Page 254 - Advanced Thermodynamics for Engineers, Second Edition

242 CHAPTER 11 CHEMISTRY OF COMBUSTION

H H

H C C O H

H H
FIGURE 11.6
Structure of ethanol molecule.

giving the enthalpy of formation of ethanol as
X X X X
ðDH f Þ ¼ DH a DHðX YÞ DH res DH latent : (11.14)
C 2 H 5 OH
This gives the following equation

2C graphite þ 3H 2 ðgÞþ 0:5O 2 ðgÞ
/2CðgÞþ2 717:2 þ 6H þ3 435:4þ O þ0:5 498:2 5½C HŠ ½C CŠ ½C OŠ ½O HŠ
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atomisation atomisation atomisation
energy for C energy for H energy for O
/C 2 H 5 OHðgÞþ 2 717:2 þ 3 435:4 þ 0:5 498:2 5 414:5 347:5 351:7 428:7
/C 2 H 5 OH 210:7MJ=kmol
ðgÞ
(11.15)

Hence the enthalpy of formation of gaseous ethanol is 210.7 MJ/kmol. This is equivalent to
5.853 MJ/kg of ethanol. The value obtained from tables is about 222.8 MJ/kmol, and the difference
is attributable to the slight variations in bond energy that occur due to the three-dimensional nature of
the chemical structure.
Example
Evaluate the enthalpy of reaction of methane, CH 4 .
Solution
This can be obtained either by using atomisation and dissociation energies, in a similar manner to
that used to find the enthalpies of formation of compounds, or from the enthalpies of formation of the
compounds in the reactants and products. Both methods will be used in this case. The reaction
describing the combustion of methane is
CH 4 ðgÞþ 2O 2 ðgÞ/CO 2 ðgÞþ 2H 2 OðgÞ (11.16)
The chemical structure of methane was given above. Hence the enthalpy of formation of the
reactants is
X X X X
ðDH f Þ ¼ DH a DHðX YÞ DH res DH latent
R
h i (11.17)
¼ DH a C graphite þ 2DH a ½H HŠþ 2DH a ½O ¼ OŠ 4½H CŠ 2½O ¼ OŠ