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Chemical Equilibrium Misconceptions

This page authored by Barry Bickmore, Brigham Young University.
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This page first made public: Nov 10, 2010

This material is replicated on a number of sites as part of the SERC Pedagogic Service Project

Summary

In this geochemistry activity, students explore a STELLA model of anhydrite-solution equilibrium. They find ways to illustrate several points about chemical equilibrium that address common misconceptions using this model. Then they write a mock research paper about addressing common misconceptions about chemical equilibrium in the classroom.

Learning Goals

Students should internalize the following points about chemical equilibrium.
  1. Equilibrium is the state where the rates of the forward and reverse reactions are equal, not when the rates reach zero. (Common misconception: Nothing is happening at equilibrium.)
  2. The equilibrium constant is equal to the rate constant of the forward reaction divided by that of the reverse reaction.
  3. Equilibrium constants can change with temperature when the activation energies of forward and reverse reactions are different. (Common misconception: Equilibrium constants are constant under all conditions.)
  4. Le Chatelier's Rule. (Common misconception: Students sometimes have a hard time getting the idea that Le Chatelier's Rule is about perturbing an equilibrium state, driving the system to a new equilibrium state.)
  5. It doesn't matter how much of a solid is present when it is at equilibrium with a surrounding solution. The concentrations of the aqueous species will still end up being the same. (Common misconception: Students usually think that if you add more solid, the equilibrium concentrations of the aqueous species will go up. This is a misapplication of Le Chatelier's Rule.)
They should also improve their skill at organizing research papers.

Context for Use

I use this in an upper division geochemistry course. It takes a couple hours of class time and quite a bit of work outside of class.

Description and Teaching Materials

In this activity, I first give the students a quick survey to assess whether they can correctly answer some questions about chemical equilibrium that target specific misconceptions. Then they do an activity where they explore a STELLA model that illustrates several concepts about equilibrium related to these misconceptions. They then take the same survey again and use the "data" from the survey to write a J. Geosci. Ed. paper about how you can use a STELLA model to address student misconceptions about chemical equilibrium. I confer with the students about their rough drafts to make sure they are explaining both the proper conceptions and the common misconceptions correctly. All this might seem like overkill, but in my experience this is what it takes to beat out some of the misconceptions students just can't seem to lose.


I am including the student handout (Microsoft Word 39kB Aug31 09), a chemical equilibrium misconceptions survey (Microsoft Word 30kB Aug31 09), some materials on how to organize scientific writing (Microsoft Word 52kB Aug31 09), and the STELLA model ( 133kB Aug31 09) the students start with. The survey is given before the assignment is introduced, and after the students start writing. That way they have some "data" to report on for their mock research papers.

Teaching Notes and Tips

One thing students almost never understand is why solid-solution equilibria do not depend on the amount of solid present. Here's how I explain it. Dissolution rates depend on the amount of surface area present, so if you take a sugar cube and smash it, it will dissolve faster into hot water. However, precipitation rates also depend on the amount of solid surface area present, since formation of the solid also occurs at certain surface sites. Therefore, if you increase the amount of solid surface area present, you increase both the dissolution and precipitation rates by exactly the same factor. Therefore, the system might come to equilibrium faster, but it won't change the final equilibrium state, where the forward and backward reaction rates are equal.

Assessment

The survey is meant to provide a quickie assessment of whether the students can correctly distinguish proper conceptions from misconceptions. I give the same one to them before and after the activity. However, even with the activity, some of them still haven't gotten over their misconceptions, so I also work with the students on their rough drafts of the writing assignment to make sure that they are properly explaining the common misconceptions and corresponding proper conceptions about chemical equilibrium.
The Organizing Scientific Writing activity may aid students in developing their scientific writing skills.

References and Resources

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