Teaching Equilibrium

I’ve just taught this topic and thought it was worth sharing my ideas as to how I introduced the topic and concept of equilibrium.

Firstly, something worth considering before you launch into the topic is when do you teach it? In my opinion it needs to go after energetics. This is because some of the concepts need a good understanding of exothermic and endothermic reactions before one can decide on the effect of temperature on the position of equilibrium. It could also be argued that an understanding of kinetics is also a necessity as an understanding of the balance between rate and yield is also necessary when looking at equilibrium case studies in industry.

It also needs to be taught before acids and bases as to fully understand what a weak acid is.

I always begin by introducing the concept of a reversible reaction and have two ways in which I do this. Firstly, I use some anhydrous copper sulfate (white) and then hydrated it to blue hydrated copper sulfate through the careful addition of water. The hydrated copper sulfate can then be turned back into the anhydrous copper sulfate through heating.

Another good demo is to use cobalt (II) chloride in ethanol—I have been purposefully vague in this description as if you would like to do this demo it is vital that you risk assess this activity yourself. More details of this reaction can be found here:

http://www.rsc.org/learn-chemistry/resource/res00000001/the-equilibrium-between-two-coloured-cobalt-species?cmpid=CMP00005957

Essentially, the cobalt (II) chloride changes colour according to the temperature. Place the solution in ice water and it turns pink, put it in hot water and it turns blue. It is also possible to see a purple colour (a mix of pink and blue).

Once the idea of reversible reactions is established, it is worth moving on an looking at what an equilibria is. My personal opinion is that the key ideas to get over is that:

1. It takes time for the equilibrium to be established.
2. The equilibrium is dynamic.

A good way of demoing this is to use two beakers of different sizes or two tanks as shown in this clip: https://www.youtube.com/watch?v=-l5zWz_TMbM&t=4s

Water is transferred from one beaker to another. One beaker can be thought of as being the reactants, the other the products. Every time water is transferred from reactants to products, it also needs to be transferred from products to reactants. Initially, more water is transferred in one direction but after a while it is easy to see that the same amount of water is transferred – in other words, the dynamic equilibrium is established.

How do you teach this topic? As ever, it would be great to hear how you approach this. Please feel free to share your results below.