Page 254 - Advanced thermodynamics for engineers
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11.2 ENERGY OF FORMATION 241
(a) H (b)
H
H
C H C H
H
H
H
FIGURE 11.5
Atomic arrangement of methane molecule.
Example
Evaluate the enthalpy of formation of methane, CH 4 .
Solution
The structure of methane is tetrahedral, and of the form shown in Fig. 11.5a. Methane is often
depicted in planar form as shown in Fig. 11.5b.
Methane can be formed by the following reaction
C graphite þ 2H 2 ðgÞ/CH 4 ðgÞ (11.11)
Hence
X X X X
ðDH f Þ ¼ DH a DHðX YÞ DH res DH latent
CH 4
h i
¼ DH a C graphite þ 2DH a ½H H 4½H C
(11.12)
¼ 717:2 þ 2 425:4 4 414:5
¼ 70MJ=kmol
The tabulated value is 74.78 MJ/kmol, and the difference occurs because the energy of the bonds
in methane is affected by the structure of the methane molecule which results in attraction forces
between molecules.
Example
Evaluate the enthalpy of formation of ethanol, C 2 H 5 OH.
Solution
The planar representation of the structure of ethanol is shown in Fig. 11.6.
Ethanol can be considered to be made up of the following processes
2C graphite þ 3H 2 ðgÞþ 0:5O 2 ðgÞ/C 2 H 5 OHðgÞ; (11.13)
