Tuesday, April 4, 2017

Modeling Reaction Kinetics

In my last post, I detailed my takeaways from a powerful workshop I attended on modeling instruction. Since attending that workshop, I find myself thinking more about incorporating the ideals of modeling into my instruction. For years I have created open-ended activities in which students explore and test hypotheses, but questions have remained. How can I make better use of my whiteboards? How can I facilitate more conversations about student experiments?

One of the first topics I applied some of these modeling ideas to was kinetics. Kinetics is one of my favorite topics to teach, so it was a perfect starting point for this new inspiration. For years I have attempted a clock reaction lab in hopes that students could use data to write a rate law. Unfortunately, the results are usually a mix of inconsistent and confusing and rarely lead to even a better understanding of rate laws in general. I have led students through at least four iterations of rate law labs, each year junking that year's plan and vowing to do it better in the future.

Here is what I tried this year: On Day 1 of the Kinetics unit, I demonstrated a clock reaction for my students by mixing a solution of potassium iodate and a solution of sodium hydrogen sulfite. It's a great hook. Then I posed the question "does the concentration of both reactants affect the reaction rate to the same degree?" I sent students into the lab with 10 mL of each reactant and some tips. I asked them to collect at least 10 data points that would support the position they took to answer the question. They completed the experiments in a spot plate, measuring the solutions by drops. Most groups took between 20 and 30 minutes to complete their data collection.

I had ordered whiteboards from The Markerboard People. Each group took a whiteboard and created a graph that showed the concentration of each reactant vs time. Without revealing the whiteboards, I asked each group to summarize their experiments. Most groups conducted similar experiments, so I asked the students to hypothesize whether or not they guessed the data, and the relationship between concentration and rate, should also be similar from group to group. They said yes. Then they revealed their boards. And the data was not the same.

I asked students to talk about their data. What did it tell them? How did they explain why their graphs looked differently? What did they notice about each other's representation of information. The conversation was fantastic. Students used math vocabulary ("It looks exponential" and "This section looks linear but we might have an error that explains that" and "why would you make the scale on the x-axis run backward?") to describe their graphs and identified, without my prompting, errors that might have contributed to poor data.

In the end, they wouldn't have been able to determine a rate law, but I'm not sure that is even what's important here. I kept coming back to the essential question: Do the reactants affect the reaction rate to the same degree? Students seemed almost unanimous that the reactants have different effects on reactant rate. That notion laid exactly the right foundation for the next day's learning about rate laws.

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