Energy
  • Proper Alignment

Then I orient them to the glassware in the back of the room, reminding them of the difference between a graduated cylinder and beaker.

I ask students about what safety equipment they need to wear.  One student responds "Goggles and aprons?" and another counters with "I can do this at home in my pajamas, do we need to wear that stuff?"

Since the lab is exceptionally safe, I permit students to not wear goggles or aprons as I assess the risk to be completely minimal.  Whether it is because they have no protective wear and are more careful as a result, this was the first day in lab all year without a single spill excepting one student in my 4th period.

At this point, I transition them back to the lab with their chosen partner.  I allow students to choose partners to put them at ease when working, and to help facilitate the communication between partners.  I find students who prefer to work together tend to do a better job of talking through obstacles and misunderstandings than assigned partners.

Baseline Lab Testing

20 minutes

Students go to the lab portion of the room with their partners and get started.  While they are measuring water or orienting themselves to the lab procedure, I am passing out the generic Alka-Seltzer tablets.

I remind students to measure the water carefully and be ready to time before adding the Alka-Seltzer for increased accuracy.  After a student in my 4th period creates a geyser by adding the tablet to the water in the graduated cylinder, I begin to issue reminders that the reactions are to take place in the BEAKER for the remainder of the day.

Students time their two trials and average the data.  They are surprised at the lack of temperature change.  I included it on the baseline as I wasn't certain if there would be a teachable moment about endo/exothermic reactions.  When students ask about it I explain that it is better to have collected too much data and be able to ignore some of it, rather than to have collected too little.

This example of student data provides a clean example, including their analysis of constants.

The only inconsistency is student definition of the reaction being complete.  Some students chose to use the end of bubbling, whereas others chose to wait for the white solid to be completely dissolved.  The directions in step 16 say to wait for both to be complete.  However, so long as students were consistent on their decided end-point in both trials, I allowed them to use either as the sign of the end of the reaction.

Lab Analysis and Inquiry Planning

25 minutes

When students complete the experiment, they clean up and focus on the constants in both trials of the lab.  One of the hallmarks of experimental design that students tend to forget is that they can only change one thing at a time.

Once students have analyzed what was the same in the two trials, they get to flip the paper over and think about what they want to do differently.  This example of student work shows that process.

This group chose to speed up the reaction, and decided to alter the water temperature.  They explain that by increasing the temperature of the water, the molecules will have more kinetic energy.  Most students understood their proposed procedural changes' connections to collision theory, but many didn't explain it well.

Below are the common methods students hoped to speed up the reaction with their rationales in parentheses:

  • Heat the water (increase kinetic energy)
  • Add more water (increase number of collisions and properly aligned collisions)
  • Break up the tablet (increase number of collisions and properly aligned collisions)
  • Dissolve in Coca-Cola (acid would dissolve tablet faster than water)

These are the methods students used to slow the reaction with their rationales:

  • Use ice/cold water (decrease kinetic energy)
  • Use less water (decrease number of collisions and properly aligned collisions)
  • Use seltzer water (carbonation would decrease number of collisions and properly aligned collisions)

With any time remaining, students are to begin crafting their procedure.  I explain that they can model it on our baseline test, but making the appropriate changes to include their new variable.  If students do not complete this step, they are to decide on a partner to finish it for homework.