Page 253 - Advanced thermodynamics for engineers

240 CHAPTER 11 CHEMISTRY OF COMBUSTION

Hence, from Fig. 11.1,
X X X X
ðDH f Þ ¼ DH a DHðX YÞ DH res DH latent (11.4)
CO 2
In this case there is a resonance energy but the latent energy (i.e. latent heat) is zero.
Then

h i h i h i
ðDH f Þ ¼ DH a C graphite þ DH a O ¼ O 2ðC ¼ OÞ DH res CO 2
CO 2
¼ 717:2 þ 435:4 2 698:1 137:9 (11.5)
¼ 381:5MJ=kmol
The tabulated value is 393.4 MJ/kmol.
Water (H 2 O)
This is formed from the hydrogen and oxygen molecules in the reaction
1
H 2 ðgÞþ O 2 ðgÞ/H 2 OðgÞ (11.6)
2
Thus

X X X X
ðDH f Þ H 2 O ¼ DH a DHðX YÞ DH res DH latent (11.7)
In this case DH res ¼ 0 but DH latent depends upon the phase of the water. First, consider the water
is in vapour phase, when DH latent ¼ 0. Then
X X
ðDH f Þ H 2 O ¼ DH a DHðX YÞ
1
¼ DH a ½H HŠþ DH a ½O ¼ OŠ ½H OHŠ ½O HŠ (11.8)
2
1
¼ 435:4 þ ð498:2Þ 497:5 428:7
2
¼ 241:7MJ=kmol
This compares well with the tabulated values of 241.6 MJ/kmol.
If the water was in liquid phase then Eqn (11.6) becomes
1
H 2 ðgÞþ O 2 ðgÞ/H 2 Oð[Þ (11.9)
2
ðDH f Þ ¼ðDH f Þ þðm w Þ h
H 2 OðlÞ H 2 OðgÞ H 2 O fg
¼ 241:7 18 2441:8=1000 (11.10)

¼ 285:65 MJ=kmol
The tabulated value is 285.6 MJ/kmol.