Comparing High Explosives with WebMO
Kate Fritts
Department of Chemistry, Millikin University, Decatur, IL 62522
Abstract
Chemistry students always ask, “When are we going to blow something up?” The ultimate task
in this lab is for the students to come up with and model their own explosive molecule and then
determine its strength relative to TNT. In reaching this goal they will have to compare the
accuracy of two different computational models, determine trends in heats of formation, and
understand the differences between combustion and explosive reactions. This activity is an
adaptation and extension of the first section of set of undergraduate labs published in the
Journal of Chemical Education.1
Key Words
Explosives, WebMO, computational chemistry, TNT, relative explosive strength
Introduction
One of the main types of reactions that introductory chemistry students learn about is
combustion. There are a number of labs that investigate the energies involved in various types
of combustion. Often, when the topic of combustion or burning comes up in class, a student
will ask about explosions.
Instructor Information
In this activity, students will build increasingly complex molecules in WebMO, run geometry
optimizations, and determine the heat of formation for the molecules using both AM1 and PM3
models. They will also use the NIST Web book to determine literature values for the heats of
formation of those molecules.
Students will then graph their data to look for heat of formation trends in both number of
carbons in a chain and number of nitro groups. They will also make a graph of their calculated
values vs. literature values for both AM1 and PM3 models to determine how well they align
with literature data.
The third part of the activity allows students to build their own molecule in WebMO and
determine its heat of formation. Students are provided with a method of using the heat of
formation to determine the strength of their explosive relative to TNT.
After completing this activity, students will be able to…

Use WebMO to determine the heat of formation.




Use the NIST Web book to find experimental values for heats of formation.
Compare experimental to computational results and determine which engine is the most
accurate.
Use the calculated heats of formation to determine the relative strengths of various
explosives.
Determine what features in a molecule contribute toward explosive strength and use that
knowledge to design a new molecule to compare to their other results.
The goals of this activity fall into two categories. The first set of goals focus on getting students
to use and evaluate computational models. They are asked to compare the results of two
different models to literature data. The second set of goals focus on getting students to think
about how the structure of a molecule affects its properties. In this instance, they are looking
at what features of a molecule contribute toward heat of formation and relative effectiveness
as an explosive.
Next Gen standards PS1.A (Structure and Properties of Matter), PS1.B (Chemical Reactions),
and PS3 (Energy) are all relevant to this activity. It also incorporates the Next Gen practices of
analyzing and interpreting data and using mathematics and computational thinking.
Background
Explosions differ from combustion because they happen at a much faster rate. Combustions
reactions are propagated by the flame front moving through the burning material. This heats
up nearby molecules causing them to burn as well. The reaction can only occur as fast as the
flame front moves. In an explosion, the reaction is propagated by a supersonic blast wave that
passes through the material.2
The strength of an explosive, relative to TNT, can be determined by calculating the heat of
explosion from the heats of formation of the possible explosive molecule and the products of
its explosion. The products of an explosion differ from combustion because the materials are
not interacting significantly with atmospheric oxygen. All of the oxygen in the products must
come from the molecule itself.
The balanced reaction for the explosion of TNT shows that it does not have sufficient oxygen to
fully oxidize the carbon and hydrogen in the molecule.
2 C7H5N3O6  12CO + 5H2 + 3N2 +2C
The Kistiakowsky–Wilson rules for determining the products of an explosion prioritize the
production of carbon monoxide, water, and then carbon dioxide. In general, more explosive
power is generated by molecules that have sufficient oxygen to fully oxidize all carbons to
carbon dioxide and all hydrogen to water.3
About the Activity
A small class of physical chemistry students was asked to complete this activity. They were able
to do all of the calculations and come up with reasonable values. However, a few issues arose
that led to some small revisions.
In the original version, students were allowed to test any molecule they chose under the
assumption that they would try to come up with the best explosive possible based on what they
had determined in the first two parts of the lab. While most students did just that (Figure 1), a
few made uninspired choices (most notably, dodecane). Most of the students chose to make
molecules that are structurally similar to TNT. The student who chose the hydrazine derivative
is working with that compound on an undergraduate research project.
Figure 1: Student generated explosive compounds
O
OH
O
-
O
+
N
+
N
N
O
O
N
O
R
-
HN
+
+
N
O
N
N
N
-
O
OH
phenazine-5,10-dicarboxylic acid
O
+
N
O
O
O
1,2,4,5-tetranitrobenzene
N
N
NH
O 2N
-
O
R
NO 2
O 2N
hydrazine derivative
+
N
O
-
NO 2
O 2N
1,2-dinitrobenzene
O
NO 2
+
O
-
N
hexanitroethane
O
+
+
N
dodecane
O
O
N
NH2
O
-
2,4,6-trinitroaniline
The student who chose to test 1,2-dinitrobenzene took the assignment a step farther and
examined the ortho-, meta-, and para-isomers. He found the ortho- arrangement of nitro
groups to give the largest heat of formation (and thus largest relative strength).
Another issue that arose was confusion regarding the relationship of heat of formation to
explosive strength. We associate large negative values of heat of formation with stability.
While a greater heat of formation indicates less stability and likely a higher relative explosive
strength, it is not the sole determining factor of explosive strength.
To address those issues, the questions were revised to have the students analyze the
expression for relative strength and determine what factors (greater heat of formation, greater
moles of gas produced, and lower molecular weight) lead to a greater strength. They are
referred back to this analysis when they choose their own molecule to analyze and then asked
to justify their choice in light of those factors.
It was also decided to add information about oxygen balance to the instructions and to ask
students to calculate the oxygen balance for their compound. The calculation is a simple
analysis of the chemical formula and will hopefully lead students to choosing better molecules
to test.
Integration into the Curriculum
There are several places in the curriculum where this activity could be used. It could fit into a discussion
of combustion reactions and the difference between explosion and combustion. The equations used to
calculate heat of explosion are very similar to Hess’s law. Using this activity in a unit with Hess’s law
would give an opportunity for an interesting discussion of how engineers and scientists use the same
information in different ways. The level of difficulty makes this activity more appropriate for higher
level high school or an undergraduate level chemistry course.
References
1. Bumpus, John A., Anne Lewis, and Stotts Corey. "Characterization of High Explosives and Other
Energetic Compounds by Computational Chemistry and Molecular Modeling." Journal of
Chemical Education 84.2 (2007): 329-32.
2. Rzepa, Henry S. "Nitroglycerin." Web. 05 Apr. 2012.
<http://www.ch.ic.ac.uk/rzepa/mim/environmental/html/nitroglyc_text.htm>.
3. Ten Hoor, Marten J. "The Relative Explosive Power of Some Explosives." Journal of Chemical
Education 80.12 (2003): 1397.