Page 168 - Chemical equilibria Volume 4

144 Chemical Equilibria
molecular structure. In practice, we are evaluating not the enthalpy but the
standard Gibbs energy at a given temperature.
Different evaluation methods exist, and are generally applied to a family
of products. Two important categories are frequently used: that applied to
metals and liquid metal alloys and that used for organic products.

For this latter family, most methods for evaluating the thermodynamic
data are inspired by what we have seen for bond energies, based on the
structure of the molecule. The most significant methods are based on what
we call the functional-group substitution method. They are founded on a
certain number of observations. For example:

– if a methyl group (CH 3) is substituted for a hydrogen atom attached to a
carbon atom, we note an increase in the standard Gibbs energy of formation
at 298 K by around 7948 J/mol;
– if a hydroxyl group (OH) is substituted for a hydrogen atom attached to
a carbon atom, we see a decrease of the standard Gibbs energy of formation
at 298 K, by around 142 120 J/mol.

Thus, when we know that the standard Gibbs energy of formation of
ethane at 298 K is -32,604J/mol. For propane, which results from the
substitution of a hydrogen atom by a methyl group on a carbon atom from
ethane, we then calculate a standard Gibbs energy of formation at 298 K of:

-32,604 + 7945 = – 34,602 J/mol

knowing that the experimental value is –23,408 J/mol.

Similarly, for ethyl alcohol, we can easily calculate a standard Gibbs
energy of formation at 298 K:

–32,604 –142,120 = –174,724 J/mol

for a real value of –168,038 J/mol.

Expanding on these observations, different tables have been drawn up,
giving the contributions of various substitutions on the thermodynamic
values. A series of particularly complete tables are given by Andersen,