Page 257 - Advanced thermodynamics for engineers

244 CHAPTER 11 CHEMISTRY OF COMBUSTION

O

C O H

C
HC CH

HC CH
C
H
FIGURE 11.7
Structure of benzoic acid molecule.

The acid is in gaseous form before the reaction, see Eqn (11.22), and thus DH latent ¼ 0. Substituting
values into Eqn (11.24) gives

X X X
ðDH f Þ C 6 H 5 COOH ¼ DH a DHðX YÞ DH res
h i
¼ 7DH a C graphite þ 3DH a ½H HŠþ DH a ½O ¼ OŠ 5½H CŠ 4½C CŠ
3½C ¼ CŠ ½C ¼ OŠ ½C OŠ ½O HŠ ½DH res Š ½DH res Š
C 6 H 6 COOH
(11.25)
Hence,

ðDH f Þ C 6 H 5 COOH ¼ 7 717:2 þ 3 435:4 þ 498:2 5 414:5 4 347:5 3 615:5 698:1
351:7 428:7 150:4 117:0
¼ 230:1MJ=kmol
(11.26)
The enthalpy of formation of the products is

ðDH f Þ ¼ 7ðDH f Þ CO 2 þ 3ðDH f Þ H 2 O
P
¼ 7 ð 381:5Þþ 3 ð 241:7Þ (11.27)
¼ 3395:5MJ=kmol

Thus the enthalpy of reaction of benzoic acid is

ðDH R Þ ¼ 3395:6 ð 230:1Þ¼ 3165:5MJ=kmol (11.28)
C 6 H 5 COOH
The tabulated value is –3223.2 MJ/kmol giving an inaccuracy of about 2%.