Note: This lesson could totally be adapted for SASB titrations as well. Here’s the lesson I used for SASB this year.
(introducing WASB Titration & pH Calculations)
Before my students came to class, they watched this video I made and took notes on this notes sheet. In the video, I do pH calculations at six different points along the titration curve, plotting each point as I go.
It’s…a lot. But the nice thing about using this video is that ALL of our class time on this topic is now them practicing, rather than me taking up multiple class periods lecturing on this.
At this point, my students have seen all the calculations, but not tried them themselves. And they’ve seen particle diagrams for SASB, but not for WASB or WBSA. I also forgot to include the idea of a “buffer region” in the video, so I briefly go over that in class.
Introductory Activity – Michael Farabaugh’s Practice
I started class by having students complete this matching activity in their groups. Farabaugh has a video explaining all of this, but I found that my students could successfully complete the matching section without it. (The writing activity that Farabaugh has posted would have been a challenge though).
The Task – Graphing the WASB Titration Curve
I got this idea from AP Chemistry teacher Rafael Betancourt on the AP Chemistry Teachers Facebook group.
The startup costs for this were about $50 because the giant, 2′ x 3′ pads of graph paper are really expensive. Luckily, we only used four sheets of it, so I’ll be able to keep using the pads for years to come. I got 0.75″ diameter colored stickers, which worked well. I wish I’d have gone with the 1″ stickers though, so they could have enough room to write the ion symbols directly on the stickers instead of relying on the key.
Students got right to work. I imagined they would lay the paper on their lab tables, but all four groups taped the graph paper to their vertical whiteboard. They’ve really taken to the whiteboard format, even when I don’t prompt them to. I love it though; they collaborate super well when at the boards.
The results are below. You can move the slider to compare what the projects looked like at the end of the first day and at the end of the second day. What I love most about this is that it encompassed nearly all aspects of the titration curves: pH calculations, the graph itself, important points like equivalence and 1/2-equivalence, and particle diagrams.
Student Misconceptions Uncovered
As a teacher, it can be disheartening when misconceptions like these crop up (did I not teach this clearly enough???). But in reality, part of the point of tasks like this are to bring these misconceptions to light, name them, and help students correct them. So with that mindset shift, here are a few uncovered student misconceptions that are worth celebrating.
- Net Ionic Equation is the same as SASB – One group had their net ionic equation written as H+ + OH– = H2O. They had separated the weak acid HA into ions on the reactant side and canceled A– on each side as a spectator.
- H+ included in particulate diagram in the buffer region – HA dissociates into H+ and A– as the acid is titrated, but the H+ that is produced is immediately neutralized by the OH–, becoming water. A couple groups were including those hydrogen ions. I pointed out the N.I.E. for the titration, showing there were no hydrogen ions produced. It felt wrong to at least one student that there were then fewer ions in the particle diagram, so we talked about how those atoms are still there, but not they’re part of water molecules, which we’re not including in the diagram.
- Equivalence point pH not in the middle of the vertical section – You can see the mistake in the “before” image of the first titration curve above. The group plotted the equivalence point, then had the graph going immediately up and to the right, instead of continuing upward before curving over.
- Difference between neutralization and dissociation – One of my groups was conflating the two of these, which led to a lot of difficulty in their calculations. I’m glad we had the chance to get this straightened out, since it’s pretty essential to everything in this unit.
Afterward, I asked a few students if they thought I should do all this next time I teach AP Chemistry. Once student replied that she thought it was worth it, even though it took a long time.
And that’s my main concern. This took two full 90-minute blocks. Overall, I think it was worth it. Between exposing misconceptions, getting practice with particle diagrams, practicing all the calculations, and comparing these curves with the SASB titration curves, there was (hopefully) a lot of learning going on.
Either way, it was definitely better than past years when I’ve just worked through the calculations in front of them and not saved time to practice.