Monday, November 25, 2013

Sometimes, I Teach Myself ...



I like to tell my students, that our time together,  ... these high school years ... are only a few moments in their future distant past.
"Most of your life will be out of school, and most of your learning will (should) happen in those years after you graduate."

Much of what seems so important now, will fade and seem silly as they become adults, but they can't see that now ... which is perfectly normal of course.

Much of what a teacher thinks is important may not strike their audience that way at all. So we constantly strive for ways to make our timeless concepts "relevant" to each new crop of kids.

Last month, in trying to do just that with an obscure chemistry concept, I taught myself something that I can't quite shake. I think not a day goes by that it slips across my mind ... just for a second, but still it's there.

We were working with the "Mole", which loosely translates into " the chemists dozen". Like a dozen always equaling 12 of anything, the "Mole" is always 6.02 X 10 to the 23rd of anything. In chemistry, that anything is generally atoms or molecules, but the concept is the same as a dozen.

With the mole and a few conversion fractions, you can figure out how many atoms or molecules are in some amount of any compound or element.

We had been working on mole to grams, mole to atoms, mole to molecules, grams to moles, etc., and they were getting it after the initial shock.

I wanted to start class with a more complicated mole problem with more conversion steps, but had not had time to choose one before class began. As I took roll, I heard 2 of my watermen students talking about the Carolina Skiff boat one had just bought. (When I call these 16 year old ,kids "watermen", you had better believe it. They are experienced boat handlers, oyster harvesters, and fishermen. Any one of them knows more about the local waters than 90% of the weekend boaters that come to Cedar Key)

Anyhow, as it does sometimes, inspiration struck and saved me from the confines of my lesson plan.

If you run a Carolina Skiff, you are probably using 6 gallon portable tanks for your outboard, so I used that as our starting amount of gasoline.
Inspired by the Carolina Skiff connection, I quickly Googled the chemical formula for gasoline and then presented them with this problem.
"How much Carbon Dioxide is added to the atmosphere if you burn a full 6 gallon tank in your Carolina Skiff?" 

We did the usual discussion of what do we need to know to solve this and I gave them the balanced equation for the combustion of one common form of gasoline.
(There are different formulas for gas, but C8H18 seems to be a nice average.)

They needed several relationships to use as conversion fractions. We discussed this and I warned them that the problem would be a "multi-stepper".
It's a small class and they sit in cooperative groups, so they can communicate and help each other on tasks like this. After that necessary introduction, they got down to it.

While they pondered their next steps, I did too. Sitting at my desk, I worked on the problem that I had just created for them.

When I arrived at the answer ... I did a double take.
115.8 pounds of Carbon Dioxide from a 6 gallon tank?
The balanced equation did produce 16 moles of Carbon Dioxide for every 2 moles of gas, but sheesh! That's a lot of Carbon Dioxide ... better run back through the process.
Yep, same thing ... I was getting 19.3 pounds of Carbon Dioxide for every gallon of gas combusted.

(Disclaimer: I've taught a wide mix of sciences for 26 years now, but Chemistry is a new prep for me since moving to Cedar Key School a few years ago. I view myself as a new, and constantly learning teacher when it comes to Chemistry and that has the side effect of occasional self-doubt)

Just to be sure, I Googled the question, "How many pounds of Carbon Dioxide are produced by burning one gallon of gasoline?"

Over and over, allowing for different mixtures of gasoline, the answer came back 19-20 pounds... so I was correct after all.
I was glad to be correct (see disclaimer), but that number ... 19 pounds.

That 19 pounds has had more effect on me than the vast Carbon Dioxide figures published in the news ... beaucoup metric tons per year for a nation or the whole planet have that disconnected mega number numbing effect like interstellar star distances and size of galaxies ... numbers so large, they don't have much relevance at my scale of existence.

But 19 pounds ... that's a frozen turkey at Walmart or almost the 20 pound dumbbell I use in a P90X workout.
I can wrap my head around 19 pounds ... it's real to me.

End of the story?

The kids worked through the problem and came up with similar answers ... which was the goal, and they were more than a little shocked at the amount of Carbon Dioxide produced by running about in the boat all day.

As for me ... those 19 pounds of Carbon Dioxide pop into my head now every time I fill up the JEEP . My brain grabs the volume numbers from the pump and multiplies them by 19 pounds.
I can't help it.

That's one of the drawbacks of learning something new ... it sticks with you.
Whether you want it to or not.






10 comments:

robin andrea said...

What a fantastic lesson, for me too. I had no idea it would be SO MUCH. Mind-blowing and startling. Thank you, teacher, thank you.

roger said...

life-long learning for sure.

Anonymous said...

Context would be interesting: What is the total of the Earth's atmosphere in pounds???

Floridacracker said...

Anon,
I'm sure that number is out there somewhere,
but greenhouse gases like carbon dioxide and methane are climate changers precisely because they are so effective at trapping heat AND acidifiying oceans. In other words, seemingly tiny amounts of greenhouse gases relevant to the vast volume of our atmosphere still can have ( and ARE having) dramatic effects on Earth's climate.

Floridacracker said...

Anon,
I'm sure that number is out there somewhere,
but greenhouse gases like carbon dioxide and methane are climate changers precisely because they are so effective at trapping heat AND acidifiying oceans. In other words, seemingly tiny amounts of greenhouse gases relevant to the vast volume of our atmosphere still can have ( and ARE having) dramatic effects on Earth's climate.

Mark P said...

That's a lot of carbon dioxide. There is some more context. For example, a cubic meter of air at sea level weights a little more than two and a half pounds. Burning a gallon of gasoline adds quite a bit of volume of CO2. (Interesting side note that may not be intuitive: the more water vapor in the air, the lighter a given volume. It's only makes sense when you realize that water is two hydrogens (lightest) and one oxygen.)

edifice rex said...

That's very interesting. Glad I work at home now!!

Sayre said...

Now THAT's a sobering answer.

Aunty Belle said...


Oh, those lucky kids,know they luved that waterman example.

But, who knows, mebbe the earth needs extra CO2 right now? Cause, ain't carbon dioxide what helps plants grow? Keeps thangs all purty an' green? Wif' more Parkin'' lots whar' trees used to be this increased CO2 may be a blessin' in that we's seen the rain forests grow faster, right? Dint NPR say a few years back how the forests wuz growing 15% faster?

I mean, iffin' all we industrialized humans is adding to the atmosphere is 2 ppm per year, could we reckon mebbe it is an unexpected aid to the plant kingdom fer a time when more land space is paved over? I dunno, of course, jes' thinkin' mebbe thar's a silver lining.

How wuz yore Turkey day? What did ya cook special??

Chris said...

More numbers ...scary numbers: http://www.skepticalscience.com/behind_the_Lines_CO2_shotput.html