6.2 Diatomic Molecules
305
Scheme 6.1 Hess law cycle
for the chemical reaction
between H 2 and Cl 2
+
+
ΔH
°
f0
Scheme 6.2 Hess law cycle
for the reaction of graphite
and oxygen
Δ
°
+
+
ΔH
°
f0
formation briefly, using two examples, namely the formation of HCl from H 2 and
Cl 2 , and the formation of CO from elemental carbon and O 2 .
Example 6.5 The H 2 –Cl 2 reaction. The Hess law cycle for this reaction is shown in
Scheme 6.1.
This Hess law cycle relates the heat of formation, H
◦
f 0 , of HCl at 0 K, 1 bar
pressure, to the spectroscopic dissociation energies D 0 of H 2 , Cl 2 , and HCl. From
Table 6.1 we have the values D 0 (H 2 ) = 4.478 eV, D 0 (Cl 2 ) = 2.479 eV, and
D 0 (HCl) = 4.433 eV. The Hess law cycle thus gives us the relation
1
2 D 0 (H 2 ) +
1
2 D 0 (Cl 2 ) − D 0 (HCl) = H
◦
f 0 (HCl) ,
so that substitution of the D 0 values gives for H ◦
f 0 (HCl) the value −0.955 eV or
−92.111 kJ mol
−1 . This value differs by less than 0.1% from the accepted literature
value of −92.127 kJ mol
−1 given in the JANAF tables.
Example 6.6 Formation of CO from graphite and O 2 . The Hess law cycle appropriate to this reaction is shown in Scheme 6.2.
This Hess law cycle requires the heat of formation of gaseous atomic carbon from
graphite at 0 K, which is H
◦
f 0 = 711.0 kJ mol
−1
7.370 eV, while D 0 (O 2 ) =
5.116 eV, and D 0 (CO) = 11.092 eV. Upon employing these values in the Hess
law cycle, we obtain H
◦
f 0 (CO) = −1.164 eV = −112.32 kJ mol
−1 : this value is
within 1.2% of the accepted literature value −113.813 kJ mol
−1 for H
◦
f 0 (CO). The
agreement in this case is not quite as good as that obtained in Example 6.5, largely
due to the less accurate data for graphite and CO. Note, however, that a difference
of only 0.015 eV in the value of D 0 (CO) (corresponding to an experimental error
of about 0.14%) would already account for the difference between our calculated
value of H
◦
f 0 (CO) and the accepted literature value given in the JANAF tables.