A Teaching Sequence for Learning the Concept of Chemical Equilibrium in Secondary School Education Marco Ghirardi,* ,†,‡,§,∥ Fabio Marchetti,* ,† Claudio Pettinari, ‡ Alberto Regis, § and Ezio Roletto § † School of Science and Technology and ‡ School of Pharmacy, Chemistry Division, University of Camerino, Via S. Agostino 1, 62032, Camerino (MC), Italy § Gruppo SENDSStoria ed Epistemologia per una Nuova Didattica della Scienza, University of Torino, 10124, Torino (TO), Italy ∥ Liceo del Cossatese e Valle Strona, Frazione Molina, 13825, Vallemosso (BI), Italy * S Supporting Information ABSTRACT: A novel didactic sequence is proposed for the teaching of chemical equilibrium. This teaching sequence takes into account the historical and epistemological evolution of the concept, the alternative conceptions and learning difficulties highlighted by teaching science and research in education, and the need to focus on both the students’ learning process and the knowledge to learn. KEYWORDS: High School/Introductory Chemistry, Physical Chemistry, Inquiry-Based/Discovery Learning, Equilibrium, Reactions, Student-Centered Learning T he concept of chemical equilibrium is a central and complex concept in chemistry. It is considered to be one of the most difficult topics in chemistry education. 1,2 Many authors have investigated students’ conceptual difficulties with chemical equilibrium, and the most observed misunderstand- ings have been summarized by several authors: students fail to distinguish between complete and incomplete chemical trans- formations; 3−7 they believe that the reverse reaction begins only when the forward reaction ends; 3−7 and students have difficulty to grasp the dynamic nature of equilibrium. 8,9 In addition, students believe that the forward and reverse reactions alternate and exist as distinctly separate events when equilibrium is attained (compartmentalized view of equili- brium) 10,11 or even the reactions are carried out in “distinct compartments”. 12,13 Sometimes students believe that the equilibrium concentrations of reactants and products are equal or proportional to the stoichiometric coefficients or fail in predicting the direction to which a system evolves, in order to adapt to changes imposed from outside. 14−17 The task of a high school teacher should be that of helping students to learn the basic concepts of a discipline. Many researchers in science education believe that learning is meaningful when the acquired knowledge is grafted onto what is already known. If such a connection is not established, learning is not meaningful and is purely mnemonic because it has been acquired passively. 18 The design and testing of teaching and learning activities in the classroom that allow for meaningful learning of the concept of chemical equilibrium should take into account the problems that arose and how they were tackled by early scientists, as there are many similarities between some of the alternative conceptions of students on science topics and the ideas of early scientists. 19 The study of the history of chemistry can help teachers design learning activities to help students understand the real nature of science and the complexity of the development of scientific knowledge and to improve their critical thinking skills. 20−24 Our teaching and learning sequence of chemical equilibrium for secondary school students is based on the following six sections: • Incomplete chemical transformation • Opposite chemical transformation • Systems in dynamic chemical equilibrium • The evolution of systems (I): from a state of non- equilibrium to a state of equilibrium • The equilibrium constant • The evolution of systems (II): from a state of equilibrium to another state of equilibrium. In this paper we present and discuss the results of the first three sections: from the idea of chemical reaction as a process that proceeds in only one direction to the concept of a system in chemical equilibrium involving two opposite reactions occur- Published: November 5, 2013 Article pubs.acs.org/jchemeduc © 2013 American Chemical Society and Division of Chemical Education, Inc. 59 dx.doi.org/10.1021/ed3002336 | J. Chem. Educ. 2014, 91, 59−65