17 Jul 2014

A cuprous azide complex: The effect of structure on the stability of the azide ion

Literature Discussion

Submitted by James Jeitler, Marietta College

This is a problem set based on the article "Energetic Cuprous Azide Complex: Synthesis, Crystal Structure and Effection on the Thermal Decomposition of HMX" in the Journal of Chemical Crystallography.  It has been used in a Chemistry Capstone course for both Chemistry and Biochemistry majors during the first semester senior year.  Biochemistry majors are not required to take Inorganic Chemistry and Chemistry majors may be currently taking Inorganic chemistry.  This paper was chosen because it introduces all students to applications of inorganic chemistry without requiring a background in inorganic chemistry.

Learning Goals: 

Students will be able to find a journal article given general information about the topic and journal

Students will be able to apply information from previous courses to answer questions based on the journal article

Students will be able to work collaboratively to answer questions

Students will be able to interpret the results of crystallographic data prresented in a journal article to identify coordination modes of the azide ion.

Students will relate Lewis structures and formal charges to the stability of an ion (molecule)

Students will compare the difference between covalent and ionic bonds between azide and different metal ions

Implementation Notes: 

This is a problem set developed for a Chemistry Capstone course for chemistry and biochemistry majors.  Students have had organic and analytical chemistry; however, they may not have taken inorganic or physical chemistry (biochemistry majors).  Students are given a general description of the topic and some information about the journal and are expected to find the article on their own.  They use this article to answer the questions.  These answers are submitted to instructor and these questions are then used as the basis for an in class literature discussion.

This article was chosen in order to introduce "Critical Thinking" (as defined by AAU&C - Critical thinking is a habit of mind characterized by the comprehensive exploration of issues, ideas, artifacts, and events before accepting or formulating an opinion or conclusion) into the Chemistry Capstone class.  The authors have synthesized a complex where the azide ligands bridge copper centers through covalent linkages.  They compare the stability of this cuprous azide complex to the stability of stictly ionic azide salts in Reference 9.  During the discussion session I ask if the students think that this is a valid comparison.  We discuss what will affect the N-N non bonding distance in ionic compounds (most focus on cation size) and then discuss what other factors will affect the N-N non bonding distance in a covalently bound complex such as the cuprous azide.  We also get into a discussion about which N-N non bonding distance the authors are referring to – Are they discussing the azides bonded to a single copper or are they discussing neighboring azides in different layers of the crystal.  This is not immediately apparent from reading the paper and we have had lengthy discussions between faculty members on this subject.  The idea of this discussion thread is to introduce the students to the possibility that the same information can be interpreted two different ways.  The last question on the problem set (When the [Cu2(dmpyz)(N3)2] complex is added to the rocket propellant HMX thermal decomposition temperature decreases.  The authors state that “…complex (1) can catalyze the thermal decomposition of HMX, and the catalytic ability enhances with the increase of mass.”  Either support this statement or dispute it.) is asked to get the students to realize that not everything in a paper may be correct.  Most student focus on the fact that less heat is needed to cause the decomposition of HMX and say that yes, the complex catalyzes the decomposition.  This leads to a discussion of what does it mean to catalyze a reaction and can there be any other explanations for why the decomposition temperature decreases.

Time Required: 
Up to one week is given for the students to find the paper and answer the questions. Two 50 minute periods are devoted to discussing the paper
Evaluation Methods: 

Problem set was graded on an individual basis.  Grades were based on correctness, depth and creativity where appropriate.  For example, in the question "What does u-1,3,3 mean" students who correctly stated the azide was bridging received partial credit.  Students who explained in more detail that one copper(I) was bonded to the first nitrogen and two copper(I) ions were bonded to the third nitrogen received full credit.

Evaluation Results: 

The first year this assignment was used students who had not completed inorganic chemistry (biochemistry majors - aprroximately 7 out of 12 students) tended to simply copy from a student who took inorganic chemistry. Students without inorganic background believed they could not answer questions about inorganic chemistry and stated on course evaluations that it was not fair that they had to read and understand inorganic chemistry.  (On other assignments - physical and biochemistry most notably, the same trend was seen).  Answers to the problem sets ranged from the very superficial to the very thoughtful and did not depend on the background of the student, rather it seemed to correlate to the desire of the student to learn.  The second time this paper was used only five students were enrolled in the course and the answers were more indepth and thoughtful.

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