Wednesday, February 11, 2015

Equilibrium Games are a Win

Water transfer equilibrium analogy.
I haven't taught equilibrium for nearly ten years!  This time around I decided to tackle the conceptual understanding of equilibrium first with three different analogies and a POGIL activity before even mentioning the words "equilibrium expression".  The outcome has been very positive from my perspective, even though my AP Chemistry students are not always eager to volunteer to be a reactant or a product in my activities.

The first thing I did was show the students the "Red Pill or Blue Pill" clip from the Matrix.  I likened equilibrium with taking the red pill because you have to open your eyes to the real world of chemistry.  We can no longer pretend that every reaction goes all the way to completion!  Maybe this scared the kids more than amused them, but I found it very funny.

Using two different sizes of straws to transfer water.
The first equilibrium analogy we did was the famous water transfer reaction with two different sized cups.  I labeled two large bowls "reactants" and "products".  The reactant side was filled half way with blue water and the product side was empty at the beginning of the reaction.  Two students volunteered to transfer water from reactants to products or from products to reactants, each using a different sized beaker (I used a 50 mL and 250 mL beaker).  When the reaction began, the product to reactant transfer was very small, but steady gained in volume as the reaction progressed.  Meanwhile the reactant to product transfer started large and gradually shrank.  After every 5 or 6 transfers we tested to see if we had reached equilibrium by measuring the volume of water transferred in each direction.  This simple and fun demo was an excellent kick off to the conceptual understanding of equilibrium.  When it was over my students understood that equilibrium is reached when the forward and reverse rates are equal, not necessarily when the amount of reactant and product are equal.

Now the students were ready to put some numbers to their water transfer equilibrium analogy.  The students conducted the classic experiment with two graduated cylinders and two different sized straws.  They used the straws to transfer water from one graduated cylinder to the other, each time measuring the volume in the two cylinders.  (By the way, McDonalds has really large straws that are perfect for this activity.)  The result is a beautiful graph of the concentration of the "reactants" and "products" as they approach equilibrium.  It only took about six transfers for the system to reach equilibrium, which was just about all the patience my students had for the water transfer with drinking straws.  I loved how the volume in each graduated cylinder was different at equilibrium, even though the amount of water transferred was the same.
Checking the volumes at equilibrium.

Penny transfer equilibrium activity.
The third activity on the opening day of equilibrium involved another transfer reaction, but this time with pennies.  The students labeled one dish "reactants" and another dish "products".  Staring with 42 pennies, they transferred pennies at a fixed rate in both directions.  Once again, the reaction started with all reactants and it gradually reached equilibrium in approximately six transfers.  In this activity, the students kept the transfer rates constant (1/4 for the forward reaction and 1/3 for the reverse reaction), but they experimented with changing the initial conditions.  They tried starting with more reactants, all products and no reactants, and an even split between products and reactants.  They also added reactants to a system at equilibrium to see how that would change the equilibrium "concentrations".  Through these variations of the penny transfer analogy, the students could see with their own eyes (and data), how a reversible reaction will reach equilibrium from many different starting conditions.

On Day Two of our equilibrium unit, I split the kids into groups of three or four and gave them a POGIL to work through.  The POGIL activity inched them a little closer toward the equilibrium expression because the examples were based on a chemical reaction ( A <-> B).  This activity is also data driven, similar to the penny and the straw water transfer, but using moles of A and B.  Each student was given a different set of starting conditions for one of two equilibrium systems.  By sharing their results, the class derived the results necessary to calculate the ratio of products to reactants.  The POGIL was the perfect transition from the reversible reaction concept to an equilibrium expression calculation using molarity, and determining the increase and decrease of the species in the system.
The class agenda for Day Three of the equilibrium unit.

On Day Three I taught the kids how to write an equilibrium expression and use it for calculations.  I was so pleased at how quickly they mastered this new skill.  I believe that the two days of hands-on activities and equilibrium analogies gave them the perfect conceptual groundwork for understanding the equilibrium expression.

Adding stress to the system by inhibiting the decomposition reaction.
The fourth and final equilibrium analogy was the "tank equilibrium".  The forward reaction volunteer assembled a film canister from a canister and a lid.  The reverse reaction volunteer took the canister apart.  All of this wass happening in a fish tank.  This fun game was the perfect introduction to Le Chatelier's Principle.  At some point in the "reaction", I dumped more product into the mixture.  The  reverse reaction picked up steam and generated reactants more quickly.  Later, I blind folded just the reverse reaction volunteer to see how this would change the equilibrium conditions.  Overall, this demo was a lot of fun and sparked the conversation about stress on a system.

Tomorrow we will head into the lab for the AP Chemistry lab kit exploring Le Chatelier's Principle using real chemical reactions.

Note:  these activities are all available on the Flinn e-learnig video library.

1 comment:

  1. Hi Sharon, I just found your blog and find it so amazing for us chem teachers trying to infuse our classes with more fun. I have a question for you: what would you suggest to be a fun demo or activity for the very first day of chem class, a powerful hook to keep students wanting for more for the rest of the year :), if you could have a post on how you handle this pivotal first day lesson it would be great!