Reversible Reactions and Equilibria
Students mainly experience chemical reactions that appear to go to completion. When they meet a reaction that does not go to completion but which has a reverse reaction occurring they find the concept difficult to understand.
One major misconception students have about equilibria is that they think equilibria positions are fixed and once achieved there is no movement of particles between the two 'sides' i.e. they believe that equilibria are static not dynamic.
Le Chatelier's Principle is a way of predicting changes to an equilibrium position under some circumstances but is generally wrongly applied due to the misunderstanding about the equilibrium position.
Rate and equilibria are often confused because students think that the rate of one reaction may change while the other slows or remains constant. They have not grasped that rate applies to the system as a whole.
It is important to use a wide range of reversible reactions to help get these ideas across to the students.
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Using Video to Illustrate Dynamic Equilibria *suitable for home teaching*
In this short video clip groups of students describe how they used video to illustrate a dynamic equilibrium. Students did this by rolling dice and, depending on their score, taking off or putting on their blazers. They noted that, on average, the number of them with blazers on or off remained roughly the same.
Le Chatelier's Principle in Colour
This is a good visual demonstration of Le Chatelier's principle using cobalt chloride and hydrochloric acid. The resource contains teacher and technician notes.
Audio on Fritz Haber
This is a short audio about Fritz Haber, who along with Bosch, shared the 1918 Nobel Prize in Chemistry for the discovery and development of the Haber-Bosch process for the synthesis of ammonia. There is also a set of notes with some good background information for teachers.
Equilibria Activities
This resource is a really useful series of practical activities looking at equilibria. It was written for gifted and talented students but could certainly be repurposed to be used with all abilities. There are both student and teacher packs.
Rate and extent of chemical change - chemical equilibrium - R2P
This resource provides a lesson plan showing how to use modelling-based teaching to teach about chemical equilibrium. The lesson outlines how to use a demonstration of the appearance of nitrogen dioxide (NO2)/dinitrogen tetroxide (N2O4) in a sealed tube at different temperatures to introduce the concept that chemical reactions can be reversible and that they reach an equilibrium which is dynamic. Another activity asks pupils to work in groups to model what is happening at a particle level in dynamic equilibrium. A research summary discusses the use of modelling-based teaching to promote successful learning about chemical equilibrium, improving pupils' understanding, reasoning and ability to make predictions.
By the end of the lesson pupils should be able to:
- describe the relationship between the forward and backword reaction rates when a reaction is at equilibrium
- use models to represent the relationship between the forward and reverse reaction rates when a reaction is at equilibrium
