Friday, November 11, 2011

Fond of bonds

Les Trois Danseuses, Picasso, 1925

O body swayed to music, O brightening glance,
How can we know the dancer from the dance?


W.B. Yeats, from "Among School Children"
***
I teach biology, or try to, anyway. What I mostly teach, I think, is a way to look at the world, a way labeled "science."

Through a series of events mostly inexplicable to me, as much related to the state biology exit exam as to the retrograde motion of Mercury, my sophomores have had essentially no chemistry before coming to my class. None...

I get to expose them to chemistry. I love chemistry. To grasp chemistry, you need to grasp bonds.

Trouble is, there is nothing to grasp--in a sense, they do no exist.
***
I had a lot of trouble with the concept of potential energy when I was in high school. A teacher would lift an object, the pontificate about how this object now had "potential" energy.

Where, I wondered (and eventually learned not to ask), was the energy?

It's like trying to "see" the energy in the water at the top of a waterfall. The water itself is exactly the same at the top as it is at the bottom. I could see that a waterfall could do work, I am not stupid, but where was it before the molecules cascaded over the dam?

I was told the energy was in the object until I finally got my first real science teacher, Ms. Lehman, a wonderful woman with a wonderfully twisted view of reality (any honest view will be twisted) and the patience of Job, who explained to me that the energy was in the position. Over and over again until I got it.

Once I internalized this, I shot through chemistry and never looked back.

It is really a simple, simple idea.
***
The waterfall analogy suffers from a major flaw--rearranged atoms do form new substances. Still, you end up with exactly the same atoms that started the reaction, just scrambled up differently.)

Every time.

If you hope to get kids (or other forms of humans) to understand chemistry, they must also internalize the Law of Conservation of Mass and Energy. This can take months, even years, especially if our children demand evidence. (Alas, we beat the snot out of the curious very early on in public education...)

We don't have that kind of time--so we (students, teachers, districts, states, with Arne's approval) fake it.

I suspect that's a major reason so many despise stoichiometry, a shame--stoichiometry is the Mikhail Baryshnikov (or Michael Jackson) of science, an intricate choreographer of seemingly impossible moves as energy flows through matter.


The moves of Michael Jackson do not "exist"--they represent the relative positions of pieces of Michael Jackson.
***

Many of us "teach" the photosynthesis/respiration equation this way:

C6H12O6 + 6O2 ⇒ 6CO2 + 6H2O + energy

Kids see "energy" the same way they see everything else there, if they see it at all. Energy is hanging out there just like the other "stuff"--heck, they've been told since kindergarten that plants convert sunlight into food.

Plant do not do any such thing--they simply rearrange the stuff around into more complex, less stable forms.

We tell the more sophisticated students that plants put energy is "in" the bonds, and they nod sagely, writing  down like ancient Irish monks bent over their vellum, recopying wisdom passed down through the ages.

I'm not here to tell you the Irish copied bunk--I do not need my Granny's leathery yellowed hand breaking through the earth, grabbing my ankle. I will tell you, though, that our student do.
***
Here's what I do.

Every time I mention a reaction that requires a net input of enery to build a bigger, less stable molecule, I stack a lab stool on top of a desk. It takes exertion. When I'm done, I have the same stuff--a desk and a stool, but I have a larger, less stable object.

If I want to break it down, I still need to put in a little bit of energy to nudge the structure. This is not a trivial matter. If a few atoms are clustered together in a non-random position, there's something about that position that allows them to "stick" together. Every reaction requires breaking up that something.

We call this activation energy.

I love watching the stool cascade off the desk, loudly bouncing along the floor until it comes to rest in a more stable position. The kinetic and sound energy are obvious, the bump up in temperature where the stool collided with the floor a little less so. ( I've been know to touch the floor at the spot of impact and pretend it is hot.)

Energy released.

The stool lying sideways on the floor is pretty darn stable, and its position pretty darn strong. This may be counter-intuitive--students confound strength of bonds with the energy released as substances break down from less stable to more stable arrangements.

This is, of course, a bit of a simplification, but unlike the concept of bond as some thing betweeen atoms, it allows for growth of a more accurate model. It also makes visible the idea of potential energy as a consequence of position, of the dance, as opposed to the stuff itself, or the dancer.




The Michael Jackson photo is from Spilled Mind, without attribution.
You cannot appreciate MJ's genius from a still.

4 comments:

Tyson said...

Beginning a force and motion unit next week with my 4th graders, so this post was good food for thought.

The quote at the beginning of the post interested me, so I googled it ;)

doyle said...

Dear Tyson,

Thanks for the words.

The poem "Among School Children" is already linked in the post. I considered putting the whole last stanza in.

It's a wonderful poem on so many levels, and amog other things, splays open the dichotomy between the classroom and the world.

doyle said...

Dear Tyson,

Lord, what a maroon I am! Of course it's Yeats; I just realized what you were winking about.

Thanks for the heads up.

John T. Spencer said...

When I was in fourth grade, I defined energy as "what makes stuff happen." To me, energy had to be movement. Slowing down and speeding up.

It was a piss-poor definition, but it beat the correct one that was so filled with academic jargon that it made no sense.

My teacher explained that energy was like fuel. I couldn't see how it was a noun and not a verb.