The first time hearing about equilibrium can be confusing for students. That’s why there are so many ways to introduce it. Many are great, but can be some set-up or a big mess. I’ve used the same little demo for the past few years and I really like the way it helps the students start to visualize. And the best new is that it takes about 1 minute to set up and 8-10 minutes to complete. Students will answer a bunch of questions. And you don’t have a huge mess to clean up afterward.
I get two clear plastic tubs, the ones that are shoebox sized. Grab three beakers, two small ones (100 mL) and one larger one (250 mL). At first, I fill one tub with water. You can label that one, “Reactants”. I suggest to the students that so far in their chemical education, reactions simply go from reactants to products. Then I pour all the water from the reactant tub to the product tub.
“Can the products go back and become the reactants?”
Generally speaking, the answer they give is no. It’s a one-way arrow. There are new bonds that are more stable. The thermodynamics don’t allow it. Asking questions like this and allowing them to process out loud is very helpful.
Then I start over by pouring the water back into the reactant tub. I explain that there are some reactions that actually can go forward and backward at the same time. I pick up the two small beakers. I take a scoop from each of the tubs, cross my arms and pour it into the opposite tub.
“Why was there no reaction from the products to the reactants at the start of the this reaction?”
The answer will be obvious. There weren’t any products to react.
I take another scoop with each beaker, cross my arms and pour it into opposite tub. This time, the products-to-reactants reaction got a few drops in the beaker, while the reactants-to-products beaker filled completely. I do the process a few more times. With each scoop, the reverse reaction is increasing in rate.
“What is happening to the rate of the reverse reaction?”
It is speeding up.
“Why?”
This is where the collision model really needs to be accentuated. At first, there were no products to react. Then there were only a few. The likelihood that they would hit each other and react was very small (as represented by a few drops in to the reverse reaction beaker). As more and more products form, the mathematical likelihood increases that atoms will collide causing a reaction.
I keep scooping and crossing as the tubs get more and more even. Eventually, I start looking at how many mL of water are in each scoop. Eventually the volumes are equal.
“Will the water level be changing any more, if I keep scooping?”
No. The reaction rates are equal.
“Did the reaction stop?”
No. It’s just that the concentrations aren’t changing any more. This allows me to introduce the concept of “dynamic” equilibrium. It doesn’t stop. It stops changing concentration. Now the kids can see equilibrium. The volumes in each of the tubs are approximately equal and to rates are equal. This is equilibrium.
Then I start over. This time the I swap out the forward reaction beaker with the large beaker. So now the forward reaction is large and the reverse is small.
“Do you think I can reach equilibrium now?”
Generally, the answer they give is no. Then they stew for a minute. Oh wait…
And I start scooping. The forward reaction is getting huge scoops and the reverse small. The volume in the reactants tub goes down quickly. It goes down so far, that the giant scoop can’t get full any more. Pretty soon I’m scraping the bottom and getting the same amount in the large beaker that I am in the small beaker.
“Are the concentrations changing any more?”
No.
“Is this equilibrium?”
Yes.
“Does equilibrium have to mean equal concentrations on both sides?”
No. It only has to be equal rates of change.
Now that they start to see it, ask them to predict what the equilibrium concentrations will be if the beakers used to scoop were swapped. If the forward reaction was the small beaker and the reverse was the large, what will the equilibrium concentrations look like? They can generally see that they will be the same concentrations, just in the opposite tub. So I start scooping again. And sure enough, it’s the same concentrations just swapped.
“If we measure the equilibrium point mathematically by the ratio of products over the reactants, how are these two reactions related?”
They are inverses.
What a huge step from just a few minutes ago. Students can now visualize reversible reactions. They recognize that it is equal rates and not equal volumes that determine equilibrium. They have a vague concept of the collision model’s effect on equilibrium. And they see that reverse reactions are inverses. That is a bunch of concepts in a short time that can be referenced in the future.
I like this better than the “snowball fight” method simply because I can have controlled conversations all the way through the demo. Not that the snowball fight is bad, I just like that I can keep them more focused on the content, and less on running around and trying to smack their buddy in the head. Plus, cleanup is a quick pour down the drain and wipe a counter. This introduction gives them a foundation and off we go into the wonderful world of equilibrium.
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