The choice of sequence of ideas in this topic is important in avoiding misconceptions and in making links to prior learning about ideas of Dynamic equilibrium and The Equilibrium Law.

Simple class experiments can be used to introduce the effect of changing concentrations of reactants and products on the position of equilibrium. The concentrations can be both increased and reduced.

What experiments might be suitable for this purpose?

Suitable experiments include:

(i) iron(III) and thiocyanate ions

(ii) dichromate(VI) and hydrogen ions

(iii) acid base indicator with hydrogen and hydroxide ions

Students then need to think about how to explain their observations. It's important for them to have a good grasp of the Equilibrium Law so that they can calculate what happens to the K_c fraction when the concentration of one of the components is changed. This will enable them to predict how the reaction system must change in order to get back to the original value of K_c.

Le Chatelier's Principle can be introduced after students have applied the Equilibrium Law to changes of concentration. Make sure that it's introduced as a helpful way of predicting the effect on equilibrium position caused by changes to concentration and pressure rather than an explanation of these changes. 

The following activity will enable you to select an appropriate sequence of ideas to develop student understanding of the effect of changing concentration on equilibrium position.

[INSERT EU_01_int]