The Decomposition of Nitroglycerin
Tom Moffett, SUNY Plattsburgh, 2010
Nitroglycerin (NG) is very unstable chemical that reacts violently if subjected to heat or
vibration. Obviously this is a chemical that must be handled with extreme care, and one that
would be unsafe to utilize in an introductory chemistry laboratory. Instead of using the chemical
it is possible to use molecular modeling software to calculate the thermodynamic and structural
properties of NG.
While the basic quantum mechanical equations have been known since the 1930s, until recently
they have been virtually unsolvable for all but the smallest molecules. In the past couple of
decades, new methods for solving the problems, and advances in computer technology have
increased the amount of chemical information which can be determined computationally.
In this experiment we will utilize a semi-empirical method to calculate the properties of
nitroglycerin. Semi-Empirical calculations use parameters derived from experiments to simplify
the quantum mechanical calculations. The results of the semi-empirical calculations can be used
as a starting point for more advanced calculations.
The enthalpy of reaction (∆Hrxn) for any reaction can be determined from measuring the enthalpy
of formation ( Hf) of each chemical in the reaction. Enthalpy of formation is the enthalpy
change that occurs when a compound is formed from the elements in their standard states. The
enthalpy formation of any element in its standard state (i.e. Al (s), Fe (s), O2 (g), etc.) is zero.
Enthalpy of formation is also dependant upon the state of the compound formed, for example
H2O (l) and H2O (g) have different values for Hf. The following equation is used to determine
enthalpy of reaction.
∆Hrxn = Σ Hf(products) - Σ Hf(reactants)
(1)
When calculating enthalpy of reaction the stoichiometry of the reaction must be accounted for.
As an example, consider the following reaction.
4 NH3 (g) + 5 O2 (g)
4 NO (g) + 6 H2O (g)
The enthalpies of formation are as follows:
NH3 (g)
O2 (g)
NO (g)
H2O (g)
-46.0 kJ/mol
0 kJ/mol
+90.4 kJ/mol
-241.8 kJ/mol
The enthalpy of reaction would then be calculated as follows:
∆H rxn = [4 ∆H f (NO ) + 6 ∆H f (H 2 O )] - [4 ∆H f (NH 3 ) + 5 ∆H f (O 2 )]
∆H rxn = [(4mol)(90.4 kJ/mol) + (6 mol)(- 241.8 kJ/mol)] -
[(4 mol)(− 46.0 kJ/mol) + (5 mol)(0 kJ/mol)]
∆H rxn = - 905.2 kJ
When the enthalpy of reaction is negative, the reaction is exothermic, meaning that it releases
heat to the surroundings. In the opposite situation, endothermic reactions absorb heat from the
surroundings and have a positive enthalpy of reaction.
The enthalpy of reaction (∆Hrxn) for the decomposition of nitroglycerin can be easily calculated
by using Spartan to calculate the enthalpy of formation (∆Hf) of each of the molecules in the
reaction:
C3H5N3O9 (l) → CO2 (g) + H2O (g) + N2 (g) + O2 (g)
Structure of Nitroglycerin:
(unbalanced)
Manipulating Molecules in Spartan
To select a molecule – Left click on it (the name of the selected molecule is indicated at the
top of the window).
Rotate a molecule – Hold the left mouse button down, and drag mouse
Move a molecule – Hold the right mouse button down, and drag mouse
Zoom in or out – Hold down the right mouse button and the Shift key, drag the mouse
Rotate a bond – Select the bond to rotate by left clicking on it, hold down the left mouse
button and the alt key, drag the mouse
Procedure:
Start Spartan by double clicking on the Spartan06 icon.
Building Molecules
1.) Begin construction of nitroglycerin by clicking on File from the menu bar, and then
selecting New, the fragment panel should now appear on the right side of the screen. (In
order to condense the instructions, the above set of commands will be abbreviated as
File→
→New.)
2.) Build nitroglycerin by first constructing the carbon backbone of the molecule. Click on
the carbon sp3 carbon atom (the one with four single bonds). Next click anywhere within
the main window, a carbon atom with four bonds should appear.
3.) Add two more carbon atoms by clicking the mouse pointer on the end of any two open
bonds on the first carbon atom. The molecule should now consist of three carbon atoms.
4.) Add an oxygen atom (with two single bonds) to each of the carbon atoms.
5.) Complete the structure by adding a nitro group to each of the oxygen atoms. Spartan has
already defined some common groups of atoms. In the fragment window, click on the
box next to Groups and select Nitro.
6.) The computer will automatically add in hydrogen atoms to any open valence spaces. The
computer will now make an initial guess at the molecular geometry by clicking on
Build→
→Minimize.
7.) Save the structure by selecting File→
→Save As
8.) Quit the builder by selecting the “view” button (the button with the black V).
Setting Up and Submitting Calculations
1.) Select Setup→
→Calculations
2.) The top box should read “Calculate: Equilibrium Geometry at Ground State with SemiEmpirical AM1”
3.) Under “Subject to”, deselect the box for symmetry, this will allow the molecule to change
symmetry point groups.
4.) Click OK.
5.) Submit the job for calculation by clicking on Setup→
→Submit. Click OK in response to
the computers notification that the job has been submitted.
6.) Note: If the job fails check the output file, Display→Output. Near the end of the file the
computer will give a reason for the failure. If “Ran out of cycles” is the reason given,
simply resubmit the calculation, and the computer will continue from the point where it
stopped. If another reason is listed, ask your instructor for help.
Examining the Results of Calculations
1.) When the calculation is finished, the computer will post a message stating that the job has
completed. Select OK. The job must be complete before continuing.
2.) Bond distances in the optimized structure can be determined by selecting
Geometry→
→Measure Distance, and then selecting the two atoms between which the
distance is to be determined.
3.) Bond angles can be determined by selecting Geometry→
→Measure Angle and then
selecting the three atoms between which the angle is to be determined. The atoms must
be selected in the order specified, the second atom is the vertex.
4.) Select Display→
→Properties, a new window will appear. Record the Energy listed, this is
the enthalpy of formation of nitroglycerin.
Calculating the Enthalpy of Reaction
1.) Sequentially build and minimize each of the product molecules. Then setup and submit
the semi-empirical AM1 calculations for each. Record the enthalpies of formation.
* To build CO2, connect two single bonded oxygen atoms to an sp3 carbon atom. Next create
the double bonds by selecting Build→Make Bond. First click on one of the open bonds on
one of the oxygen atoms, then click on an open bond on carbon, a double bond should now
appear. Repeat to create the second double bond.
Name: ____________________________________
Enthalpies of Reaction -Data Sheet
Section: ______
CHE 101
Decomposition of Nitroglycerin
Molecular Geometry of Nitroglycerin
Nitroglycerin Bond Lengths
Nitroglycerin Bond Angles
C-C: ____________
C-O-N: ______________
C-O: ____________
O=N=O: ______________
N-O: ____________
C-C-C:
C-H: ____________
N=O: ____________
Enthalpies of Formation
Nitroglycerin
_______________
CO2
_______________
H2O
_______________
N2
_______________
O2
_______________
______________
Name: ____________________________________
Enthalpies of Reaction –Questions
Section: ______
CHE 101
1.) Balance the reaction for the decomposition of nitroglycerin.
2.) Calculate the enthalpy of reaction for the decomposition of nitroglycerin using the enthalpies
of formation that you calculated with Spartan. Is the reaction exothermic or endothermic?
3.) What is the enthalpy change when 4.25 kg of nitroglycerin detonates?
4.) The computer incorrectly calculated the enthalpies of formation of oxygen and nitrogen,
what should these values be?
Name: ____________________________________
Enthalpies of Reaction –Questions
Section: ______
CHE 101
5.) If 75.0 g of copper (II) sulfide reacts with 40.0 g of oxygen and forms 54.0 g of copper (II)
oxide, what is the percent yield? (Hint: this is a limiting reagent problem.)
2CuS (s) + 3O2 (g) → 2CuO (s) + 2SO2 (g)
Name: ____________________________________
Enthalpies of Reaction – Pre-Lab Questions
Section: ______
CHE 101
1.) Draw Lewis structures for the products of the decomposition of nitroglycerin.
2.) Given the following reaction:
3 BaO (s) + 2 Al (s) → 3 Ba (s) + Al2O3 (s)
The enthalpies of formation for each compound in the above reaction are listed below.
BaO: -553.5 kJ/mol
Al:
Ba:
Al2O3: -1676 kJ/mol
0 kJ/mol
0 kJ/mol
b.) Determine the enthalpy of reaction for the above reaction. Is the reaction exothermic or
endothermic?
c.) Calculate the enthalpy change when 35.0 g of barium oxide reacts with an excess of
aluminum.