Different models used to interpret chemical changes: analysis of a curriculum and its impact on French students' reasoning
We present an analysis of the new French curriculum on chemical changes describing the underlying models and highlighting their relations to the empirical level. The authors of the curriculum introduced a distinction between the chemical change of a chemical system and the chemical reactions that account for it. We specify the different roles of the three identified models: a thermodynamic one based on the reaction quotient and the equilibrium constant, a kinetic macroscopic one based on the rates of the opposing reactions, and a kinetic microscopic one. According to our analysis, interpreting the end-point of an incomplete chemical change should offer an opportunity to use the three identified models. We have investigated the reason(s) the students (grade 12) gave to explain why a chemical change remains incomplete, and whether they used one explanatory model more than another. The two macroscopic models were not used by a majority of students, and no student referred to the microscopic kinetic model. A rather high proportion of students held static conceptions of the equilibrium state. These results are discussed.