Diet Coke/Mentos Final Exam

This is about a week old, but I hadn’t talked about it here yet.

One of the advantages of being the only teacher for a course that has no curriculum is that I can do just about anything as long as I can justify how it fits into the course as described in the course catalog.

One of the topics we did in my Honors Chemistry II class this spring was kinetics. Inspired by this video, I decided that the final exam should involve a Diet Coke/Mentos experiment.

The objective of the experiment was to come up with a rate law for the “reaction” (which is actually physical rather than chemical) between Diet Coke and Mentos.

We assumed that the pressure inside the Coke bottle is directly proportional to the rate of the reaction. The pressure of flow through an opening is inversely proportional to the cross-sectional area, so I had them make a calibration curve for pressure vs. height by drilling holes of different sizes in the bottle cap, and plotting the cross-sectional area of the hole vs. height of the spray. (Height was measured by observinging how high the spray was next to a telephone pole, and then using an exactly-5-foot-tall student standing by the pole to estimate the height.)

Now that the students had a calibration curve for height of spray vs. rate of reaction (albeit in strange units), they got a data point (keeping cross-sectional area constant) for the height of the spray with twice as many mentos, and another data point for the height of the spray with half the concentration of Diet Coke (by diluting it 50/50 with water).

I had them do the experiment as a class. (I had to explain how to plot the correlation curve and the relationship between pressure, cross-sectional area and reaction rate, but I had them make all of the decisions about the actual experiment, right down to having them tell me what size drill bits to use.) After they obtained their data, they had to individually determine the rate law from the data and write up the experiment as they would write up an entry in a lab notebook.

The experiment was a success. The kids learned some important things about working in a large group, making estimated measurements, and using a correlation to measure one thing and calculate another, even though they didn’t fully understand the relationship between what they were measuring and what they were calculating. And they had a blast doing it. We also found out that the reaction is first-order in Mentos and zero-order in Diet Coke.

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