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Recently I participated in an APTeach discussion that focused on using activities to teach chemical kinetics and address common student misconceptions involving the topic.

One resource we reviewed was the sequence provided by the Course Exam and Description (CED) that was released by the College Board. That document follows the order of most textbooks, beginning with experimental rate laws, followed by collision theory and reaction mechanisms.

As I followed along in the discussion, I found myself reflecting on the traditional sequencing I once used in my own classroom, and how the experiences of my students inspired me to change to an entirely different order of topics.

Previously, when I started my chemical kinetics unit in the traditional order, I consistently saw that students were confused about why the data did not match the balanced equation of the chemical reaction. My sense was that this confusion also led to reduced comprehension about the relationship between experimentally determined rate laws and reaction mechanisms.

Because of my own limited experience beyond organic and inorganic chemistry, I sometimes struggled to answer students’ questions with anything more than simple explanations. So, taking a different approach than many of my peers, I began to use my own misconceptions of mechanisms and rate laws to change my approach to teaching chemical kinetics. I now start by discussing the concepts of collision theory and reaction mechanisms, and then teach about using experimental data to determine the rate law using methods of initial or integrated rates.

Teaching the topic of mechanisms before rate law

Most recently, I’ve been using the first two days of the unit to introduce students to collision theory and teach them how to collect data on rate of reaction. Many of my students’ only experience working with the concept of rate was calculating speed in their middle school science classes, so I have them measure the rate of reaction using a variety of data.

For example, my students measure the carbon dioxide gas produced through a reaction of hydrochloric acid and marble chips, and collect data on change in absorbance when bleach and a blue dye are mixed. During these activities, students are learning how to operate lab equipment and collect data to determine the change in rate of reaction over time for an overall reaction. This introduction to basic lab techniques has also improved student success when working to determine the rate law based on experimental data.

After exploring the concepts of overall rate of reaction and the stoichiometric relationship of consumption and production, I have my students explore collision theory using the childhood board game, Hungry Hippos, as a model for effective collisions. Using this sequence of activities, I’ve found that my students gain a solid understanding that reactions must have proper orientation and sufficient energy in order to produce a successful collision. They also gain insight into how changing variables can affect the rate in terms of frequency and forcefulness of collisions.

Next, I have students learn about the chemical reaction for cellular respiration — something many of them learn in their earlier science courses (see Figure 1). Students are tasked with drawing a particle diagram to show what must happen for cellular respiration to occur. This leads many students to the realization that it is nearly impossible for a molecule as large as glucose to collide with six molecules of oxygen at exactly the right orientation, and with sufficient energy, to produce a reaction. Some students even predict that this reaction would not occur (not realizing it is the process required for energy production in aerobic living organisms). I also have them review diagrams from OpenStax for glycolysis and citric acid cycle (Kreb’s cycle). Based on what they find there, I have them write out the series of reactions that occur. This allows them to show how the reactants of one step (such as glucose to pyruvates) produce the reactant that begins the next step (such as pyruvate to acetyl-CoA).

Figure 1. The author’s students used cellular respiration as a model to understand why collision theory requires that the overall reaction occur as a mechanism that includes the steps of glycolysis, Kreb’s cycle, and electron transport chain.

During the five minutes I give them to use the diagrams and write the mechanisms, I often guide students to think of each transition as a separate step that needs a reactant and a product. From these diagrams, I encourage students to focus on the major reactants and products of each step and to think about how the steps are connected. It is important to note that each name listed in the diagrams is a more complex organic formula, but that they are changing to a vital structure each time the name changes in the process.

While their examples are not complete at the end of this brief activity, the experience does enhance their understanding of several multi-step processes occurring in a very complex reaction. Since students have no prior knowledge of mechanisms, this provides an entry point for discussions of much simpler reaction mechanisms addressed in AP Chemistry, as well as a way to introduce intermediates and biological catalysts that can be seen throughout the images provided in the OpenStax sources.

Teaching the concept of mechanisms before experimentally-determined rate laws is a great opportunity for reminding students that experimental data not only helps predict possible mechanisms by analyzing reactions for potential intermediates, but also helps determine whether or not a mechanism is valid. The lessons address the fact that during reaction mechanisms, valid mechanisms must match the overall reaction with fast and slow steps in order to write the rate law. Students also learn about intermediates and catalysts with energy profiles of multi-step mechanisms.

When I introduce experimentally-determined rate laws (whether calculated using initial rates or integrated rate law methods), students learn to analyze the experimental data to determine simply whether or not a mechanism is valid. While this order may not work for all classrooms, my students had huge success with it, reflected in their scores on our chemical kinetics tests. I plan to continue using this sequence when teaching kinetics going forward.

Activities to support learning

Unit 5: Chemical Kinetics of the CED suggests using 13-14 class periods (assuming 45-minute periods that meet five days a week) to cover the topic. However, only 7% of the multiple-choice portion of the AP test focus on these concepts. So, in my classes, I cover the unit in only 10 days, which allows more time throughout the school year to focus on other units that are emphasized more heavily on the AP exam, or that are more challenging for students. This unit is broken up into three major sections: Introduction to Rates and Collision Theory, Reaction Mechanisms, and Experimentally-Determined Rate Law.

I use AACT Kinetics activities based on the sequence that emphasizes mechanisms before experimentally-determined rate laws to create a flow that fosters deeper understanding of how to use data and observation to apply kinetics. Typically, I modify the activities to be more open-ended, which is a focus of our district’s science curriculum. In addition to the activities provided, there are generally 1-2 days within each topic used for other aspects of learning, such as direct instruction, guided practice, individual practice, or assessment. Here are the activities I’ve used in my classroom:

Introduction to Rates and Collision Theory (CED topics 5.1 & 5.5)

Figure 2. Students playing “Hungry Hippos” as an introduction to collision theory.

  • Ted Ed video “How to speed up chemical reactions (and get a date)” by Aaron Sams (8 minutes)
    • I love watching this cringy video with my students, as it connects the effects of changing temperature, concentration, and catalyst to the rate at which a chemical reaction occurs in terms of frequency and energy of collisions.
  • Student Misconception Alert! Since this unit emphasizes rate, it is important to spend time making sure students understand that rate changes over time. Students also struggle with the concept of initial rate, instantaneous rate, and overall rate of a reaction.

Reaction Mechanisms (CED topics 5.4, 5.6, 5.7, 5.8, 5.10, and 5.11)

  • Cellular Respiration (Figure 1) (15 minutes)
    • Using the overall reaction for cellular respiration and images that show glycolysis, Kreb’s Cycle, and electron transport chain, students write possible mechanisms.
    • Student Misconception Alert! Students think that only one step of the process occurs at a time, like a repeating cycle, rather than occurring constantly with multiple steps occurring at any given moment.
  • Rate Determining Step (15 minutes)
    • Found in the AACT Reaction Mechanisms Lesson Plan, with both teacher and student instructions.
    • Instead of using the student guide, students can draw a storyline to show slow and fast steps, along with the addition of a catalyst

    Experimentally-Determined Rate Law (CED topics 5.2, 5.3, and 5.9)

    • Sulfur Clock Lab (30 minutes)
      • Adapted from the AACT lab: Starch-Iodine Clock Reaction
      • Due to supply availability, I’ve adapted the lab to use sodium thiosulfate and hydrochloric acid.
      • Extended to select a valid mechanism based on the experimental data.
    • Crystal Violet Lab (30 minutes)
      • Uses spectroscopy to collect data to use the integrated rate law to determine reaction order.
      • Students do not determine a valid mechanism, since rate order with respect to sodium hydroxide is not determined.
    • Student Misconception Alert! It is important to emphasize that the rate law based on the rate-determining step of the reaction mechanism needs to match that of the experimentally-determined rate law. This helps reinforce that the mechanism must be supported by experimental data.

    Collaboration and reflection

    Teachers are always looking for new ways to foster the best experience for their students. It never hurts to get ideas from other teachers on ways to transform your teaching to improve student learning.

    For new teachers, or teachers who struggle with the concept like I did, thinking about how to sequence each unit to deepen student understanding can require a bit of a journey. In my case, I started by using my own misconceptions about reaction mechanisms to redesign the unit, and also ran ideas past several other teachers to get their input and gather new lesson ideas. I’m sure there will be things I need to modify for next year, but kinetics is already off to a fast start this year!