Dust Off Your Molecule Kits!

Molecular kits are a large investment for a chemistry classroom and often they only get used once a year during a VSEPR activity. My students love to play with molecular kits as it reminds them of building with LEGOS. With COVID restrictions limiting many of my lab experiences in class, dusting off my molecular kits and using them throughout the year seemed like an attractive venture. Here are three ways I incorporated my molecular kits into lessons that were not VSEPR-related.

Intermolecular Forces

acetic_acid_water.jpg

Figure 1: Vinegar solution of Acetic Acid and Water with IMFs 

OK - this one is kinda cheating as intermolecular forces (IMFs) is a very close topic to VSEPR. This year I had students build familiar molecules with their kits like caffeine and acetic acid (vinegar). Chocolate or sucrose would be great molecules to build as well. Students looked up the chemical structure on their Chromebooks and built a molecule using their molecular kits. We built the molecules in the cafeteria so I could give them salt and vinegar chips and some soda to drink because I wanted them to experience/taste the molecule as they were building them. Once the molecules were built, they found a partner and labeled the IMFs between the molecules with colored tape (Figures 1 and 2).  A longer piece of tape and shorter dashes represented a weak IMF whereas a short piece of tape and thick dashes represented a stronger IMF. We finished with a gallery walk where students explained and justified the IMFs they found between molecules. It was a great activity. Students enjoyed building a "real" molecule instead of the one central atom structures in the VSEPR unit. This activity helped students see how intermolecular forces are formed BETWEEN molecules. Since we used tape for the IMFs, students clearly saw how an IMF is not a "bond." It was fun to debate all the different ways a hydrogen bond IMF could form. It helped solidify my student's understanding and definition of hydrogen bonds. 

caffeine.jpg

Figure 2: Caffeine molecules with IMFs labeled.

Modification for virtual students: I provided my virtual students a picture of two similar molecules on a Google Slide (or Classkick page) and they drew in the IMFs present. Follow-up virtual activities: Playmada Collisions has an IMF game that is great as a follow-up. I modeled the differences in tape length in my activity after Playmada's representation of IMFs in their game. The Concord Consortium has a great Intermolecular Force tutorial with excellent simulations to help students visualize IMFs. Next time: After my last class completes this activity, I think I will have all my students tape their molecules to one of the large whiteboards in class. Then I will challenge them to use a whiteboard marker and draw in all of the IMFs they can find over the next few days. I think it would be powerful for them to see the web of IMFs in a larger molecule sample.

Strength of Acids

acid_strength_molecules.jpg

Figure 3: Strength of acids using different bond pegs

Determining the strength of acids based on the molecular structure is very confusing to students. Sometimes the ranking is based on the bond length and other times is it based on electronegativity, producing opposite ranking results. This year I had students make the acids HF, HCl, HBr, HOF, HOCl, and HOBr using the molecular kits. We used a space-filling peg for the smallest halogen, F, to represent a short bond length. We used a normal bond peg for the medium-sized Cl. The long double-bond peg was used for the large halogen, Br, representing a long bond length. For the oxyacids, we used a space-filling peg between the H and the O (Figure 3). Once they built them, they ranked the molecules from weakest to strongest acid. This activity helped students see the differences in ranking. The bond length was important when the H was present in the bond like with HF, HCl, and HBr. In the oxyacids, the bond length between the H and the O stayed the same so electronegativity differences accounted for the ranking. This was a quick build with many "Ah Ha" moments. 

Modification for virtual students: I created these acids under a doc-cam for my virtual students to see and they ranked them on their Classkick page. Follow-up virtual activities: Playmada Collisions has an Acid-Base game that is great as a follow-up. While the focus is on neutralization and conjugate acids and bases, it does briefly mention bond length and strength. I like how the game shows H⁺ requiring more energy to remove in a weak acid and less energy to remove in a strong acid. Next time: If it doesn't take too long, I plan to have my students make oxyacids with varying amounts of oxygens and different halogens. Then I would have them rank them and justify. Since tape worked so well with my IMF activity above, I plan to have my students draw different-sized bond dipoles on tape and place the tape on the appropriate area of the acid. I think this would help them visualize the electronegativity trend better.

Titrations

Using large magnets on my whiteboard, I have modeled different acid-base titrations for my students in the past while we take notes. This year with molecular kits, I had students model the titration with me at their desks. Please see the video below showing a student quickly modeling a weak acid titration and its connection to a pH graph and relative amounts of [HA] and [A^-]. I liked how this was more hands-on than when I demoed it on the board. Every student was doing it as they took titration notes. Their notes included the pH graph, particulate diagram, major species, and pH calculation for each unique area. I felt that students understood the particulate diagram in their notes better because they had to physically make the water and the conjugate base every time they added an OH^- ion from the buret. 

Modification for virtual students: Virtual students used different colored pieces of paper or candy to model. Follow-up virtual activities: I did not have a virtual follow-up for this activity. My favorite titration simulation no longer works because of Flash, but I found a video of it, Titration Animation, on Caleb Arrington's YouTube Channel. Next time: I plan to repeat the modeling process with the students a second time with double the acid in the flask. My goal is to show my students how the pH graph changes based on concentration change. I wouldn't do a full set of notes, just a quick run-through like in video 1 below. Then I would have them superimpose the pH graph of the more concentrated acid over the less concentrated acid to see the difference. 

Video 1: Quick run-through of modeling a titration with Molecular Kits on Melissa Hemling's YouTube Channel (accessed 3/18/2021)

Do you use your Molecular Kits in lessons other than VSEPR? Share your ideas by commenting below!