Tuesday, July 24, 2012

#ModChem Day 11

Modeler's Log, Day 11--

It was unusual how we had a sneak peek at today's workshop day, because we actually had to prepare for today's session a couple of days ahead of time. Due to the constraints of time in our workshop format, we had a multi-day lab that needed to be done for us so that we had the products at the start of unit 7. Since we meet for 8-hour sessions each day, it wasn't possible for us to do the lab all on one day, though we sometimes progress through more than one unit in a single workshop day. At any rate, day 11 has moved our particle model back in the direction of energy considerations, but not without a twist--

The day started off with a discussion of Gregg Swackhamer's Cognitive Resources for Understanding Energy. It is very challenging to put a summary of what he has to say about energy into words; however, the main points that we discussed dealt with:
  • The need to have a better way to represent energy in teaching science
  • There are not multiple "forms" of energy, there is only energy
  • Traditional approaches to teaching the energy concept result in circular reasoning or misconceptions of energy
  • Energy is not a substance, but it is substance like, and can be treated as being stored or transferred
  • The language used to talk about energy must not oversimplify nor mislead; it must be clear
  • Energy should be represented in multiple ways, not merely quantitatively through equations
Throughout the 40-page article, Swackhamer details many aspects of energy in various contexts and how to think about energy for understanding. Ultimately, we were all able to identify with the need to better teach energy concepts to students so that they have a conceptual model to apply in a variety of energy contexts.

Our paradigm lab in this unit is the Iron Nail in a solution of copper (ii) chloride. I wonder what will happen when this dark grey nail is submerged in this blue solution for a while???

We started by massing out the copper and iron before combining everything with water into a beaker. Then we combined everything in the beaker and made some observations. The beaker was left alone for a while and then the next day we removed the nail. Over time, the color of the solution and contents of the beaker changed. Once we removed the nails, we isolated all the "orange solid" that was in the beaker and had gathered on the nail. We washed this solid with water and acid, and then we put it on a hot plate to dry for a while. After all was said and done, we massed the nails again and the orange solid that had formed. The comparison was very interesting!

Upon speculation of what the orange solid could be, we reasoned that it was most likely solid copper metal. This led us to infer that copper and iron had switched places in the beaker, thus producing new substances that weren't there before. Specifically, based on our previously devised model of charged particles and ions, we decided that copper ions had become atoms on the surface of the nail where iron atoms had become ions and went into solution. After we whiteboarded our particle diagrams of this change, we concluded that the switching of places and changing of substances in the beaker amounted to what we wanted to call a "reaction," which is a term not previously used in our sequence. We defined a reaction as: a rearrangement of particles to form different substances.

We used our symbolic representations of elements and knowledge of how to write chemical formulas to represent what took place in the beaker. There we had it, a chemical equation for a reaction! One issue existed though, we weren't able to decide whether the iron in solution was in +2 or +3 ionic form, so we weren't sure how to balance the equation or predict the products. So, like good modeling students, we wrote out both possibilities on whiteboards and compared to our empirical data. The data showed that the ratio of iron to copper particles in the reaction was 1:1, and thus we were able to conclude the reaction that happened in the beaker:

The debriefing that led us to this reaction, the way to represent it with an equation, and a definition for chemical reaction took a fair amount of time, especially because we had a lengthy debate about the way to represent the ions in solution using our particle diagrams. Eventually, we arrived at consensus about what took place and how to represent it. Then, our day finished up with crafting whiteboards where we deployed our model of chemical reaction to represent the substances involved in a variety of reactions using particle diagrams. This would set us up for our next participant-led questioning session that will take place on day 12.


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