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.

Kinetics and Guided Inquiry Are Not Good Friends

I just wrapped up a three week unit on kinetics with my AP Chemistry students.  As part of the unit we attempted two kinetics experiments from the set of sixteen recommended labs in the AP Curriculum.  The first one was the Rate of Decomposition of Calcium Carbonate, and the second one was the Kinetics of Crystal Violet Fading.  Both of these experiments exposed the students with the data-driven nature of rate laws, using two different techniques.  The difficulty I faced was trying to push my students towards inquiry without telling them what to do!  Let's face it, teachers like to tell their students what to do.  When a lab group starts floundering, I nearly have to restrain myself to keep from bailing them out too soon.  But with the kinetics experiment, my students struggled to understand how to collect the data in a reproducible and consistent way.  With only one week of kinetics under their belts, my students struggled to understand what data to collect, and then what to do with it to derive a rate law for the reaction.  Add equipment difficulties to the conceptual challenge of this experiment and the results can be very frustrating and confusing to my chemistry students.

The first lab we tackled was the Rate of Decomposition of Calcium Carbonate.  I used the Flinn kit and lab handout, Publication No. 7648.  The experiment is actually pretty straight-forward:  students react solid calcium carbonate, in the form of marble chips, with hydrochloric acid.  The goal of the lab is to determine the order of the reaction with respect to HCl.  By changing the [HCl] the students can plot the data and determine the effect of the concentration on the rate of a reaction and calculate the order, in the end writing the rate law for the reaction.    In the introductory activity the students all ran the experiment using 6M HCl.  The reaction rate was monitored by measuring the gas produced from the reaction using a plastic syringe to trap the gas.  We ran into problems right away!  Only one group got decent results.  The syringes leaked or stuck to the sides, which prohibited them from getting an accurate measure of the volume of gas produced.  The one lucky group with the working apparatus was so pleased with their work; they had a lovely graph of the progress of the reaction.  The other three groups fooled around with their apparatus until the leaking stopped and the plunger was free to move, eventually resulting in something that resembled a typical graph of reaction rate over time.  At the end of day one, only one group walked away smiling, the other three were very frustrated and annoyed that their apparatus failed.  (Next time I'll try water displacement to collect the gas.)

After some discussion of the results and the goal of the experiment on Day 2, we charged ahead to the guided inquiry part of the experiment.  In this phase of the lab, the students were asked to design an experiment to determine the effect of concentration on the reaction rate.  It was obvious to me:  run the experiment again with different concentrations of HCl (and I just happen to have 2 M , and 4 M HCl prepred for you...).  The only catch was that two of the groups had to monitor the rate of the reaction by measuring the loss of mass of the system, while the other two groups were required to measure the volume of gas produced (just like the introductory activity).  To make it fair, I had the groups draw from the lucky beaker to determine what direction they would go with the experiment :  mass or volume (I included one "wild card" allowing the group to choose).  At first, the groups that drew the mass cards were annoyed that they had to change the experiment.  However, the leaky gas collecting apparatus made for a very difficult experiment for the groups who got the volume cards.   Even the lucky group from Day 1 was foiled by leaky and uncooperative syringes in the guided inquiry phase.  Adapting the experiment to measuring mass took some tweaking, but produced decent results if the reaction mixture was stirred consistently.  The volume groups limped along with the experiment, getting mostly straight line plots for their reaction progress, rather than a graceful curve.  In the end, the data proved to be consistent in three of the four lab teams regardless of the technique they used.

I walked away from this experiment with another good reminder of how difficult inquiry is for both the students and the teachers.  These students in my AP Chemistry class are the best science students in the school, who are experts at solving problems in class.  However, when I put them in the lab without step-by-step instructions, they struggle to create a solution.  What I need is MORE TIME to develop the lab skills and hands-on problem solving that the students gain from inquiry.

In the interest of time, I decided to use a step-by-step procedure to conduct the Kinetics of Crystal Fading experiment.  In this experiment, the students used spectroscopy to monitor the color change over time of the blue dye when it reacts with hydroxide ions.  I have a set of Vernier colorimeter probes in my lab, and the software package on Logger Pro includes this experiment.  I chose to go with the "standard lab experience" for the spectroscopy data collection.  I'm so glad I made this choice because something happens to students when you say the word spectroscopy.  They get this fearful look on their faces and they groan audibly.  Why do they dread this wonderful lab technique so much?  Maybe it's the "black box" effect, with this machine taking readings that they don't understand.  Maybe it's the quantity of data produced that they have to graph.  Maybe it's the new terminology that throws them off.  I'm not sure, but I saw it happen to my students with this experiment.   Even with step-by-step instructions, the lab took two days to complete!  The nice outcome here is that all the groups got beautiful data from the experiment that they could analyze using the integrated rate law graphing technique. I was happy to have real data to analyze, rather than another set from the book.  I'll save the guided inquiry for another day, and just tell them how to do the spectroscopy experiment for now.  Maybe I'll have them design their own lab using this technique AFTER the AP exam.