Reflections from Kinetics - Solidifying Why I Teach Method of Initial Rates AND Integrated Rate Laws

kinetics demonstration

Obvious answer to my title: Because the College Board (CB) tells me to. Moving on to a more philosophical response...

The first year I taught AP chemistry, I focused on the nuts and bolts of unit design with support from a few friends (“Do I have materials? Quizzes? Labs? Tests?). The second year and beyond, I worked to smooth out nuances, and I'm sure your story is likely similar if you've had the luxury to teach the same course multiple times. Here are a few things that are on my mind now as I reflect on teaching kinetics a few times now. (Quick context: as I shared in my AP Chemistry - Scope and Sequence post, students engage in this unit after stoichiometry, intro to spectroscopy, gases, and reactions units.)

Noticing in year one: I taught my students how to use the method of initial rates. I taught my students rate laws. However, they strugged to differentiate when to use what method. Upon further probing, they struggled to articulate why one might use one method over the other. They could parrot back some ideas ("The rate law tells you about the particles involved in the rate determining step of the reaction."), but I wasn't convinced of mastery and connections. However, I realized this pretty late in the game, mopped up the mess the best I could, and moved on to race through content.

To be honest, until I observed these qualities in my students, I did not realize I suffered from the same short-sightedness. Why is the method of initial rates useful? Why do I teach multiple methods in such a jam-packed course, where from my very limited perspective, the emphasis on the exam, sans maybe ONE multiple choice question, is on application of Integrated Rate Laws? While I’m sure there are many reasons, I realized I wanted my students to take away the notion that different techniques are useful based on the data immediately available, and there are limitations on each method. I made a mental list of considerations for next year (slightly cleaned up for your viewing pleasure):

Flash forward one year: My students engaged in the same introductory lab “factors that impact reaction rates”, and then I lectured on the Method of Initial Rates - heavily  based upon Aaron Glimme’s lecture posted online.

What was new-ish: I figured I could then just hammer my students with problems, right? Bummed that my textbook didn’t provide the kind of practice I wanted for my students (too focused on integrated rate laws), I went to the internet. I found a treasure trove of “Advanced Starters for Ten” within the Royal Society of Chemistry’s (RSC) Teaching Resources”, warm ups on a myriad of topics developed by  Dr. Kristy Turner, RSC School Teacher Fellow 2011-2012 at the University of Manchester, and Dr. Catherine Smith, RSC School Teacher Fellow 2011-2012 at the University of Leicester. I gave pages 1 and 3 as a homework set from the Kinetics document. This “10-20 minute” activity served to be pretty challenging for my students (based on the number of kids who sought out help), but it opened up dialogue among my students and I.

Additionally, I engineered a mash-up lab where students practiced the method of initial rates, based upon a lab from Flinn and a lab from Paul Price (here is a link to a video to get the gist). Students had to use the method of initial rates in the formation of a precipitate. I tailored the awesome resources roughly based upon what I felt my students needed - to understand WHERE the initial rate came from, and more practice. (Note 1: Shout out to a student from the previous year’s AP class who optimized the conditions for me - I totally didn’t have time. Note 2: Do you see the pun in the title of this post now?). In my mind, here lay the foundation for my students to be able to articulate pros and cons of the Method of Initial Rates.

After this lab, my students learned about the Integrated Rate Law, and we had more discussions to continue to flesh out pros and cons of each method after engaging in the "classic" Crystal Violet Lab. (As I write this now, I wonder why I did not give my students an empty version of this table above to fill in throughout the unit???? Note to my future self here, and if you try this, let me know how it goes.)

While no learning sequence is perfect, I am proud of the clearer vision of this progression from Method of Initial Rates to Integrated Rate Law. The previous year  lacked vital practice and thus time for students to create and build a schema. By the end of the unit and even the school year, more students made deeper connections than the previous year. So why do I teach both methods? To help my budding scientists make intelligent choices in the lab, and experience a small slice of that in a controlled fashion. I think that's what CB wants too.

The ever pertinent question - how did this translate to AP results? As I look at the data, I want to tell you it translated to higher scores. To be perfectly candid, for the short answer questions labeled kinetics, the data is about the same- both years between 63% and 73% scored in the highest fourth. Could it have helped in the multiple choice? I’ll never know. One: There were fewer than 5 multiple choice questions on kinetics last year, so no general data. Two: I only received data for 16/20 students from last year - the rest were swallowed up by the CB, never to be found after multiple phone calls to CB and support from my school’s test administrator.

Next time: J. Chem. Ed. Pick- How to improve results from the classic crystal violet integrated rate law lab.
Thanks for reading! What do you notice in your classroom as you teach kinetics? Also, what might you add to my pros/cons table above?