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VII.E.2. (II.C.4.) (XIV.I.)

Species with well-known enthalpies of formation

In 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
Element Species Enthalpy 0K unc.
H H2 0.000 0.000
Li LiH 140.624 0.040
Be Be 324.000 5.000
B BF3 -1136.000 0.800
C CO2 -393.510 0.130
C CO -110.530 0.170
C H2CO -108.700 0.500
N N2 0.000 0.000
N NH3 -45.940 0.350
O O2 0.000 0.000
O H2O -241.806 0.029
F F2 0.000 0.000
F F 79.380 0.300
Na Na 107.500 0.700
Na Na2 142.070 1.200
Mg Mg 147.100 0.800
Al AlF3 -1209.320 2.500
Al Al 330.000 4.000
Si SiF4 -1615.000 0.800
Si SiH4 34.700 1.500
P P4 58.900 0.300
P P 316.500 1.000
P PH3 5.400 1.700
S S 277.170 0.150
S SO2 -296.810 0.200
S H2S -20.600 0.500
Cl Cl2 0.000 0.000
Cl Cl 121.310 0.008
Cl HCl -92.310 0.100
Br Br2 30.910 0.110
Br Br 111.870 0.120