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.
















"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