Grade 12 French Students’ use of a Thermodynamic Model for Predicting the Direction of Incomplete Chemical Changes

International audienceThe authors of the current chemistry curriculum -implemented in grade 12 in France- provided a criterion of change allowing predictions of direction of chemical changes and pointed out the difference to be made between experimental facts and models. A study analysing part of the curriculum content and the effects of teaching this content on students' reasoning was conducted. The content analysis presents the functioning of the thermodynamic model which highlights the links to be made between the experimental situation and the model when predicting the direction of a chemical change. This functioning specifies the role of the chemical equation and that of the criterion of change (comparing the reaction quotient to the equilibrium constant) and stresses the crucial points that may lead to misunderstandings. Written tests were administered to students after teaching to determine how they predicted the direction of a chemical change, whether they made a relevant choice between using the chemical equation and using the criterion of change and a clear distinction between the experimental situation and the thermodynamic model. Few students had a good understanding of the respective roles of criterion and chemical equation. A majority used the criterion to predict the direction of chemical changes relevantly but correct answers were not widespread. Two particular mistakes, the modification of the expression of the reaction quotient and the prediction of a change despite a missing reactant revealed that students do not properly understand the difference and the relationships between the experimental situation and the thermodynamic model