‘Expanded LCP’ or ‘Q versus K’ explanations?

Written by Adrian
On February 02, 2015
Categories: AP | AP TOPIC 13 | Big Idea 6

It’s never been OK, on the AP exam, to offer a simple, ‘because of Le Châtelier’s principle‘ argument as the explanation for the shift of an equilibrium position caused by an external stress. This is in much the same way that it has never been acceptable to say, ‘because as you go across the period first ionization increases‘ as an explanation for the trend in first ionization energy from Na to Ar. In short, one can’t state the ‘trend/rule’ as the ‘reason’.

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Fair enough, but what has always been acceptable is to use the words, ‘because of Le Châtelier’s principle‘, followed by a more expansive explanation that includes the idea that the stress must be relieved, and that whatever the shift in equilibrium subsequently is, that stress relief was the reason for the shift. This is what I call ‘expanded LCP‘, and is an explanation that shows an understanding that simply stating the rule does not.

In the new curriculum we find a couple of LO’s that deal with such explanations. Firstly, LO 6.8;

The student is able to use Le Châtelier’s principle to predict the direction of the shift resulting from various possible stresses on a system at chemical equilibrium.

As far as this LO goes, it seems like business as usual and we should assume that explanations in the ‘expanded LCP’ spirit should still be entirely acceptable. However, we also have LO 6.10;

The student is able to connect Le Châtelier’s principle to the comparison of Q to K by explaining the effects of the stress on Q and K.

The inclusion of the specific language of 6.10 seems to make it inevitable that we are going to see questions in the future, that specifically ask the kids to answer questions regarding equilibrium shifts in terms of changes in Q and K. This has not been directly targeted in the past, but 6.10 looks like a direct attempt to force kids into using a Q versus K explanation as opposed to an ‘expanded LCP’.

6 Comments

  1. Dr. Daniel Price

    I am in agreement with the College Board–if indeed the trend is as you describe it–on this matter. Absent a quantitative treatment of equilibrium, Le Châtelier’s Principle is a means of making some predictions regarding the reestablishment of equilibrium at a new position, but AP students have the advantage of Q and K as a means of predicting outcomes. I can imagine some situations in which Le Châtelier’s Principle will fail to predict the ‘shift’ (the changes in species’ concentrations); one might argue that such situations are contrived, but it is equally fair to argue that many current applications of LCP are contrived so as to permit accurate predictions via LCP. [I shall now attempt to rewrite my presentation on equilibrium stresses without resorting to LCP–focusing exclusively on Q and K–and will let you know whether I am satisfied with the result.]

    Reply
    • Adrian

      I’m really only concerned with what’s required in order to get full credit on the AP exam, so in that regard the niceties of the advantages and disadvantages of one approach over another is somewhat trivial to me. It’s my guess that LO 6.10 means that I might be forced into Q v. K, in a way that I would not have been previously.

      Reply
  2. Dr. Daniel Price

    I do not believe that we are in disagreement. If the CB includes language, or expects language, that specifically uses the term “Le Châtelier’s Principle”, then I am duty bound to include the term. If not, however, we may (time will tell) find it easier to approach LCP strictly in terms of Q vs. K, with manifold benefits: a consistent means of predicting the ‘stress relief’ and a reinforcement of the Q vs. K ratios.

    As always, I thank you for your insight.

    Reply
    • Adrian

      Daniel – I absolutely agree that we are NOT in disagreement! Apologies if my reply suggested that we were, that was not my intention.

      I think that the CB will increasingly move toward the exclusive use of ‘Q versus K’ over LCP, but at the same time (suitably fleshed-out) LCP explanations will often be 100% acceptable in many answers. The only exceptions will be where the question EXPLICITLY asks for ‘Q versus K’ type explanations.

      Of course, given the CB’s record in failing to communicate such things explicitly, we may still get into ‘justiplanation’ type discussions/arguments down the road! Yikes!

      Reply
      • Dr. Daniel Price

        In a larger sense, it does not matter whether a student (or a teacher) uses Le Châtelier’s Principle or Q vs. K as a *predictor*–neither a memorized ‘shift’ nor an appeal to mathematics would represent any sort of *explanation*. A genuine explanation would require a particle-level justification (e.g. in A + B C + D, ‘increasing concentration of B allows more particles of A and B to react, creating more of C and D’). I expect Q vs. K to provide more accurate predictions, and accurate predictions should help students to formulate the necessary particle-based justification. [Formulating particle-based justifications from first principles or from the outcomes of memorized LCP ‘rules’ has proven a challenge at times.]

        Reply
        • Adrian

          Agreed, but I for one certainly don’t want all ‘explanations’ going back to first principles – that would make teaching chemistry in high school a very different job, and frankly not one I would have much interest in. Having said that, with the emphasis on ‘particulate diagrams’ in the new curriculum, I think that there is a general move in that direction. I think that’s a huge mistake, since I think it swaps one problem for another, where a lack of content depth replaces any gain in understanding.

          Reply

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