Chemical kinetics is an important topic that is reinforced across the undergraduate chemistry curriculum, but previous research indicates students tend to have difficulty developing a sophisticated understanding of reaction rate. In this qualitative case study, we characterized how two students conceptualized reaction rate in the context of reaction coordinate diagrams. Analysis involved using the knowledge-in-pieces perspective to model reaction rate as a coordination class. In short, a coordination class is a type of concept that involves the combination of an inferential net (a group of knowledge elements used to draw conclusions) and extractions (observations made related to the target concept). Between the two students we noted multiple distinct operationalized definitions for reaction rate; using the language of coordination class theory, these can be described as concept projections. Moreover, results indicate metonymy served as a cognitive construct that guided students' reasoning, in which a word or phrase was used to reference a larger knowledge system. On the basis of the analysis, we discuss the importance of metonymy in supporting students' problem solving related to reaction rate. As a practical consideration, we encourage interested instructors to review the Supporting Information for this work, which provides a short summary of the main findings and suggestions for practitioners.