Mechanisms are part of the high school curriculum in many countries. Although research shows that university students struggle with mechanistic reasoning, very little is known about learning mechanisms in high school. Understanding the cognitive resources that high school students intuitively activate is critical to fostering meaningful learning. A readily available tool for modeling dynamic processes are stop-motion animations (SMAs). In this study, 55 high school students were asked to create SMAs of their intuitive ideas of nucleophilic substitution reactions. In a second step, the students evaluated two pre-made SMAs modeling the textbook-based stepwise (SN1) and concerted (SN2) mechanisms. Within the student-generated SMAs, SN1- and SN2-like mechanisms were equally distributed; after viewing the textbook-style animations, most students rated an SN2 mechanism as more likely. However, no group modeled both types of mechanisms or reasoned that both mechanisms were possible. Students used diverse chemical concepts as well as plausibility in their reasoning. However, simultaneous movement of atoms, conformational changes, and the idea of competing and boundary mechanisms do not seem intuitive. In conclusion, SMAs were found to be an appropriate tool for activating, analyzing, and discussing students’ resources “on the fly”, and these ideas can serve as a starting point for promoting productive mechanistic reasoning.

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