CHANGES IN CONCENTRATION AND IN PARTIAL PRESSURE IN CHEMICAL EQUILIBRIA: STUDENTS’ AND TEACHERS’ MISUNDERSTANDINGS

Abstract

We discuss two questions that deal with changes in concentration or partial pressures of the gases involved in chemical equilibrium systems. This paper analyses the responses and reasons given by both high school and fourth-year university students as well as by both pre-service and inservice teachers. The sample was taken from the Spanish context, where Le Chatelier’s principle is compulsory for Grade-12 chemistry students as well as for first year university students. It is reported that Le Chatelier’s qualitative statements were the main and almost exclusively conceptual tools used to predict equilibrium shifts when changing pressure, volume or mass. In some cases, incorrect rules led students to state correct shift predictions. Changes in concentration caused by a variation in the volume of the equilibrium vessel were mainly associated with equilibrium shift mass changes. Equilibrium law was not used at all by students; a minor number of teachers did mention the equilibrium constant in their explanations. Thus, few correct answers were ascertained. Gas behaviour misunderstandings were one of the most important obstacles when coping with changes in partial pressure due to changes in both mass and volume. Many responses concentrated on physical behaviour of gases, instead of going beyond using the equilibrium law. Many teachers erroneously associated changes in partial pressure caused by a variation in the volume of the equilibrium vessel with equilibrium shift mass changes. Eventually, this paper presents evidence, both theoretical and empirical, to conclude that Le Chatelier’s rules should be abandoned completely in chemical education. Thus, it suggests the teaching of these problems on the basis of the equilibrium law, and the focussing on the ‘history’ of the system under consideration. Obviously, such an approach will not solve all the problems one has with Le Chatelier’s rules, but it is a promising approach. [Chem. Educ. Res. Pract., 2004, 5, 281-300]