Water transfer analogy
There are a number of ways of involving students in illustrating a dynamic equilibrium. One of them is the transfer of water between two tanks.
Activity directions
Start with two tanks of water, A and B. These could be large beakers, troughs or ice cream containers. One tank is half filled with water and the other tank is empty. One student uses a 250 cm3 beaker to transfer water from tank A to tank B. Another student uses a 100 cm3 beaker to transfer water from tank B to tank A. The students transfer water simultaneously or take it in turns. A point is reached where the transfer of water doesn't change the volume of water in each tank.
Understanding the analogy
What does the volume of water transferred from one beaker to another represent?
The volume of water transferred represents the rate of reaction.
When has the system reached equilibrium?
When the volume of water in the tanks is constant.
What do you know about the volume of water transferred at this point?
The volume of water transferred between both tanks is equal.
The procedure can be repeated starting with tank B half full of water and tank A empty. Everything else is kept the same.
What feature of an equilibrium is illustrated by this procedure?
That an equilibriun can be established starting from either reactants or products.
How will the volumes of water in the two beakers at equilibrium compare to when the equilibrium was established starting with water in tank A?
The volumes of water in the tanks will be the same.
Like all analogies care must be taken to identify its strengths and weaknesses in relation to the concept it is used to illustrate, and to prevent the analogy reinforcing student misconceptions.
What misconceptions might this analogy inadvertently reinforce?
The idea that the forward and reverse processes are separate reactions that occur in different compartments. The idea that forward and reverse reactions are sequential rather than simultaneous changes.