13 Jul 2012

High Energy Density Materials: A laboratory and literature investigation

Literature Discussion

Submitted by Kevin Hoke, Berry College
Categories
Description: 

The synthesis of the nitrogen triiodide ammoniate shock-sensitive explosive is a simple laboratory exercise, but it does require a lengthy time for the material to dry before it is active.  This activity uses that time to have students investigate some simple thermodynamics behind their explosive, as well as consult the literature on high energy density materials from the work of Karl O. Christe.

There is also a shorter version of the activity posted as an in-class activity that omits most of the literature investigation.

Learning Goals: 

By completing this activity:

  • The student should correctly use a table of bond enthalpies to estimate the change in enthalpy for the decomposition of an explosive material.
  • The student should realize that small quantities of a high energy material (HEDM) release significant amounts of energy.
  • The student should develop an awareness for safer approaches to handling an HEDM.
  • The student will be able to find and analyze at least two papers from the primary literature on an HEDM 

 

Implementation Notes: 

This is an activity that I use in our upper-level inorganic chemistry course. It touches on several different learning objectives, but I use it in support of a lab activity in a junior/senior-level course that has a fair deal of literature usage. I developed this activity to use some time in lab that would otherwise be wasted waiting for a contact explosive to dry. Certainly, the activity can be done independently of the lab exercise.

I find that the students need much more review of general chemistry principles than I would care to provide in class, so having them work a "simple" bond enthalpy calculation (and associated thermochemistry) helps catch typical processing errors. For example, a bond enthalpy calculation is not products minus reactants if the Table of Bond Enthalpy consulted uses positive values, representing heat needed to break a bond. Unfortunately many students have memorized the wrong algorithm for this scenario. 

The reaction they are investigating involves the breaking of a N-I bond. Many tables of bond enthalpies lack a value for this bond, hence my suggestion that they use a "reasonable assumption."  This allows them to decide what estimated value that they would accept as reasonable, usually by extrapolation from the values that are provided.

Most of the students in my course were recently in a physics course and remember that kinetic energy calculations for an object are best done with mass expressed in kilograms. Some students may have forgotten, though.

Before the lab, I sternly warn the students of the consequences for inappropriate use of chemical reagents. I also relate some relevant experiences from graduate school. 

In advance of the lab meeting, the students are given a 2007 perspectives article by Karl O. Christe on some high energy density materials. This article has pictures showing the significant damage to a fume hood in his lab caused by a small amount of material detonating. The students also compare the energy given off by a gram of an explosive material to the kinetic energy of a bullet. Several questions in the activity have the students discuss lab safety. 

The students have some experience with literature searches, but they are not efficient in doing so. As a result they are very slow to access and read the primary literature, but I view this as an ongoing lesson that extends throughout the course.I monitor them and give suggestions when they try to find an earlier paper by Christe on the N5+ cation. Although it is a short article, they are slow to process it, so I do not ask many questions about it. There is a tie-in to some lecture material on super-acids that can easily be omitted.

 

 

 

Time Required: 
90 minutes
Evaluation
Evaluation Methods: 

We discuss the simplest questions together after completion, noting the common pitfalls. I collect the students' work to see their individual perspectives on lab safety. They will see some content from the activity later on an exam.

 

Evaluation Results: 

Quantitative analysis of learning outcomes is not available yet. I have some in-class observations:

Students in general chemistry struggle with distinguishing an enthalpy calculation using heats of formation from one using bond enthalpies. When I see the chemistry majors  as juniors and seniors, I see that this error persists. In working this activity, students review this concept and correct their misunderstanding. I still need to follow up later to see the extent of their retention of this concept.

Students do well in discussing how to use personal protective equipment and minimal quantitites of material in order to reduce risk. 

Students stuggle with literature searches. I do not think this activity has enough support of this learning goal. I would suggest using another LO for this purpose.

 

Creative Commons License: 
Creative Commons Licence

Comments

I have updated the materials to incorporate revised thermodynamic data and equations, along with better sources for additional information.

I created a new, shorter version of the activity (and instructor's notes), which I am now submitting as a new LO (as an in-class activity). I also took the opportunity to correct an error in the original activity and update the description.

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