Students were allowed to keep working until they had correct pH values, so they were graded on participation. Worksheets were collected at the end in order to construct the titration curve.
This could be collected and graded for correctness.
Students were evaluated on similar questions on the subsequent exam. Most students (12 out of 15) scored 11-13 points on a 13 point question where they had to solve for the pH in the four regions of a strong acid titration curve. 8 out of 15 recieved full credit on a question where they had to calculate the pH in the buffer region of a weak acid titration curve.
In this in-class activity, each student calculates the inital pH, equivalence volume, and pH at the equivalence point for both a strong acid-strong base and a weak acid-strong base titration.
In addition, each student is assigned a unique volume before the equivalence point and a unique volume after the equivalence point for each titration curve.
The data from the class is then assembled in Excel to construct the two titration curves.
This forces each student to do the calculations for each of the four regions of both types of titration curves. This activity could be used to introduce titration curves or to reinforce previously covered lecture material/problem-solving. It could also be switched to do a strong base-strong acid or a weak base-strong acid titration curve.
The constructed titration curves can be used for further discussions of the differences between a strong acid and a weak acid in terms of initial pH, the rapid-rise portion of the curve, and the pH at the equivalence point.
A student should be able to
- determine the pH of a strong acid solution
- determine the pH of a weak acid solution using Ka
- use stoichiometry to calculate equivalence volumes for acid-base titrations
- employ limiting reagent calculations to determine acid or base concentrations for different regions of a titration curve and determine pH
- determine the pH of a weak base solution using Ka, Kb
notecards with assigned volumes
computer for entering volume and pH data
This could be done as an in-class activity (I used a 3 hr lab period - most students took less than 2 hrs) or as a take-home assignment. Students were allowed to use their notes and textbooks. I did not strictly forbid them from working together, but I did tell them that I wanted them to be sure that they could do all of the calculations themselves.
I had an Excel spreadsheet of the correct pH values for each volume (attached). Students were allowed to come check their work with me and continue working if their answers were incorrect. I was also able to help them if they got stuck.
Attached are the student worksheets, the class titration curves, and the Excel file I used to calculate the correct pH values. I chose volumes and molarities that would give me an appropriate number of volumes before the equivalence point. Volumes and molarities should be adjusted as needed for the size of your class.
I used whole number volumes, but I think it would be better to have smaller volume increments near the rapid-rise portions of the curves so it doesn't look like the data "jumps" as much.
I graded each student’s problems as I would any other homework assignment, and they averaged about 80% on that part of the assignment. The other half of the total points for the assignment came from in-class participation.
We had a rich conversation about this article in class; it was probably one of the most interesting literature discussion conversations I’ve had. Although this was the only introduction to Pourbaix diagrams in the course, 12 of 15 students correctly interpreted a “standard” Pourbaix diagram on a course assessment.
This set of questions is based on a single figure from Rountree et al. Inorg. Chem. 2019, 58, 6647. In this article (“Decoding Proton-Coupled Electron Transfer with Potential-pKa Diagrams”), Jillian Dempsey’s group from the University of North Carolina examined the mechanism by which a nickel-containing catalyst brings about the reduction of H+ to form H2 in non-aqueous solvent. Figure 3 in the article presents an excellent introduction to the use of Pourbaix diagrams and cyclic voltammetry to determine the mechanism of a proton-coupled electron transfer reaction central to the production of hydrogen by a nickel-containing catalyst.
Students should be able to:
- identify atoms in a multidentate ligand that can coordinate to a metal as a Lewis base
- outline the difference between hydride addition to a metal and protonation of a ligand in terms of changes to the overall charge of the complex
- analyze a Pourbaix diagram to predict the redox potential and pKa of a species
I have discussed the challenge of integrating literature discussions into my inorganic course in a BITeS post and the VIPEr forums. Each spring I try something a little different. This year I used three articles from the literature to frame our review of course material at the end of the semester, with each literature discussion occupying a one-hour class meeting.
In each case, the students completed problems before coming to class. While these problems were based on the journal articles, they did not require the students to read / consult the journal articles in order to complete the assignment. The students brought an electronic or paper copy of the article to class. I usually put students in groups (approximately 3 per group) and gave each group new questions to work on, which did draw from the article. After some time working in groups, each group presented their material to the rest of the class.
In implementing this particular literature discussion, I didn’t have any further questions for them. I walked through some of the other figures from the article (especially Figure 1). We discussed the authors’ use of color in creating Figure 3. We also reviewed the significance of horizontal vs vertical vs diagonal lines. Because I had not covered Pourbaix diagrams in the course, the activity was a good introduction to the concept.
Because these problems don’t require consultation with the article, they are suitable to use on an exam.