29 Jun 2015

High Energy Density Materials: Bond enthalpy and safety considerations

In-Class Activity

Submitted by Kevin Hoke, Berry College
Categories
Description: 

This is a shorter version of a previously published Learning Object. This version focuses on bond enthalpy calculations and has students think about the risks and safety precautions for the synthesis of an explosive material (nitrogen triiodide). 

There is also a longer version of this activity posted as a literature dicussion.

 

Learning Goals: 
The student will:
 
1) Calculate the approximate enthalpy change for the decomposition of nitrogen triiodide, using a table of bond enthalpies.
 
2) Relate the release of energy for a small quanity of explosive material to the kinetic energy of a bullet.
 
3) Consider the importance of safe practices when studying explosive materials and how that needs to balance the amount of information that can be learned.
Equipment needs: 

The activity itself requires nothing more than a table of bond enthalpies and access to a laboratory handout and journal publication.

Implementation Notes: 
This is an abbreviated version of 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. 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 and shortened accordingly. 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. 
 
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.  Although it is a short article, they are slow to process it, so I do not ask many questions about it. This article has pictures showing the significant damage to a fume hood in his lab caused by a small amount of material detonating. I am very fond of the figure showing hood damage in a research laboratory so I kept this part in the short version. However, this portion of the activity could also be omitted or used with the much longer version of the activity posted as a literature discussion. 
 
 
Time Required: 
I would allow 30-40 minutes for this version of the activity if students don't get stuck!
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. 
Creative Commons License: 
Creative Commons Licence

Comments

This is a shorter version of a previously published Learning Object. I am also posting this as a separate Learning Object so that it can be categorized as an in-class activity instead of a literature discussion.

The VIPEr community supports respectful and voluntary sharing. Click here for a description of our default Creative Commons license.