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I've ran into conflicting information on how to calculate the Gibbs free energy of fuels during combustion per unit mass, volume and mole.

A sample solution for hydrogen would be really appreciated!

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Just a tip: it would probably improve your question to include some of this conflicting information you've found, citing sources when possible. –  David Z Dec 5 '10 at 0:48
Also, an answer to this question is going to be highly dependent on the conditions under which the process takes place. I assume the process at least takes place at constant temperature and pressure (hence why one would use the Gibbs Free Energy and not the Helmholtz free energy) –  Jerry Schirmer Dec 5 '10 at 1:33
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1 Answer

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"Calculating" is some word that can understood very differently.

  • from "scratch" ie ab initio, only using physical values, nothing

from chemistry level. This was possible when I studied for the

molecule H2+ only. Today computers maybe are able to do such

calculations for molecules somewhat more complicated, but I do not know.

  • from spectroscopic data, this is rather easy for 2-atomic molecules,

when spectroscopic data are simple. (statistical mechanics)

  • Method of Benson.

this is an incremental method, the increments are derived from

experimental values for organic molecules where thermodynamic values are

known. The method is a kind of inter/extrapolation from known

to unknown. The increments are such for atoms, atom groups and

types of bonds, sometimes some intramolecular non-bonding interactions

are put into calculation. One needs the the structural formula to start with,

of course the Benson method is for organic molecules only.

The incentive is, that since the 30ties frequency of measurements of

heats of combustion and specific heats has decreased, since the 50ties,

it is almost zero. For the basic molecules, especially those conected

to mineral oil technology most thermodynamic values exist. But outside of this,

You look for data in vain.


  • Analogon to Born-Haber cycle

This is a method used when You know all other

thermodynamic values. Not often appicable.

To convert values per mole in values per mass or volume

is elementary, isn't it?



I forgot another method:

So called "force field calculation". Molecules are modelled

mechanically, between atoms (and atom groups) potentials

for vibrations and internal rotations and nonbonding interactions

are set up and calculated for a set of initial distances and

angles. This is repeated for different sets of distances etc.

Some method for search of overall minimum for total energy is applied,

because the number of variables is several dozen even for a

molecule of half a dozen atoms this is the crucial part.

When I studied in the 70ties, the big machine of our university

ran some hours to calculate a molecule of 12 C atoms.

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