Hydrocarbons

Computational Study of Heats of Formation and Combustion of Hydrocarbons

Jonathan Smith

In this investigation, we will use the molecular modeling suite, Gaussian 03, to investigate a series of hydrocarbons some of which are important fuels. We will calculate the molecular structure (geometry), a theoretical value for the heat of formation, and heat of combustion for these compounds. The values we calculate are far from exact values. Within any computational software, we are using a very approximate model to describe the interactions between positively charged nuclei and electrons that make up the molecule. Each level of approximation is called a model chemistry and comparisons can be made between the structures and energies of molecules all calculated with a particular model chemistry. It is of interest to compare these theoretical values to experimental values when they are available. This comparison can validate the use of a particular model chemistry for a class of related molecular systems for which there may be no experimental values or to calculate properties not easily measured by experiment. Some model chemistry's provide us with data which is easily related to tabulated experimental data. Molecular geometry given from calculations can be compared with x-ray or NMR determined structures when they are available. Another property which semi-empirical model chemistry's provide is the theoretical heat of formation. This value can readily be compared with tabulated values in the CRC handbook and online at the NIST Webbase. Values determined from the AM1 semiempirical model chemistry have been found to differ on average by 5-10 kcal/mole from experimental values.¹ Heats of formation can be used to determine heats of combustion. These are particularly useful when examining fuels such as ethanol and octanol.

Read the section on thermochemistry in the text in preparation (McQuarrie and Simon 5-10, 5-11). Write out the equation for a hydrocarbon combustion reaction and indicate the quantities we will have to compute to determine the heat of combustion for each compound listed below. As well, you will need to look up the heat of formation for each of the compounds below and tabulate them so that we have a reference point for our calculations. In addition to the below compounds come up with 3 additional compounds that you will study with our model chemistry. For these calculations we will also compare the results from two different semiempirical model chemistry's the AM1 method and the PM3 method. In preparation for this investigation you will need to set up your laboratory notebook with a title a short description of what we are trying to accomplish in the investigation as well as the equation for combustion. Tabulate the data you look up for each of your compounds.

Compounds

ethylene	ethanol
benzene	methanol
naphthalene	butanol
anthracene
phenathrene
pyrene
toluene
"Kerosene" C₁₁H₂₄
"Octane" n-C₈H₁₈

1. Dewar, M. J. S.; Zoebisch, E. G.; Healy, E. F.; Stewart, J. J. P. Journal of the American Chemical Society 1985, 107, 2902-3909.
Notes: Hyperchem

Steps

First, it is important to become familiar with WebMO. Navigate to the login page by first visiting the chemistry department's computational server homepage at http://pandora.chem.gac.edu/ . Next, follow the “Computational Tools” link and then the “WebMO” link. This should bring you to a login screen that requires a username (your Gustavus email name) and password (provided).

Once logged in, click on the “New Job” button. This will bring you to an interface where molecules can be constructed. Click the “Open Editor” button to begin this process. You will need to build H₂O, CO₂, and O₂ (create a separate job for each molecule). Visit http://webmo.net/ for a detailed set of instructions to aid in this building process. Before closing the editor, click the “Clean-up” tab followed by “Comprehensive”. You can then close this window and select the “Continue” arrow. On the next page you can select “Gaussian” for the computational engine (see http://www.gaussian.com/ ) and click the “Continue” arrow. The next screen allows you to configure the Gaussian job options. Begin by entering a unique name for this job to make it easier to identify in the future. Then, change “Calculation” to “Geometry Optimization” and the “Theory” to “AM1.” Clicking on the “Continue” arrow will then submit the job for processing. The “Job Manager” page does not refresh itself so hitting the “Refresh” button will give you a current status of the calculations.

Once these calculations complete, tabulate the bond angles and bond lengths. Do they correspond to your expectations? Now, repeat the calculations but substitute “Vibrational Frequency” for “Geometry Optimization” in the type of calculation. You can view the vibrational modes by opening the completed job and selecting “view” by the desired vibration. Record the frequencies for these molecules. How do these match with experimental data (see )? You should then perform “Optimization and Frequency” calculations on these molecules in order to acquire enthalpy values. Gaussian 03 calculates these values using statistical mechanics.

Repeat these calculations using “PM3” as the model chemistry instead of “AM1.”

From these calculations record the energy and enthalpy for each compound. Note that these are not the heat of formation but you are using differences so they are equally valid in the thermochemical calculations. You will compare these with those tabulated in your Excel spreedsheet to verify its operation. Do one or two examples in the spreadsheet to verify the validity of your values and the calculations in your spreadsheet.

Build your remaining compounds and carry out calculations on each.

Use Excel to graph the heat of formation calculated for each molecule using both methods (AM1 and PM3) versus the experimental values you looked up ().

Add columns in your Excel spreadsheet to calculate the heat of combustion for your compounds as well as the heat of combustion per gram.

Which compound is the most efficient fuel?

Discuss the advantages/disadvantages of ethanol, 85% ethanol, 10% ethanol as fuels.

Discuss the accuracy of your calculations and compare the results.

Complete your discussion and tabulate your results in your notebook to be turned in by the next laboratory period following the directions outlined in the syllabus. Note any trends you observed. Submit the values for the compound(s) you chose to http://pandora.chem.gac.edu/pchemsubmita.html using the appropriate entry fields. Post these results as soon as possible. You will need to discuss and tabulate the other data posted (See: http://pandora.chem.gac.edu/cgi-bin/datasaved.pl?file=Hydrocarbons) here by others in your own laboratory report.

updated September 13, 2006