Friday, December 28, 2012

Random thoughts on teaching (thermodynamics for chemists)



The course in question
I recently finished co-teaching Nanothermodynamics - a P-Chem course for nanoscience students covering statistical mechanics, thermodynamics, diffusion, and kinetics.  This is the third time I have taught it and the first time I have been really happy with the way my part went.  I have also gotten the best teaching evaluations ever, so I know the students were happy with it as well.  This blogpost is about why I think it went well and some general musings about teaching in general and teaching thermodynamics in particular.

Repeating questions
I use peer instruction so my "lecture" periods consists mostly of me asking questions that the students answer using Socrative.  This year I decided to ask questions about material covered in previous lectures - either the exact same question or a variation of previous questions - and it was a real eye-opener.

Fundamental questions that had received near 100% correct answers one week received at most 50% correct answers one or two weeks later.  Clearly the students had done the reading for a particular lecture period but that does not mean they remember it after a few days.  Sometimes when I used the exact same question they would remember that the answer was, say, "A" but could not really remember why.  So it's not that they are not paying attention.

Less is more
So I decided there was a few key concepts that needed to be reviewed periodically until they "got it" and that was the connection between the equilibrium constant $K$ and the standard free energy change $\Delta G^\circ$ and a molecular understanding of $\Delta H^\circ$ and $\Delta S^\circ$.  So I started each lecture period with  a few questions such as this one.

Sometime I would spend as much as 50% of the "lecture" period on review.  This means something else has to be covered in less detail and this forced me to think much more deeply about what concepts are most important. (It makes it a lot less painful to cut things when you have amble data that 80% won't remember it for more than a few days.) And I think this is why the course was so successful this year: I had, for the first time really, thought very carefully about what to teach and why.

The "textbook" is a problem
Think about the first step in the "design" of a course: pick a textbook.  The textbook typically defines what you teach, in what order you teach it, what problems you assign and, as a result, the exam.  At best, lectures cover the most difficult parts of the chapters or, at worst, is a mad Powerpoint-fueled dash to cover it all.  Often each chapter is given the same number of weeks of coverage regardless of content.  I know because I have done all these things myself at some point.

Most textbooks on a particular topic have very similar content.  This is not, in my opinion, because textbooks authors have, through exhaustive trial-and-error, converged on an optimum solution but due to a variety of other factors.  It is primarily because the audience/customer is not the student but the instructor, because the customer is the one choosing the book, and the customer is a very conservative person for a variety of reasons.

The main reason is that the customer usually has taught the course before and wants, for whatever reason, to change textbooks without making major changes to the course.  Furthermore, many instructors do have a "favorite topic" and will not pick the textbook unless that topic is covered in some detail.  As a result textbooks rarely leave anything out, no matter how irrelevant the author personally thinks it is.

I would argue that courses end up covering way to many topics, many for no other reason than that they appear in the textbook, and that these topics are in textbooks for no particularly good reason.

A vicious Carnot cycle
The Carnot cycle is in most physical textbooks and is generally a very difficult and abstract concept to do with the maximum efficiency of heat engines.  In Molecular Driving Forces, one of the few thermodynamics textbooks that looks quite different from the rest, it is included in Chapter 7 called "The Logic of Thermodynamics" where concepts like heat and work are introduced.  The first page of the chapter has pictures of pistons.  The opening paragraph states that these new additions to the "toolkit" are "crucial for understanding cyclic energy conversion - in engines, motors, refrigerators, pumps (including your heart), rechargeable batteries, hurricanes, ATP-driven biochemical reactions, oxygen transport around your body, and geophysical cycles of carbon and water, for example."  These things are never mentioned again in the rest of the remaining 27 chapters as far as I can tell.

I am not saying these topics are unimportant to chemists, but they are nowhere near as important as say, the relationship between $K$ and $\Delta G^\circ$.  However, if you spend more time on the Carnot cycle students will think that it is, and as a result will not really understand either.  Example: Who's afraid of Big Bad Thermodynamics?

Tl;dr
For the last few years I have been focussing on how I teach by using simulations and peer instruction.  I still think these tools are important; for example, polling the students proved they needed key concepts repeated a few times before they "sink in" and I challenge you to test this yourself with your class - the tool is freely available.  But using these tools to teach overly abstract concepts that you or your colleagues never utilize in your jobs only because they appear in the textbook won't get you much further.  It's time to take a cold hard look at what you teach and why.

Thermodynamics for the average chemist: some recommendations
* Most chemists think in terms of molecules not equations

* Most chemists would like to understand how to use an equation properly before worrying about where it came from.  Consider deemphasizing derivations.

* Most chemists deal with the molecular interpretation of reactions and binding, not phase transitions.

* Most chemists work in solution where volume changes are usually negligible and are usually trying to shift the equilibrium towards products.  Consider deemphasizing the concepts of work and efficiency.

* Most $\Delta G^\circ$ measurements are done by measuring $K$.  $\Delta S^\circ$ is obtained either by measuring the temperature dependence of $K$ or by measuring $\Delta H^\circ$ calorimetrically and solving for $\Delta S^\circ$ knowing $K$.  Consider deemphasizing $\delta S=\delta q_{rev}/T$.  Consider introducing $K=e^{-\Delta G^\circ/RT}$ as early as possible.

* Most measurements ultimately deal with $\Delta G^\circ$.  Consider deemphasizing concepts related to $\delta G=0$.

* The conformational entropy is important but almost never discussed in textbooks.

* The most often used standard state is 1 M ideal solution and the most often used activity convention is the solute convention.  Consider deemphasizing the rest.

* Enthalpy changes are dominated by $\Delta H^\circ(T=0)$ but this term is generally glanced over in most textbooks.  So students generally have a poor molecular understanding of  $\Delta H^\circ$.

More posts one statistical mechanics can be found here.
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