Once you’ve gotten to macro chemistry it’s a lot easier to really show off some phenomena. I get really into phenomena for intermolecular forces. They’re REALLY fun!
My teaching style has a lot fewer demos than the chemistry classes I took as a student. That’s probably because I started teaching with no chemicals and no lab in my first two (formative) years. Once I got to a school with a lab, it was a lot easier to do demos, but I found myself spending WAY TOO MUCH time trying to figure things out.
So I tested a bunch of things. Tried to figure out what worked and what didn’t work. So here, I’ll be sharing the phenomena I’ve used for intermolecular forces.
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The Breakdown
I teach intermolecular forces in only five lessons. We do
- Bond Polarity (electronegativity difference)
- Molecule Polarity (using SNAP to find symmetry)
- Dipole-Dipole Forces & Hydrogen-Bonding (where H-bond is described as an extra strong dipole-dipole forces)
- Dispersion Forces (comparing fluorine to iodine)
- Effects of Intermolecular Forces (on boiling point, surface tension and vapor pressure)
If you want to see exactly how this fits in my curriculum, grab the entire outline for free here. We also do 2 intermolecular forces labs: like dissolves like, and the penny drop lab.
Bond Polarity
Head down to the gym, or if you’re crazy like me, literally buy a rope. You may also want a stopwatch. To get bond polarity to be something visual, I have the kids play tug-of-war. Yep! We line up in the hallway (usually because my classroom is’t long enough). Without really saying much I have the same number of kids on each side. I tell them we want a “fair fight.” The boys, usually 15 or 16 years old, are very quick to decide if the teams are fair based on what they’ve seen from each other in the school’s weight room. The battle goes on for quite some time. This clearly would be the nonpolar bond.
Then we play again, but this time I make sure the teams are off balance. Usually I put the less athletic kids on one team against the athletes. I sometimes even phrase it that way, “athletes vs. non athletes.” The non athletes are usually upset by this, citing that it’s “unfair.” Thankfully for me, they’ve made the point for me. We have a polar bond.
Depending on the length of time I have in class, I may put all the kids in the class against a team of two. Typically they can quickly take the entire rope from the team of two. And we have an ionic bond.
The stopwatch is to time how long each of the rounds takes. Obviously, bonding doesn’t really have a winner and loser the way tug-of-war does, so we compare times instead.
The Discussion
I take it back to the classroom and compare the three games we played to bonds. We relate the strength of the team to electronegativity & ionization energy. I have kids comment on how easy or hard a round was and explain what side they were on.
I describe bond polarity as the “BEND.” That’s the Bond Electro Negativity Difference.
Molecule Polarity
This one isn’t as fun – I’ll just say it straight out. I put Lewis structures on the board in two groups: polar and nonpolar molecules. But I don’t label them. I put the kids in small groups and they have to figure out how the molecules were categorized. I give them a few index cards that they’ll write their answer on. They bring me the card one at a time. The first team to figure out they’re categorized based on symmetry wins. It’s a silent game. They are supposed to talk to their group, not to me or other kids. If I find it’s taking too long, I’ll drop some key words on the board, or maybe even add the axes of symmetry to a molecule or two.
This one is not really a phenomena, per se, but it does get them thinking and talking about the molecule symmetry.
Dipole-dipole and H-bonds
Now that we know polar molecules it’s time for a real intermolecular forces phenomena. There’s two options here depending on where you want to go with it. A good long term phenomena is comparing the boiling points of water and dihydrogen sulfide. They won’t necessarily get this one right away – but if you like a cliffhanger, this is a good one.
I prefer to do the cards again. But this time you’ll put up only polar molecules, but in two different categories: those with dipole-dipole forces, and those with hydrogen-bonding forces. The kids, knowing what you learned yesterday will again, go looking for symmetry and they’ll find that all the molecules are asymmetrical. (Well as long as you draw water at a right angle, like I ALWAYS make my students do.) If they look hard enough, they’ll notice all the molecules in one group have fluorine, oxygen and nitrogen. It also helps if all the molecules you choose are polar with a hydrogen!
Dispersion Forces
If you have money to spend and time on your hands, make jello. Have the kids play with it! What happens when you bang the cup of jello? Then chop up the jello a bit and bang it again. It wiggles! Just like electron clouds do when you bang into them! Then of course, let the kids eat it. Be mindful that some kids, based on their personal beliefs won’t eat the jello due to the presence of gelatin. They may not even want to touch it so maybe pair them up with another student so they can observe.
You could demo this same thing with water in a ziplock bag! You can even pass out ziplock bags and have students prep them themselves. Try it with different sizes of bags to represent the different sizes of atoms. Again, I like to compare fluorine to iodine, and we really focus on size. Please anticipate that kids will slap these ziplocks VERY HARD and a few will turn into water balloons. Maybe make it a demo you do yourself if that’s something you’re afraid of.
Effects of Intermolecular Forces
Last up is a classic. You’ll need either a chalkboard or a dark colored sheet of construction paper. Soak a cotton ball with each of the liquids and swipe along the surface. You can also just splash some premeasured amounts of each liquid on the surface. You’ll use three chemicals: acetone, rubbing alcohol and water. Then you’ll watch their evaporation rates. This is also an excellent phenomena for vapor pressure in your gases unit if you prefer to do it there. During the lesson you’ll piece together that the strength of intermolecular forces can vary and is based on molecular structure. For reference, I define vapor pressure as the willingness of a substance to evaporate and become a gas, instead of the classic definition you probably know. This helps the kids to realize that a substance doesn’t have to boil to evaporate. I often relate it to drying dishes, rain puddles and snow.
Alongside all the intermolecular forces phenomena, we do two labs: like dissolves like, and the classic penny drop lab. The kids get a really good picture of intermolecular forces from doing these labs, especially alongside the demos!
