Monday, June 20, 2016

Hydronium Ion: Yes or No?

Why make things more complicated? OR They deserve a good answer!
Do you use the hydronium ion when writing a weak acid equilibrium system? This is a question I want to ask the crowd at the next ChemEd to find out where chemistry teachers fall in this debate. As I head into my twentieth (yes, that’s a really long time) year of teaching chemistry, I can honestly say that I am a complete hydronium ion convert.

 I started off, back in the early years, using the H+ ion exclusively because it was easier and quicker to write.
The acid dissociation equation without the hydronium ion notation.

I can imagine my younger chemistry teacher self justifying this choice by saying “Why make things more complicated? Isn’t a weak acid system challenging enough for my students?” And let’s face it, writing out the equilibrium system with the water and hydronium ion requires not only more effort every time, but also an explanation (the first time, and then at least once a day) about why the water in this case is actually part of the reaction and not an “innocent bystander” like in other aqueous solutions. Streamlining the acid dissociation equation with the H+ ion makes the weak acid the star of the show, with the emphasis on the loss of the proton from the acid. Referring to H+ as a proton also requires a bit of explanation and a few moments of digesting by most high school chemistry students, but they usually get the hang of it after a few examples.

And now, many years later, I have joined the hydronium ion camp.

The acid dissociation equation with the hydronium ion notation

The transition happened when I returned to AP Chemistry after a ten-year gap in teaching the course. In my new school, I used Chemistry & Chemical Reactivity by Kotz, Treichle, and Townsend for AP Chemistry. This excellent book uses the hydronium ion exclusively throughout the book when describing acid/base equilibria. In an attempt to be consistent, I adopted the practice of writing the hydronium ion, along with the many side conversations about what it is and why it’s there. To my delight, I found that my students engaged in more discussion about the chemistry of the system because of this simple (and some teachers would say insignificant) change in notation. Rather than making the weak acid the star of the show, the hydronium ion helps to make the process of proton transfer the important feature in the reaction. When students can see water as a weak base in the reaction, they have crossed an important hurdle to understanding the equilibrium system.

Dr. Binyomin Abrams, Boston University
I recently had the pleasure of discussing the hydronium ion with Dr. Binyomin Abrams of Boston University. Dr. Abrams makes a good case for including hydronium ion in chemical reactions. First of all, the proton doesn’t just fall off the acid, it is transferred to the water molecule in a chemical process. Dr. Abrams uses the analogy of dropping a pencil. If you are holding a pencil in your hand, usually you wouldn’t just drop your pencil on the ground for no reason. However, if a classmate asks to borrow your pencil, you would purposely hand it over to the person. We can imagine something similar happening on the molecular level between a weak acid and a water molecule. The proton from the acid is transferred from the acid to the water in the solution. Additionally, including the chemical process of proton transfer in the weak acid system lays an important foundation for more complex reaction mechanisms. Esterification is a good example of how the proton transfer process in a weak acid can help lay a foundation for a more complex reaction mechanism with a similar action of lone pairs on the oxygen atom of an alcohol.  After my brief conversation about the hydronium with Dr. Abrams, I held my head a little bit higher as a hydronium ion enthusiast.

But there are also good reasons not to use the hydronium ion. In high school chemistry, there is a fine balance between telling the students enough information to understand the chemistry and giving them too much information about what is “really happening”. We have all had to retract a statement because we dove into an explanation that goes beyond the scope of a high school class. The glazed over look that comes from the “long” explanation of what’s really happening in the system is a red flag for me. But in the case of the hydronium ion, can we say with confidence that it actually exists and is the best explanation of the proton in a weak acid system? In a letter to J.Chem. Ed (The Solvated Protonis NOT H3O+!), Dr. Todd Silverstein from Wilamette University argues that the hydronium ion is not an accurate representation of the species; it actually exists as a proton that is surrounded by many water molecules. And, why give a proton special treatment when we exclude the solvating water molecules around other ions such as Cu+2 and Co+2? His letter effectively wiped that self-righteous grin off my face. I can’t look in the mirror and say “at least I’m telling my students the truth about the weak acid system” because the real truth is much more complex than a one-to-one proton transfer. Just like the famous courtroom scene from the movie “A Few Good Men”, Dr. Silverstein is saying to me “You can’t handle the truth!”

I can hear the arguments from a pair tiny chemistry professors on each of my shoulders. On one side I hear “The hydronium ion is always the best choice, your students deserve it.”; while the other side is saying “Why do you make things more complicated than necessary, stick to what they need to know.” For now, I’m listening to the hydronium ion argument because I believe that it best achieves my ultimate goal which is to teach my students how to think like a chemist.

Wednesday, June 15, 2016

Poly Who?

My titration lab apparatus at the end of the titration. Notice the MANY data points recorded!
Last week I got the chance to titrate a polyprotic acid. I found this a refreshing diversion from the annual acetic acid titration I do with my honors chemistry class. Not only was I interested in titrating a “new” acid with two acidic protons, but this was the first time I performed a lab as a student for many (read very many) years. I experienced a wide range of emotions during the lab period that included fear, anxiety, frustration, confidence, excitement, and even wonder. Looking at this titration through the eyes of a student was such an amazing opportunity for me. I found myself spending as much time observing the classroom dynamics as I did performing the experiment. The undergraduates in the class are terrified of me. Can you imagine the strange looks I got when I started taking pictures of the TF as she demonstrated the proper use of volumetric pipet? The guy across the bench watched me suspiciously as I photographed my lab apparatus from different angles. When I tried to explain to him that I write a chemistry teaching blog, things just got worse. Anyway, this alien life form (a person way beyond twenty-five years of age) has landed in their lives to make the summer lab course a bit more entertaining.

Katherine demonstrating the use of a volumetric pipette
The first thing that happened during the lab period was a flood in our lab. The TF’s had to scramble around transferring all the chemicals and equipment from our lab to another teaching lab for the day. Each of the students had to go in shifts to get equipment from our lockers for the experiment. This was such a perfect example of why teaching a lab course is challenging! Things like this happen all the time; as the teacher you have to be ready to adapt and move forward. Without much fanfare, the lab started right on time in the new location and the TFs quickly got everyone up and running in the new space.

Every lab starts with verbal instructions from the teacher. Today was no different. Katherine gave a review of the safety considerations, pointed out the location of chemicals and special equipment for the day, and she demonstrated how to use a volumetric pipet. The students stood in their spots around the lab, afraid to move closer to get a better look. I know that the students in the back couldn’t see what she was doing, but they held steady at their lab stations.

Everyone is very shy during the first lab. The guy in the back can't see anything, I'm sure!
I appreciate that she did not assume we all knew how to use the volumetric pipets. Something that seems so obvious to the instructor can set the students back during an experiment. She also pointed out the three buffer solutions to use for calibrating the pH meter along with the instructions. Once the pre-lab instructions were complete, everyone began to gather equipment and get organized.

Buffer solutions that we used to calibrate the pH meter.
Calibrating my pH meter proved to be difficult. I hailed Katherine at least three times for help because my meter was not cooperating. Getting used to new equipment requires patience and multiple attempts, something I usually don’t have to draw upon in my own teaching lab. My pH meter would not calibrate, even after several cycles through the three buffer solutions. Katherine gave me a different meter to use, luckily we only have fifteen students in the class; the new one worked well and I was soon calibrated and ready to titrate. Katherine and Nick hustled around the lab helping everyone get their meters calibrated and trouble shooting the equipment. Once everything was in place and ready to go, I stopped to appreciate the student experience that requires learning the correct use of the lab equipment, doing it wrong, trying again, replacing a faulty meter, and repeating the process. Making mistakes does not come naturally to these talented young science students. Yet, here they are in a lab course that forces them to open up the possibility of not only making mistakes but also fixing them.
Nick and Katherine, our wonderful TFs for the class.

It wasn’t long after I finished calibrating my pH meter that I was ready to titrate my sample of glycine hydrochloride. That competitive side of me came out after the first data point. Naturally I wanted to get the best results in the class. Why wouldn’t I, I’m a chemistry teacher after all! I tried to listen to my own advice, which I wrote in my lab notebook before the lab: “go slow and be patient so you get good results”. I took MANY data points until I found the first equivalence point. My heart raced a bit when the pH started to climb dramatically after each additional drop of NaOH. I got obsessed with trying to get the indicator to turn pick after an addition of a single drop. Why not geek out on the lab? I had four hours to do it and no reason to rush! Even after all these years of using burets, I still made the mistake of turning the stopcock the wrong direction at least twice. Once I passed the first equivalence point, my attention waned a bit, and I actually forgot about the second equivalence point. Many years of titrating vinegar has emblazoned a single spike into my brain. Luckily the students across the way were discussing the pH of their second equivalence point, and I snapped back into focus. The resolution of my second equivalence point is not that good because I was adding too much base with each data point. So much for getting the best looking graph in the room.
Starting a second trial, notice the pH of the original acid solution.

We stayed after the lab to analyze the data with Katherine and Nick at the ready to help us plot graphs of the data. I have been terribly spoiled by the logger pro interface that we use in my lab. No need to plot graphs or calculate first or second derivatives with logger pro, it all happens within the program if you know how to ask the right questions. For this experiment, I was grateful for the refresher course in calculating and plotting the first and second derivative graphs for my titration data. Doing the graphs in Excel is very easy, the only laborious part is entering all the data (which I had plenty of, at least until about 25 mL of NaOH added). I am a big advocate for teaching my students important skills through the context of chemistry class; Excel is top on my list of tools I want my students to master in my class. I am so excited to see how it is taught and applied to a college course. Now I will have a better understanding of how best to prepare my students for advanced study in chemistry.
Graph of my titration of glycine hydrochloride with NaOH.

As you can see from my graph, I got a really nice first equivalence point and a pathetic blip for the second equivalence point. Upon further analysis, I determined with confidence that my first equivalence point happened after 20.68 mL of NaOH, which allowed me to determine the pKa of the first acidic proton. I decided to use this data point to derive the second pKa value rather than rely on the graph because the transition for the second equivalence point was not distinct. When I compare my results to the literature values, well, let’s just say that I didn’t have the best results in the room. Enthusiastic science teacher: yes, great lab results every time: not a guarantee.