Species with well-known enthalpies of formationIn designing a reaction to determine the enthalpy of formation of a species, atomization reactions are often used. There are several reasons to NOT use atomization reactions such as
1) the enthalpy of formation of an atom may not be as well known as a compound containing that atom (such as Boron),
2) an isogyric (same spin on each side) reaction usually gives better results
For example, if we wanted to calculate the enthalpy of formation of CH3Cl
we could use an atomization reaction:
CH3Cl = C + 3 H + Cl
or we could use an isogyric reaction:
CH3Cl + H2O = H2CO + HCl + H2
While the experimental uncertainties in the enthalpies of formation of the products are very similar for both reactions, the calculated reaction enthalpy is expected to be closer to experiment for the second, isogyric, reaction.
The following list suggests, for each element, some species with well defined enthalpies of formation.
Enthalpies in kJ mol-1