19 Mar 2011

The [XeF]+ Cation and Ion-Pairing in [MF6]– and [M2F11]– Salts (M = As, Sb, Bi)

In-Class Activity

Submitted by Maggie Geselbracht, Reed College
Categories
Description: 

This Lewis structure and VSEPR problem is based on a paper from Inorganic Chemistry in 2010 reporting the crystal structures of a series of salts of the [XeF]+ cation.  The [MF6] and [M2F11] anions (M = As, Sb, Bi) were used as counterions, and in all cases, the [XeF]+ cation interacts with the anion via a weak bond between the Xe and a fluoride of the anion to form an ion-pair in the crystalline solid.  These somewhat unusual ions provide an interesting application of the predictive powers of Lewis structures and VSEPR theory to molecular structure, backed up by experimental data on bond distances and bond angles and can provide an opening to the descriptive chemistry of the heavier pnicogens.  These ions also provide interesting examples to discuss the model and utility of assigning formal charge in Lewis structures and how to describe the bonding in molecules with expanded octets. Although not specifically addressed in the attached problem, which focuses on the bismuth salts, one could extend the discussion to periodic trends in these salts with different counter ions from the pnicogen group.  The reference to the original paper is Inorg. Chem. 2010, 49, 8504-8523.

 

AttachmentSize
Microsoft Office document icon XeFBiF6.doc31.5 KB
Learning Goals: 
In answering these questions, a student will:
  • Practice drawing Lewis structures for molecules with expanded octets and assigning (sometimes unusual) formal charges
  • Gain experience applying VSEPR to predict the molecular structure and approximate bond angles in different molecules
  • Relate the predictions of relatively simple theories to actual experimental results obtained from X-ray crystallography
  • Learn the names of the heavier pnicogen elements
Implementation Notes: 

This was assigned as a problem to be worked at the board by small groups during a conference session for a second-year inorganic chemistry course. 

 

Time Required: 
~30 minutes
Evaluation
Evaluation Methods: 

This was done as an in-class activity during my conference section, so I did not collect or formally evaluate student work.  I made the solutions available after class for students to check on their own time.  During the conference, I circulated among the groups, answering questions and gently guiding them towards the right answer if necessary.  The solutions to this activity are posted as a Problem Set learning object type (see Related Activities above) and can be downloaded by VIPEr Faculty Users that are logged in.

To assess the learning goal of whether this activity helped students learn the names of the pnicogen elements, I asked a Clicker Question in lecture the day after this conference activity on which group of elements belonged to the pnicogens.  The choices were (A) As, Bi, P; (B) Bi, Sb, Te; (C) As, Bi, Pb; or (D) S, Se, Po.  Out of 19 students, 11 (58%) chose the correct answer.

 

Evaluation Results: 

The students had a more difficult time with these problems than I had anticipated, in particular coming up with a Lewis structure for the [XeF]+[BiF6] ion-pair.  A useful hint that I would provide was to consider that there is a new “covalent” bond that forms between the ions, or at least covalent in the sense of a shared pair of electrons in the Lewis structure.  Some groups would draw Lewis structures without the correct number of electrons, caught most easily by summing the formal charges or they would propose Lewis structures with “less favorable” distributions of formal charges.  This led to quite interesting discussions about whether or not formal charges were “real” and if not, then why did we bother with them.

 

Creative Commons License: 
Creative Commons Licence

Comments

In spring 2012, I asked this question on a problem set in my Introductory Inorganic Chemistry course (second year). This question was 1 of 8 on the problem set that covered Lewis structures and molecular shapes. I do not grade all of the questions on the problem set, but this was one that I did grade.

Of the 25 students, only 2 students answered the entire problem correctly with the last two parts (c and d) causing the most problems for students.  Looking at the results for each of the four parts in more detail: on part a, 22 of 25 students, or 88% correctly drew the Lewis structure and predicted the shape of [BiF6]. Nearly all the students, 23 of 25 or 92% answered part b correctly, the Lewis structure of the [XeF]+ cation. Only 4 of 25 students answered part c correctly, and another 2 students earned partial credit for their answer.  On part d, 9 of 25 students or 36% answered correctly and another 9 earned partial credit for their answer. The majority of wrong answers on part c involved bonding the xenon directly to bismuth.

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