Modern Engineering Thermodynamics

15.10 Entropy Production in Chemical Reactions 623
Since all of the enthalpy values used thus far in this chapter are based on a standard reference state of T°=25.0°C
and p°=0.100 MPa, it would be convenient to be able to shift our absolute entropy scale to this reference state,
T °. Todothis,wedefines° to be the absolute molar specific entropy at the standard reference state and use
Eq. (7.35) with p = p°=constant to obtain
s° =sp°, ð T° Þ abs
=
Z T°
°
c P ðdT/TÞ
Values for the s° of various compounds can be found in Table 15.7. The absolute molar specific entropy at any
other state at pressure p and temperature T is given by
sp, ð TÞ abs
= s°+Δsðp° ! p, T° ! TÞ
where Δs represents the change in molar specific entropy between the state at (p°, T°) and that at (p, T). For an
incompressible substance, this becomes
and for an ideal gas it becomes
sp, ð TÞabs
= s°+
incompressible
substance
sðp, TÞabs ideal = s°+
gas
Z T
T°
c P ðdT/T
Z T
T°
cdT/T ð Þ
Þ− R lnðp/p°
Þ
If these substances have constant (or averaged) specific heats in the range of T° to T, thentheseequationscan
be integrated to give
and
sp, ð TÞabs
incompressible = s°+c lnðT/T°
Þ (15.23)
substance
½sðp, TÞ abs Š ideal
gas
= s°+c p lnðT/T°
Þ− R lnðp/p°
Þ (15.24)
Table 15.7 Molar Specific Absolute Entropy and Molar Specific Gibbs Function of Formation
at 25.0°C and 0.100 Mpa
s o
g o f
Substance
kJ/(kgmole ·K) Btu/(lbmole·R) MJ/kgmole Btu/lbmole
Carbon monoxide, CO 197.653 47.219 −137.150 −59,003
Carbon dioxide, CO 2 213.795 52.098 −394.374 −169,664
Water, H 2 O(g) 188.833 45.132 −228.583 −98,333
Water, H 2 O(l) 70.049 16.742 −237.178 −102,036
Methane, CH 4 186.256 44.516 −50.751 −21,834
Acetylene, C 2 H 2 200.958 48.030 +2011.234 +90,015
Ethylene, C 2 H 4 2111.548 52.473 +68.207 +29,343
Ethane, C 2 H 6 2211.602 54.876 −32.777 −14,101
Propane, C 3 H 8 270.019 64.361 −23.316 −10,031
Butane, C 4 H 10 310.227 74.146 −16.914 −7,276
Octane, C 8 H 18 (g) 466.835 111.576 +16.859 +7,253
Octane, C 8 H 18 (l) 360.896 86.256 +6.940 +2,986
Carbon, CðsÞ 5.740 1.372 0 0
Oxygen, O 2 (g) 205.138 411.029 0 0
Hydrogen, H 2 (g) 130.684 31.234 0 0
Nitrogen, N 2 (g) 191.610 45.796 0 0
Source: Van Wylen, G. J., Sonntag, R. E., 1976. Fundamentals of Classical Thermodynamics, SI Version, second ed. Wiley, New York, p. 496
(Table 12.3). Copyright © 1976 John Wiley & Sons. Reprinted by permission of John Wiley & Sons. Data on C, O 2 ,H 2 , and N 2 are from the
Journal of Physical and Chemical Reference Data, 11, Suppl. 2 (1982). Used with permission.