Eutectic Surprise

I tried out a new class activity in my G-Chem Honors class yesterday. We had just finished covering colligative properties the class before, which included calculations on boiling point elevation and freezing point depression and making thermodynamic arguments to explain these phenomena.

 

To incorporate my research interests (chemical origin-of-life) and motivate the students as to why this might be interesting, I had them read a paper before class (“Prebiotic Synthesis of Adenine and Amino Acids Under Europa-like Conditions”; Levy, M.; Miller, S.M.; Brinton, K.; Bada, J.L. 2013, Icarus, 609-613.) I annotated the paper so that they would focus on the key paragraphs in the paper; in my experience first-year students can read scientific papers if provided this sort of scaffolding support.

 

At the beginning of class, I briefly discussed the role of heat-shock proteins in prevent the cell contents of organisms from freezing at temperatures below zero Celcius. Then I divided them into small groups and let them loose on a worksheet. The first bit required them to recall how to calculate the freezing point depression of a solution (in this case the NH₄CN solution in the paper they read). I discovered that some of the students got stuck even though they should have known how to do this from the examples in class and the previously assigned homework. After some quick reminders everyone was up and running.

 

I hoped they would be able to puzzle out a simple binary eutectic diagram (shown above). In the worksheet, I had defined the eutectic point and the eutectic temperature, and then asked them to work out how to label the appropriate parts of the diagram given the eutectic temperature and the freezing points of two substances (HCN and H₂O). One group figured this out relatively quickly but the others struggled a little. The next instruction was to have them figure out the approximate mole fraction of each substance, assuming all “curves” were straight lines. I assumed they would use a simple ratio guesstimate but the students were either stumped or attempted to use y = mx + b equations (which would have worked but were terribly slow). I had to step in before they tied themselves in knots. They were surprised at my simple explanation. I had momentarily forgotten that students often turn to an algorithmic procedure before thinking about a guesstimate, while I usually do the opposite.

 

The point of this was to have them think through what would happen if you started with a 0.1 M HCN solution and then progressively lowered the temperature. Because I was running out of time, I only gave them a few minutes to think about in their groups before walking them through what they would expect to observe. I think the students got my main point, which is that one substance (in this case, the solvent water) solidifies, its mole fraction decreases, resulting in the solution following the diagonal line down to the eutectic point. I should say I think they were surprised, but understood the implications. (I didn’t have time to double-check this with a follow-up question.)

 

We ended briefly with discussing the difference between a eutectic low temperature approach to making adenine from HCN versus a high temperature approach. The students were able to make connections to collisions and thermal energy, which is good because this discussion was meant to be a segue to our kinetics module coming up next. We briefly discussed why guanine is also found in the mixtures (students looked up the structures) but we ran out of time and weren’t able to discuss other aspects of the paper.

 

All in all, I was reminded that even after many years of teaching, I overestimate how much students can do even when I have the Honors section and that includes many academically cream-of-the-crop students. (The performance on the first exam over a week ago was strong.) I did tell the students that they were helping me refine a new class activity I had designed, and I’m sure they were relieved to know that none of this eutectic stuff will be on class exams. Clearly, I have more work to do, and it put a dampener on my desire to completely overhaul my G-Chem class next year with these sorts of activities. I need to remember: Baby steps!