New Vistas in Reaction Kinetics

As a holdover from grad school, I still get a little pain in my stomach whenever someone mentions “kinetics studies.” I never had the displeasure of running one myself, but I’ve heard many stories of others’  painful nights camped out at the NMR, running hours-long kinetics runs on slow reactions. And really, not a whole lot has changed with respect to reaction kinetics over the years. Sampling rates have gotten larger, and the repertoire of analytical methods used to follow concentration(s) has grown, but the underlying theory of reaction kinetics has largely remained the same.

Historically, the development of reaction kinetics has been a story of increasing cleverness. At some point, someone figured out that using a reactant in “drowning” concentrations causes its concentration to remain basically constant over the course of the reaction, removing its influence on the reaction rate—and thus was born the “isolation method.” Yet another clever chemist figured out that only initial rates are necessary to determine kinetic orders, provided multiple runs of a reaction are feasible—and thus emerged the “method of initial rates.”

Why stop there? Increasingly complicated mechanisms (especially catalytic mechanisms) have created a demand for ever more clever methods of kinetic study. Plus, technological advancements are pushing the Δt between data points ever smaller and the sizes of data sets ever larger. Concentration versus time data are basically continuous these days (as are rate versus time data), so why not use the entire span of a kinetics run to the best of our ability? A recent article by Blackmond shows just how far this approach can take chemists studying reaction mechanisms. With data for just a couple of cleverly structured reaction runs, one can propose pretty good guesses for reaction mechanisms. Continue reading →

Advertisements

Kinetics and Statistics Collide

It may not be a stretch to say that the study of reaction kinetics has claimed more hours of chemistry graduate student labor than any other enterprise. Waiting for a reaction to go to “completion” could require hours or even days, and one must keep a watchful eye on the data collection apparatus to avoid wasted runs. There’s a good chance that guy who’s reserved the NMR all night long is battening down for a kinetics run.

All of that effort, of course, leads to supposedly valuable data. The party line in introductory chemistry courses is that under pseudo-first order conditions, one can determine the order of a reactant in the rate law just by watching its concentration over time. We merely need to fit the data to each kinetic “scheme” (zero-, first-, and second-order kinetics) and see which fit looks best to ascertain the order. What could be simpler? The typical method—carried out by thousands (dare I say millions?) of chemistry students over the years—involves attempting to linearize the data by plotting [A] versus t, ln [A] versus t, and 1/[A] versus t. The transformation that leads to the largest R2 value is declared the winner, and the rate constant and order of A are pulled directly from the “winning” equation.

Continue reading →

Chemical Education Roundup, 6-12-2011

This week has been an interesting one in chemical education. I know I promised interactive concept mapping on the web like two weeks ago, but things have gotten a little crazy as I’ve gotten stuff together for a publication (and realized the massive amount of work I have to do to make the publication complete). I’m going to go ahead and stamp it with a “Coming Soon” label.

At any rate, the Journal of Chemical Education was abuzz this week with a debate about the role of the rate-limiting step assumption in enzyme kinetics. Definitely worth a read if you’re a biology-leaning chemist with an interest in Michaelis-Menten kinetics.

In Science, experimental philosophy in the social sciences came under the gun this week, as Shaun Nichols and David Carmel debate the role of surveys in social-science experiments. In education, the value of triangulation has been recognized for a long time as a means to support survey data. Student performance data, qualitative observations, student interviews, focus groups, and a loooong list of assessment techniques (including, but not limited to, surveys) may all be used to judge the effectiveness of a classroom intervention or change. In fact, when such data are missing, raised eyebrows are the norm. Personally, I learned this lesson the hard way on my first publication… 😛

Speaking of classroom assessment, this JCE paper outlines a qualitative approach to assessing “inquiry-based” teaching methods, which involve open-ended problems that demand application of the scientific method to reach a reasonable solution. The authors argue that most current assessment techniques are inappropriate for inquiry-based activities (IBAs), advancing the “mental models” framework as a theoretical basis for assessment of IBAs. Critically, the goal is to shift the focus of assessment toward the learner and away from content exposure (and other irrelevant measuring sticks).