Le Chatelier’s Principle is easy. To you. But for whatever reason, it can take kids a while to see how perturbations to a system in equilibrium cause shifts in the quantities of the products and reactants. Mostly I wanted to honor my college professor who liked to use the word “perturbations”. But Le Chat can be tricky. Years ago, I started introducing the idea with a silly little drawing on the board, and I use it to this day. Let me introduce you to the “Ants on a Teeter Totter Principle”.
I draw (at least attempt to draw) a basic teeter totter with an equal number of ants on each side. I am not sure why I chose ants and not something with a little more mass, but oh well. It’s easy to see that the system could be in equilibrium. You could also draw fat ants and skinny ants. Ask the kids if those could reach equilibrium. They can, just not in the same location on the board. Once the teeter totter is in equilibrium, stress it out.
A bird comes along and picks off one of the ants from the product side. Is the system in equilibrium any more? Nope. Now comes the key. Which way do all the ants have to walk to regain equilibrium? The direction that they are walking indicates which way the reaction must shift. If they are walking towards one side, then that side is increasing in concentration. In this example, the shift would be toward the right or the product side.
Now draw the system in equilibrium again. This time add an additional ant to the reactant side. Which way do the ants walk? This identifies the shift. Again, they have to walk right.
It takes the students about five whole seconds to see the pattern. Then I start giving them real chemical equilibria and asking them to identify the shifts. The basic shifts are almost automatic now. Add a reactant, shift right. Add a product, shift left. Remove a product, shift right.
This is where the Ants on a Teeter Totter Principle ends. Once the stresses on a system include adding solids, catalysts, and changing volume it gets trickier. (Though feeding all the ants Red Bull does behave a little like a catalyst. But now the story is just getting weird.) But since I have used the ants to help with the simpler shifts, they can focus on the more challenging aspects of the principle.
I love it when I watch the kids taking their chapter test and they make a little teeter totter with their hands, tip it out of equilibrium, and then watch as they envision little ants walking up their arm as the shift brings the system back into equilibrium. They are now visualizing a challenging subject in a tangible way. When they do that, performance increases.
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