Effect of lone pairs in VSEPR now irrelevant to the AP exam??

Written by Adrian
On August 18, 2015
Tags: AP | VSEPR

I’m not sure, but there is evidence to suggest that just might be the case.

There’s a nagging irony about the LO’s in the new AP Chemistry curriculum when compared to the hopelessly inadequate course descriptions of the legacy exam. It’s something that I’ve been talking about since the inception of the new course, and it’s this; the new LO’s ought to provide some significant, specific improvement in terms of informing us about what can (and cannot) be examined, but some potentially odd interpretations of them are worrying. As such, in reality, we won’t REALLY know what each LO specifically means, until we have a decent bank of old (new) exams that help to illustrate the LO’s more completely. This is really the opposite of what the LO’s should do, since ambiguity ought to be removed, not introduced, by a more detailed CED!

So, as I continue to think about each LO, I still to get the occasional, worrisome feeling that I’m not really sure about how a particular LO might manifest itself on the exam. However, nothing has (yet) compared to the stunning jolt that I got a few days ago when I posted a fairly innocent question about the 2015 exam, that was answered in a way that I would never have dreamed by Russ Maurer. Here’s what happened.

I posed this question.

Screen Shot 2015-08-15 at 3.04.35 PMRuss offered this explanation.

Screen Shot 2015-08-15 at 3.06.24 PM

At this point we should pause, and examine what Russ is saying.

He’s saying that if a student knows that there are 4 pairs of electrons in total around the O atom, but knows nothing about the 3D nature of molecules and knows nothing about the extra repulsive force of lone pairs, that they would select a bond angle of 90 degrees. We know that a bond angle based upon the traditional, simple understanding of 4 bonding pairs would be approx. 109.5 degrees. Halfway between those two numbers is 100, the lower number of the range accepted.

Russ goes on to ask us to consider the idea that a student might think there were only 3 bonding pairs of electrons around the central atom. In that case the bond angle would be 120 degrees, and halfway between that number and 109.5 degrees (from above), is 115 degrees, the higher number of the range accepted.

Those two situations, taken together Russ argues, would give the range of 100 to 115 degrees. And you know what, he’s right! His analysis does fit with range given as an acceptable answer, but respectfully it is INCREDIBLY esoteric thinking that I think is outside the realm of what I might call ‘normal’ analysis – it works, but it’s a VERY, VERY odd way of thinking about this problem.

It’s compounded of course by the fact that there are NOT 4 bonding pairs around the oxygen atom in ethanol, nor are there 3 bonding pairs around the oxygen atom in ethanol, so using these as markers of some range makes no sense to me.

But even forgetting Russ’ ‘outside of the box’ thinking, why on EARTH would the College Board be OK with the idea that you could go ABOVE a bond angle that could possibly be generated from 4 bonding pairs of electrons around the O atom, i.e., why could you POSSIBLY allow a bond angle above 109.5 degrees? What is even more staggering, is that Russ’ analysis (as he points out), totally ignores the extra repulsive effect of the lone pairs! This is STAGGERING to me.

(Please note that I completely agree with using a range in questions like this, AND I fully understand that bond angles in real molecules offer differ slightly from what simple VSEPR might predict, but this magnitude of range seems bizarre to me, and it appears to abandon a bunch of fundamental principles).

Russ then does a quick analysis of all of the new, released AP questions that deal with VSEPR, and he points out that it appears that no consideration is being given to the extra repulsive power of lone pairs, and that such massive, weird ranges of acceptable answers are effectively removing the need for such knowledge on the new exam! Stunning if true. Stunning. I absolutely defy anyone to read the EK’s or the LO’s relating to VSEPR and then reach this conclusion. Russ may well be right, but as I say, there is no way that any sane person would glean this information from the CED – hence my continuing worry.

Even though Russ’ analysis is apparently ‘correct’, it’s highly unlikely that I could bring myself to stop teaching VSEPR in a traditional manner, for two reasons. Firstly I would be far too nervous to accept at this nascent stage that such a modified/diminished treatment is definitive – there’s just not enough evidence from old exams yet. Secondly, I’m not sure that I could abandon the traditional treatment with a clear conscience. If Russ’ analysis proves to be correct in the long term, just WHAT is happening to AP chemistry content here??

 

6 Comments

  1. Rick Sanders

    Compare the bond angles of H2S to H2O, carboxylic acids to amides, and NH3 to NBr3 then contrast with PH3 and PBr3. In short, lone pairs have a much less significant effect than is stated in texts, but when comparing analogous species in terms of atomic radius (H2O to NH3), then the slight drop of a couple of degrees is apparent. When comparing atoms that differ greatly in atomic radius, the VSEPR prediction of a smaller angle due to extra lone pairs breaks down. Also, the presence of resonance structures can dramatically influence the measured bond angles. It’s not as simple as saying two bonds and two lone pairs is always 104.5 degrees, as that predicted angle is more often false than true when moving outside of the second period. For example, OF2 is 103 degrees (close to what VSEPR predicts in theory) and OCl2 is 111 degrees (in the opposite direction of what “the extra repulsive effect of lone pairs” would suggest). As with all theories, there are limitations to the model, so is it better to go with the theory that predicts the wrong angle in some cases, or the actual collected data? To be fair to the student who must predict without having the data to refer to, who may have been exposed to only the theory or may have seen the measured angles of actual molecules to know that the theory is a ballpark estimate, a reasonably broad range of angles should be considered for the grading. The range given above credits the student with the “extra repulsive power of lone pairs” reasoning as well as the student who considers the issues of atomic radius and knows that the textbook angle is not accurate for all species.

    Reply
    • Adrian

      Rick I KNOW all of that, that’s NOT the point! I agree with a range (as I say in the post), AND FULLY understand that models have limitations (I’ve been teaching for 26 years), but to answer your question about actual data or predicting wrong angles, in the context of the AP exam, I choose wrong angles EVERY time!

      Reply
      • Reed Konsler

        But, you don’t give wrong answers and neither would your students.

        Every student in your course, if they internalized what you teach, answered “104.5” or something close to that for that question. They have all been given credit for the question.

        It seems like what you’re irritated by is that their “better” (in your opinion) understanding of Chemistry is not differentiated by the exam. A bunch of derps that answered “113” got credit, also. WTF?

        Now you’re really wrestling with a ghost. You’re making the argument that the model of reality you teach your students is “better” (more accurate, detailed, intellectually appealing, traditional…) than another model. Not only is it better…but better enough that your students ought to get credit where others would not.

        A key point is that a student giving the **empirically observed** bond angle (not one using simple VSEPR or one complicated by lone pair and steric considerations) will receive credit. Any student not giving the **empirically observed** bond angle receives credit to the extent the model they have in their head predicts the observed angle “reasonably enough” as part of a comprehensively accurate system.

        Rick has noted, and you’ve agreed, that VSEPR theory complicated by lone pair considerations sucks at predicting angles that aren’t in the 2nd row or where there are resonance considerations. It also doesn’t account for sterics, ring strain, and other multi-center geometric considerations.

        So, if you stand back, Occam’s Razor would support cutting out lone pair and steric considerations in most predictive questions. I think it would still be reasonable to ask a MC question or one which asked for explanation (“why is the HOH angle less than the HCH angle…”).

        It becomes a question of what you need to know vs. what is an interwebz question. A competent Chemist looks at ethanol and recognizes the lewis structure is showing 2 tetrahedral and one bent center, and could build one out of molecular modeling stuff if needed.The exact number is an Interwebz question.

        Reply
        • Adrian

          We need better questions. The question should say, “What does simple VSEPR theory predict about the value of the C-O-H bond angle in ethanol?” I need the exam to reflect what we ALL teach, not some esoteric ideas that someone, somewhere is teaching kids about the real world!

          IMO, this is a dangerous precedent. There could be all KINDS of things that happen in the real world that are NOT currently being considered for/getting full credit on the exam where, if we apply the CB’s ‘logic’, now ought to be.

          Reply
          • Reed Konsler

            I think you avoided my point. I think you’re arguing that you want there to be an exact-correct answer so that you can differentiate your student from another less-well-prepared one. If the question offers too wide a range for “correct” you can’t do that. I’m sort of like…[shrug]…I think you’re going about the AP sporting event the wrong way. My purpose is to provide my students a slate of models and answers which will tend to make their outputs match expectations closely enough to score the point.

            What I teach gets them the point. Score, done. Next question. It’s not necessary for me to demand *any* point be a trick-shot. Most of the players won’t even make the free-throws. How many “180” responses do you think that question got? I’m guessing a great deal. It’s just one free-throw after another. A good student will differentiate themselves based on reliability, not precision.

          • Adrian

            I think we are destroying too many fundamentals on the new exam. VSEPR (for example) is only a model (which has limitations) I know, but it serves children well at this stage in their education. I believe that I am trying to preserve some small aspects of such a course that will turn out to be much more valuable to them than the current flavors of the month.

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