College Board runs AP Insight, a website full of teaching and assessment tools for teachers and students of AP courses. The site focuses on specific "challenge areas" that tend to give students trouble on the exam. Chemical kinetics is one of the five challenge areas in AP Chemistry. My students and I have been working our way through one of the teaching and learning activities called Concentration vs. Time. The graphical analysis, guided-inquiry questions, and application to past and future content are seriously challenging, and my students report higher levels of understanding than in past semesters.
The "Concentration vs. Time" activity is broken into three tasks: 1) Graphing concentration vs. time data given for three reactions, 2) Analyzing the graphs with guided inquiry to help students understand graphical connections to differential rate laws and then integrated rate laws, 3) Making predictions about a reaction and devising a plan to test the predictions.
Task 1) Graphing concentration vs. time data given for three reactions: In groups of three, students graph concentration vs. time for a zero, first, and second order unimolecular reaction. The students then work together to devise a list of how the three graphs are similar and different. Finally, the students write statements summarizing what the graphs mean about the relationships between concentration and time. My students never truly understood the significance of zero order reaction rate being independent of concentration. Some also never understood fully that average rate IS the rate for a zero order reaction while average rate is virtually meaningless for first and second order reactants. They pointed out those important concepts to me this time!
Task 2) Analyzing the graphs: After a short lecture and practice interpreting other graphs and data, the students view a table with zero, first, and second order rate laws and integrated rate laws. The guided-inquiry questions lead them to understand the differences in changes in concentration vs. time in the graph is also displayed mathematically in the rate law. The integrated rate laws for zero, first, and second order reactions are given in slope-intercept form, which is not how they are displayed on the AP exam's resource page. Students are asked to use their understandings of slope-intercept form and the given integrated rate laws to derive graphs for first and second order reactions. In the past, I taught the integrated equations and showed the graphs simultaneously. The method of asking the students to derive the graph again deepened their understandings. This ability to visualize a graph seems to have helped them in reading and understanding "word problems" as well. I'm not being asked what number matches what variable anymore.
Task 3) Making predictions: Students read the description of a reaction and make predictions about the reactions order. They then devise a plan to test the hypothesis. Admittedly, two and a half snow days cancelled this activity for my students. I will try it out next year.
I guess I'll find out in July if using this activity was the best use of short time! The exam may not have many kinetics questions, but if it does...we're prepared!