Physical Chemistry II Laboratory
Energy of Isomerization for Naphthalene to Azulene: Bomb Calorimetry
and Theoretical Calculations
Goal:
Determine the ∆Uiso for naphthalene converting to azulene. You will
obtain this value by measuring the energy of combustion of the two
substances in a bomb calorimeter. You will also compare these values to
theoretical calculations in an attempt to validate the theory.
Procedure:
Follow procedures as outlined in the calorimeter manual for operating the
device. Be sure to transfer a precise amount of water into the Dewar flask.
You will need 450.0 grams measured precisely. This will need to be
consistent throughout all the calorimetric runs. Also, be sure that the
water placed in the flask is a few degrees cooler than room temperature.
The calorimeter can be interfaced to a computer to obtain the thermogram
(plot of Temperature vrs. time).
First calibrate the calorimeter by using benzoic acid as the standard.
Next prepare a pellet of either azulene or naphthalene (see instructor for
assignment). Use approximately 150 mg. Export both thermograms to
Excel and overlay them unto one plot that uses an entire sheet of paper.
Show detailed divisions on the y-axis so that the temperature change can
be determined precisely. This is explained in the calorimeter manual.
Before leaving lab, fill out the data form for this experiment and hand to
the instructor. This form is how the groups share info with one another.
Analysis:
The analysis of this lab can be performed by using one simple equation:
heat lost = heat gained
Here the heat is lost by the sample burning and the wire burning. The
heat is gained by the water in the dewar and the calorimeter itself.
−qsample − qwire = qwater + qcalor
Note sign convention will result in positive quantities. Expanding gives:
comb
 − =
−  nsample ⋅ ∆U sample
qwire
[ Ecalor ⋅ ∆T ]
Here are the definitions of the terms of this equation:
nsample ≡ # moles of sample (positive)
comb
ΔU sample
≡ molar energy of combustion for the sample (negative)
qwire ≡ heat lost by part of wire that burned (negative)
Ecalor ≡ energy equivalent of the calorimeter (positive)
ΔT ≡ temperature change from thermogram (positive)
Please note that the simplification of the right hand side of this equation
comes about since we use precisely the same amount of water in each
run. Otherwise, you would have to take into account differences due to
the heat capacity of water.
Using the data from the standard, solve for the energy equivalent of the
calorimeter (using the literature value for the molar energy of
combustion). Now using data from your compound, solve for its molar
energy of combustion (using the Ecalor determined above).
Finally, compute the molar energy of isomerization (naphthalene to
azulene) by partnering with another group.
Theoretical Calculations
1. Use Gaussian to calculate the absolute energy for each molecule and
take the difference to obtain the molar energy of isomerization. This
requires an optimized geometry and frequencies to be calculated (OPT
FREQ) so that the thermochemical information is evaluated. Your
instructor will assign a theoretical model for you to use. This
information is labeled “Sum of electronic and thermal Energies”
in the output file and is in atomic units.
2. Take the difference in energies in atomic units (azulene –
naphthalene) then convert the energy difference into to kJ/mol. How
does this difference compare to your value obtained experimentally?
3. What are the point groups of these two molecules?
4. Explain the difference between the electronic energy computed by
Gaussian and the thermochemical energy from a FREQ calculation.
Why did we use the latter? How much error would have been present
of we instead used the former?
Summary
Be sure to discuss your results and compare them to literature values.
Use percent difference when comparing. How successful was your
experiment? Can you explain any errors?