26 Sep 2012

The Lewis Dot Structure(s) of Nitryl Fluoride

In-Class Activity

Submitted by Sheila Smith, University of Michigan- Dearborn
Categories
Description: 

This is the In Class Activity that I use to review the concepts of Lewis Dot Structures, LDS, (connectivity, resonance, formal charges, etc.) learned in General Chemistry and to introduce new ideas of resonance contributions to the character of the molecule.  The question itself is apparently very simple, but the discussion that it produces can be quite rich and brings in both new and old ideas of LDS, providing both a good review and a good segue into advanced ideas of Lewis Dot Structures.

Learning Goals: 
  • A student should be able to predict the connectivity of a Lewis Dot structure based on a formula; largely, this involves choosing the appropriate atom as the central atom based on electronegativity trends.
  • A student should be able to draw/choose the best LDS for a molecule based on the ideas of formal charges.
  • A student should be able to draw resonance structures for the LDS of a molecule , including less favorable structures, but eliminating any impossible/illegal structures  (This is the first difference between what I do in Gen Chem and what I do with my sophomores/juniors; I want my students to draw all possible resonance structures, even those that are unfavorable and would therefore have been ruled out in Gen Chem; this requires the students also to differentiate between what is only poor, e.g. a structure with an unsatisfied octet, and what is actually not allowed, e.g, a structure with five bonds to nitrogen)
  • A student should be able to rank the relative favorability/stability of the various resonance structures available to a particular compound.
  • A student should be able to write a representative equation showing the relative contributions of all of the possible resonance (This is the "new" idea" that I am introducing at the sophomore/junior level; it is not expected that students get this far on their own in the In Class Activity, but some students who have been reading ahead may approach this)
Equipment needs: 

A Periodic Table

Implementation Notes: 

I do this In Class Activity in my sophomore/junior level inorganic (Descriptive) course before I ever say a word about Lewis Dot Structures.  I gave the students about 8 minutes to work on this activity individually.   I had the students change writing utensils after their initial independent work, so that I could see what they had gotten on their own before the group discussion.  But I allowed them (expected them) to correct their work and add to it as we discussed the problem as a group.  Collecting this data allows an assessment of what the level of recall of this material was on the part of the students, but also allowed me to see a flaw in the way that I framed the question, which I will correct in future uses of the assignment, and which I discuss on my answer key.

Time Required: 
30 minutes of total class time, individual and group discussion
Evaluation
Evaluation Methods: 

I assess this in two ways: 

  • I collect the handouts to see the initial individual responses of the students.  Since this is an In Class Activity, students know that they are graded based on participation, and can therefore be trusted not to change their answers.  This assessment gives me an understaning of their backgrounds and recall of  Lewis Dot Structures from Gen Chem.  (It also led to the discovery of an interesting twist that I had not anticipated).
  • I give a similar question on an upcoming exam with a different molecule.

 

 

Evaluation Results: 

A detailed description of the student responses is given in the key, but I was shocked by how poorly students performed on the simple LDS drawing. I was most surprised that 8 of 18 gave an unexpected connectivity that I had not considered. 

Creative Commons License: 
Creative Commons Licence

Comments

I was so intrigued by this problem that I had to know what my students would do for it, but since I am not lecturing right now, I gave an ungraded "pop quiz" at the start of my two lab sections this week. All I asked them to do is "draw the complete Lewis Structure of NO2F;" I did not imply the connectivity.

I had 9 juniors take my quiz in the 2nd semester organic lab course. 5 got it perfectly correct and included resonance, while 1 got the connectivity right but forgot resonance. 1 student expanded the octet and drew 5 bonds to N, while 2 drew the "unexpected" structure.

The next day, I had 20 first year students take my quiz in our general chemistry laboratory. 5 got it perfectly correct and included resonance, while 7 got the correct connectivity but forgot resonance. 2 students expanded the octet while 6 had the wrong connectivity (mostly a variant on the "unexpected" structure but with various connectivities).

For what it's worth, although I did not attempt to draw the Lewis Structure before knowing the answer, I am pretty sure I would have drawn the "unexpected" structure for the same reasons that your students did (formal charge argument).

Seemingly "simple" problems often reveal much about student and facutly expectations and biases. I love this problem.

Adam

Sheila,

I will share this with my colleagues teaching intro chem this semester at Wooster.

Sibrina

It's interesting to draw a comparison between NO2F, and nitrous acid (HNO2), which does adopt the "chain" structure.  This comparison lends itself nicely to a discussion of how electronegativity differences can influence structures: the relatively electropositive hydrogen prefers the protic environment of the oxygen, whereas the electronegative fluorine prefers the "hydridic" environment of the nitrogen.

Austin-  this is an excellent idea and one I had not thought of!  As I said in my post re this LO, I sort of stumbled upon this issue; it was not a planned incursion.

I would love to see youdevelop an LO related to this one in which you discuss that variation.  It would make an excellent test question as a followup to the in-class exercise I designed.

 

I would argue that this is dominated by pi-bonding effects. The reason F forms a bond to N is that it can form a partial pi-bond to it. H in HNO2 is incapable of pi bonding, so the pi bonds are left to the N-O bonds. In contrast, an O-F bond is awful from a pi-bonding perspective. Both pi and pi* orbitals would be filled, leading to a massive repulsion. O-F is a crazy weak bond for the same reason O-O and F-F are (both in the 30s of kcal/mol) because of pi-pi repulsion. 

Thank you, Sheila, for your presentation on this topic today at BCCE.

This is an interesting example of how students can evaluate resonance structures. The "chain" form of NO2F possesses no formal charges, but the "central N" structure should have resonance stabilization. Perhaps that principle is worth reinforcing with students: structures having multiple (resonable) resonance forms may be more stable.

Of course, this would also be an interesting computational chemistry exercise, comparing the relative energies of the two forms, as well as their predicted bond orders.

Thankd for sharing this example!

I think I found the magic words...  this term, I told students that nitryl fluoride has "a single central atom".  No more linear structures.  Sadly, this kills a good conversation, but the students find it less confusing.

With this change, I got 3 correct answers, 7 students had one of the two best structures but neglected to show resonance, 5 students violated the octet rule on N, and 3 students violated the Law of Conservation of Matter (not enough electrons).

Scott, 

 

Lori Watson was going to do just that (calculations), but I think she got too busy and forgot about it.  If that's your thing, I would love to see such data and link it to this LO!

 

Nancy, This is an excellent point, since one of the goals of the exercise is to get my sophomores and juniors to consider that ugly doubly bonded F resonance structure that they would have been taught NOT to draw in Gen Chem (because it is not the ground state).  The point that this "ugly" resonance structure actually contributes in an important way to the stabililty of the compound is exactly the point that I want the students to get.  

And honestly, before reading your comment, I didn't have a good reason why it was important, so now I can add this to my discussion with my students.

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