Page 230 - Advanced Thermodynamics for Engineers, Second Edition

218 CHAPTER 10 THERMODYNAMICS OF COMBUSTION

The heat of formation of CO may be evaluated by reaction (Eqn (10.19a)) and then used in reaction
(Eqn (10.19b)) to give the heat of reaction of that process. From experiment the heat of formation of
carbon monoxide (CO) is 110.5 MJ/kmol and hence, for reaction (Eqn (10.19b)), the energy released is

Q p ¼ H P H R ¼ 393:5 110:5 ¼ 283 MJ kmol (10.20)
1
Hence the heat of reaction for CO þ O 2 /CO 2 is 283 MJ/kmol.
2
Theheatsofformationofanycompoundscan beevaluatedbybuildingupsimplereactions,havingfirst
designated the heats of formation of elements as zero. This enables the enthalpy of formation of various
compounds to be built up from component reactions. Enthalpies of formation are shown in Table 10.2.
These enthalpies of formation can be used to evaluate heats of reaction of more complex molecules
e.g. for methane (CH 4 ) the equation is

CH 4 þ 2O 2 ¼ CO 2 þ 2H 2 OðgÞ
giving

Q p ¼ðDH f Þ þ 2ðDH f Þ ðDH f Þ
25 CO 2 ðgÞ H 2 OðgÞ CH 4 ðgÞ
¼ 802279 kJ=kmol:
This gives an enthalpy of reaction per kilogram of CH 4 of 50,142 kJ/kg.

Table 10.2 Enthalpies of Formation of Some Common Elements and Compounds
Species Reaction State DH f (kJ/kmol)
e Gas 25 C, 1 atm 0, element

Oxygen, O 2
e Gas 25 C, 1 atm 0, element

Hydrogen, H 2
Carbon, C e Gas 25 C, 1 atm 0, element

Gas 25 C, 1 atm a 393,522

Carbon dioxide, CO 2 C þ O 2 /CO 2
Carbon monoxide, CO C þ O / CO Gas 25 C, 1 atm a 110,529

Water vapour, H 2 O H 2 þ ½O 2 / H 2 O(g) Gas 25 C, 1 atm a 241,827

Water (liquid), H 2 O H 2 þ ½O 2 / H 2 O(l) Liquid 25 C, 1 atm 285,800

Nitric oxide (NO) ½N 2 þ ½O 2 / NO Gas 25 C, 1 atm a þ89,915

Methane, CH 4 C þ 2H 2 / CH 4 (g) Gas 25 C, 1 atm a 74,897
Ethane, C 2 H 6 2C þ 3H 2 / C 2 H 6 (g) Gas 25 C, 1 atm 84,725

Propane, C 3 H 8 3C þ 4H 2 / C 3 H 8 (g) Gas 25 C, 1 atm 103,916

Butane, C 4 H 10 4C þ 5H 2 / C 4 H 10 (g) Gas 25 C, 1 atm 124,817

Isooctane, C 8 H 18 8C þ 9H 2 / C 8 H 18 (g) Gas 25 C, 1 atm 224,100

Isooctane, C 8 H 18 8C þ 9H 2 / C 8 H 18 (l) Liquid 25 C, 1 atm 259,280
Methyl alcohol, CH 3 OH C þ 2H 2 þ ½O 2 / CH 3 OH(g) Gas 25 C, 1 atm 201,200

Methyl alcohol, CH 3 OH C þ 2H 2 þ ½O 2 / CH 3 OH(l) Liquid 25 C, 1 atm 238,600

Ethyl alcohol, C 2 H 5 OH 2C þ 3H 2 þ ½O 2 / C 2 H 5 OH(g) Gas 25 C, 1 atm 234,600

Ethyl alcohol, C 2 H 5 OH 2C þ 3H 2 þ ½O 2 / C 2 H 5 OH(l) Liquid 25 C, 1 atm 277,000

a
note: the values of DH f given in the tables of gas properties are based on 0 K.