Chemical Equilibrium ‘There seems to be no topic in freshman chemistry that presents more difficulties to students than chemical equilibrium’ Chemical equilibrium is a central organising concept in the teaching of chemistry. It would be only when the students had simultaneously understood all the interactive, energetic and dynamic aspects of chemical reactions that they could be said to have developed an integrated conceptual understanding of chemical reaction.

Guldberg&Waage 1879 Horstmann 1873 Van’t Hoff 1884

Some problems, indeed 1. Introduction of chemical equilibrium usually is preceded by a discussion of reaction kinetics. However, the assumptions underlying the kinetic derivation of the Law of Mass Action are not simplistic. 2. Much attention is traditionally paid to Le Chatelier’s principle. However, the use of Le Chatelier’s principle has been criticized in the chemical education literature. Stimulating the formation of misconceptions among students. 3. The Equilibrium law is normally presented as a fact per se or as an empirical law. Thus, a thermodynamic approach to teaching equilibrium appears as a post-hoc justification.

Common misconceptions Students do not discriminate between reactions that go to completion and reversible reactions. Students fail to distiguish betweeen rate (how fast) and extent (how far) of reaction Students may believe that the rate of the forward reaction increases with time. Students may believe that the forward reaction goes to completion before the reverse reaction commences.

Students often fail to understand the dynamic nature of the chemical equilibrated state. at equilibrium no reaction is taking place equilibrium is sometimes seen as oscillating like a pendulum Students may have a compartmentalised view of equilibrium, as somehow involving two disconnected sides of the reaction (‘left reaction and right reaction’) Students may believe that mass and concentration mean the same thing for substances in equilibrium. At equilibrium the concentrations of the reactants equal the concentrations of the products.

The equilibrium constant changes with change in concentration or volume of reactants. - lack of understanding of stoichiometry A large value of the equilibrium constant implies a fast reaction. A catalyst affects the rate of the forward and reverse reaction differently. Many problems arise from use of terms of commonly used in the everyday language: reversible, affinity, shift, stress, balanced.

Le Chatelier’s principle ‘When a stress is applied to a system, the system responds in a way to try to relieve the stress’ Students tend to apply the principle without full understanding. In particular, they often apply the principle outside the area in which is valid (variation in pressure or temperature). Students tend to misapply kinetic arguments to explain changes in a state of equilibrium Students prefer applying Le Chatelier’s principle in a qualitative way with respect to use the Equilibrium Law for quantitative prediction of equilibrium point .

Teaching Chemical Equilibrium Core concepts Reversibility of chemical reactions What happens if you add a reactant to an equilibrated state? Describe the reversibility of a chemical reaction in corpuscolar terms Dynamic equilibrium Simulation and analogies ‘The Apple war’ In silico simulation

Students need to subscribe the idea that the forward and reverse reactions are taking place simultaneously (conceptual change) Students should produce graphs of reaction rates, tracing the amounts of substances as a function of time. Teachers and authors of textbooks should be very careful about the use of specific language in this domain. Apply the equilibrium Law to various types of systems (acid/base, electrochemistry, biochemistry) - avoid fixed rules and algorithms Most of the difficulties in this area are mainly due to the abstract nature of the chemical equilibrium concept.

It is better to confront students with the complexity of phenomena, than to teach them simplified truths. Ex. It is not possible to predict the changes in equilibrium upon Temperature changes without knowing if the reaction is exothermic or endothermic.

Selected readings