CE 561 Lecture Notes
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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.
we know the rate constant for an elementary reaction in one direction, we need to know the
reaction thermochemistry (
) to get the equilibrium constant and the rate constant in
the reverse direction.
There are essentially four ways to obtain this information:
From the literature.
Of course, this is the best way because it is the easiest and often the most
The first place that one should generally look for thermochemical data is in
the NIST Chemistry WebBook, available at
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
Another online source of thermochemical data useful for high-temperature
chemical kinetics is the HiTempThermo page maintained by Sandia National Laboratories at
Another valuable source is the thermochemical
NIST has also compiled a
This is usually the most reliable method, but also the most difficult, and is
usually not an option.
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
1 or 2 kcal/mole.
This is generally much easier than experiments, but in many cases
is still less reliable.
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
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.