Day13 - CE 561 Lecture Notes Fall 2009 Day 13: Estimation...

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CE 561 Lecture Notes Fall 2009 p. 1 of 15 Day 13: Estimation methods for thermochemical and rate parameters To apply transition state theory and the various unimolecular rate theories, we need to know something about the structure and energies of the reactant(s), product(s), and transition state. If we know the rate constant for an elementary reaction in one direction, we need to know the reaction thermochemistry ( H , S , C p ) to get the equilibrium constant and the rate constant in the reverse direction. There are essentially four ways to obtain this information: (1) From the literature. Of course, this is the best way because it is the easiest and often the most accurate method. The first place that one should generally look for thermochemical data is in the NIST Chemistry WebBook, available at http://webbook.nist.gov . This database contains thermochemical properties for more than 7000 small organic and inorganic compounds, and includes the entire contents of several other databases. For hydrocarbons, the thermodynamic database supplied with process simulators (i.e. Aspen or HYSIS) can be a useful resource. Another online source of thermochemical data useful for high-temperature chemical kinetics is the HiTempThermo page maintained by Sandia National Laboratories at http://www.ca.sandia.gov/HiTempThermo/ . Another valuable source is the thermochemical property database assembled by Burcat, which is available online at ftp://ftp.technion.ac.il/pub/supported/aetdd/thermodynamics/ . NIST has also compiled a comprehensive database of gas-phase chemical kinetics, available at http://www.kinetics.nist.gov . (2) From experiment. This is usually the most reliable method, but also the most difficult, and is usually not an option. (3) From ab initio molecular orbital calculations. These methods can provide accurate structures and vibrational frequencies for small to moderately large molecules and transition states . For small molecules, energies can be calculated with ‘chemical accuracy’, which usually means ± 1 or 2 kcal/mole. This is generally much easier than experiments, but in many cases is still less reliable. (4) Empirical estimation methods. There are some simple “quick and dirty” methods of estimating thermochemical quantities for molecules for which no data are available, and for estimating reaction rates for elementary reactions that have not previously been studied. These are not always reliable, but they can often provide better estimates of a rate constant than (a) blind guessing, or (b) neglecting the reaction (effectively assuming a rate constant of 0). In the literature, you will find that option (b) shows up pretty often. This makes no sense, because even if we have to purely guess a rate constant, we can almost always make a guess that is closer to correct than guessing zero.
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Day13 - CE 561 Lecture Notes Fall 2009 Day 13: Estimation...

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