The Faint Young Sun Paradox

I had been suspecting that the last couple of weeks, students in my Honors General Chemistry class might be slacking off and not preparing adequately for class. I can tell by the increased number of blank stares when I go through something that should have been familiar if students followed the preparation instructions beforehand. There is usually some reading, a few basic problems to try, and occasionally an additional reading or a video to help motivate the topic to be discussed. As the semester draws to a close, students increasingly (as they get busy) try to skip the reading and go straight to the problems only going back to the reading if they can't do the problems.

We had just finished looking at acid-base aqueous equilibria and the plan was to put things together by having the students go through an exercise where they can apply what they learn to study an interesting problem that has both relevance today and in the distant past. It's kind of a neat exercise that I put together (if I do say so myself!) Starting with just four equilibria one can derive a relationship between the partial pressure of CO2 and ocean pH to a reasonably good approximation, and I guide the students through this. They aren't strangers to this sort of exercise. I had a similar guided exercise the week before on deriving O2 saturation curves for myoglobin and hemoglobin the week before after we finished the chapter on chemical equilibrium.

Below are the instructions to my students in preparation for class:
 
To integrate the material you have learned in Chapter [number provided here], you will be figuring out the relationship between the partial pressure of CO2 in the atmosphere and how that affects the pH of the ocean on the early Earth. In preparation for class:

1. Read the wikipedia entry on the "faint young Sun paradox". You should be able to tell me what it is, and how CO2 might help "solve" the paradox.
2. Look up the polyprotic acid dissociation constants for carbonic acid (Ka1 and Ka2), the solubility product equilibrium constant (Ksp) for CaCO3, and the Henry's Law constant for CO2. [Explicit instructions of where to look were provided.]
3. Be sure to bring a device that allows you to make a data table and construct a graph since you will be plotting P(CO2) versus pH for the early ocean.
4. There will be a worksheet in class that takes you through the multiple steps you will need to plot the graph. If you don't finish it in class, it will be turned in as homework at the beginning of next class.
5. Optional: 2-minute animation on the effect of ocean acidification on shell formation of marine life -- we'll be looking at the same chemical reactions, but in much more detail, in class today. [A youtube video link was provided here.]

My class has frequent low-stake quizzes. This semester I am counting the top 20 (for 10% of the total grade) but I give the students 25-30 quizzes throughout the semester. They take place right at the beginning of class and take 3-5 minutes so the students are very used to the drill as they are quizzed 2 out of every 3 class days on average. On Friday morning I decided to check how many of them actually did the reading in #1. My quiz question: What is the faint young sun paradox?

This is a case where if you did the reading you would know exactly what the answer is. If you didn't, you would have no idea. Some students looked completely befuddled, others started writing immediately. I could tell within the first 10 seconds who had done the reading. Grading the quizzes after class was hilarious. (I either give full credit, half credit or zero credit for a quiz so I can grade them very quickly. The students write their answers on an index card.) About half the class easily got full credit. The other half got zero credit with answers ranging from nonsensical to amusing. I got a nice picture of a sun wearing shades, several apologies for not doing the reading, some who honestly said they didn't know, and some that tried to make up scientific sounding but completely wrong answers. However one answer was so amusing it made me laugh out loud and I decided to award half-credit just because it was witty and written in just a couple of minutes at best:

The "faint young sun" paradox originates from ancient Greek philosophy and the concept of existance. The paradox itself questions the ability of our sun to flicker and faint despite being a young sun, son.

[The student spelled existence incorrectly.]

Given I was grading this at the end of the day after a bunch of Friday afternoon admin meetings, this was welcome relief! A nice way to end the week.

And if I successfully get you to look up the faint young sun paradox after reading this blog, I've done my job as a teacher!