Let us consider an example of Step 3 of indirect determination of the enthalpy of
formation with the following (which is also a simple reaction):
CO þ 1 = 2 O 2 ! CO 2 þ Q to be removed
The Q to be removed is found experimentally in this case to be 393,520 kJ/kmol. The
enthalpy balance is
h
0
f
CO
þ 0 ¼ h
0
f
CO 2
þ 282; 990
That is,
h
0
f
CO
¼ À393520 þ 282990 ¼ À110; 530 kJ/kmol
Another example,
CH 4 þ 2O 2 ! CO 2 þ 2H 2 O þ Q to be removed
The Q to be removed is found experimentally in this case to be 890,868 kJ/kmol. The
enthalpy balance is
h
0
f
CH 4
þ 0 ¼ h
0
f
CO 2
þ 2 h
0
f
H 2 O
þ 890; 330
That is,
h
0
f
CH 4
¼ À393520 þ 2 À285830
ð
Þþ890330 ¼ À74; 850 kJ/kmol
Through such a research program, the enthalpy-of-formation values for most
hydrocarbons have been compiled and can be found in Table 9.3 including
n-Octane (liquid).
h
0
f
C 8 H 18 l
ð Þ
¼ À249; 950 kJ/kmol
9.8 Combustion Chemical Reactions and Enthalpy Balance
263