Page 149 - Chemical equilibria Volume 4

Determination of the Values Associated with Reactions – Equilibrium Calculations 125
=
0
1
−
Δ h
571,70 kJ.mol ²
R
6 298
The inverse reaction of the reaction of dissociation of hydrogen is:
H 2(gas) = 2H(gas) [4R.7]
Δ h 0 = 871.86 kJ.mol − 1
R
7 298
Finally, we have the reaction of sublimation of graphite:
C(graphite) = C(gas) [4R.8]
Δ h 0 = 716.682 kJ.mol − 1
R 8 298
The sum of reactions [4R.4] to [4R.8] gives us the following for the
dissociation of methane:

CH 4(gas) = C(gas) + 4 H(gas) [4R.9]

This sum gives us:
−
1
Δ h 0 = 1663.39kJ.mol
9 298
R
This sum can be considered as being four times the energy of the C-H
bond, which therefore is:
−
ε = 415.85 kJ.mol
1
−
CH
Note that many bond energies use the sublimation of graphite, for which,
as pointed out above, it is difficult to obtain a value. This accounts for the
sometimes-divergent values noted between different sources of values of
those bond energies – the divergence stems from a difference in the choice
of the sublimation energy of graphite. Thus, the value of the C-H bond is
given as 388.74 kJ/mol by the Handbook der Physik, based on a value of
627 kJ/mol for the sublimation of graphite, whereas Pauling, in 1945, gave
the value 364.91kJ/mol, based on graphite sublimation at 578.74 kJ/mol,
and in 1948, Gerö and Valatin gave the value of 405.46 kJ/mol, based on
709.35 kJ/mol for graphite sublimation. The value calculated above is
716.68 kJ/mol, and it is this reading which we use as the basis for the values
reported in Table 4.4.