Teaching Equilibrium in Year 12 New Zealand Curriculum Level 7 –The Structure of Matter Develop an understanding of and use the fundamental concepts of chemistry (for example equilibrium and thermochemical principles) to interpret observations. Covered by Chemistry Achievement Standard 2.6 –Describing and interpreting chemical equilibrium –Writing equilibrium constant expressions –Predicting changes in concentrations of reactants and products when conditions change.

Introduction Equilibrium can be a difficult concept for students to grasp Constructivist approach to building conceptual knowledge – Establish prior knowledge and understanding of equilibrium processes – Work from more familiar physical processes up to chemical equilibria – Working in groups through worksheets/practical experiments to build own understanding

1. Physical Equilibrium Brainstorm/Discussion on Equilibrium – Establish prior knowledge Physical Equilibrium – Deal with physical processes first as they may be more familiar and more easily understood – Compare water in open beaker with water in a sealed beaker Evaporation versus evaporation/condensation In the closed system the two processes are occurring at same time – dynamic equilibrium

2. Chemical Equilibrium Make comparison between the physical equilibrium just studied and a chemical equilibrium, e.g. Cr 2 O 7 2- (aq) + OH - (aq) ⇌ 2CrO 4 2- (aq) + H + (aq) orange yellow – Can change between colours just by adding acid or base – Develop the idea that the two reactions are occurring at same time – a dynamic equilibrium – Also that at equilibrium the concentrations of each species do not have to be the same (a common misconception) This could be done by having two different solutions – one which is orange with a trace of yellow and one which is yellow with a touch of orange – but both at equilibrium

3. Model of Chemical Equilibrium Involves two students using straws to transfer water between two pots Measuring the height of the water allows students to follow the progress towards equilibrium Eventually both are transferring the same amount of water and equilibrium is reached Different starting points lead to the same equilibrium position

4. Equilibrium Changes First need to introduce – the equilibrium constant and explain what information it does and does not give. (extension for able students - look into deriving the expression) – Le Chatelier’s Principle Have a range of experiments available that students can work their way through to look at the effects of temperature, pressure and concentration on the equilibrium position. – Make predictions – Carry out experiments – Rationalise observations

5. Equilibrium Changes The earlier pot and straw experiment can also be used to show – the effect of adding or removing reagents or products to a system at equilibrium (add more water) – Speeding up the rate of either the forward or reverse reaction, i.e. temperature (one large straw, one small straw) – The effect of a catalyst – speeds up both reactions (two large straws) The effect of adding reagent or product can be seen through the colour changes in the chromate/dichromate equilibrium The effects of changing pressure or temperature can be seen in the NO 2 /N 2 O 4 equilibrium (videos also available on YouTube)

Reflection Students compare their current knowledge with their previous ideas Evaluate which experiments were useful and identify any which did not help Special Needs – Large font worksheets available. Large font labels on any chemicals. Be aware colour changes may not be visible. – may need to work in groups or pairs during practical work. – ESOL – Have sheets with additional vocabulary/terminology required for the unit available beforehand. Could work in pairs with an advanced student who could peer teach/explain methods etc.