Page 131 - Essentials of physical chemistry

The Second and Third Laws of Thermodynamics 93

TABLE 5.1
0
Selected Values of H in kJ at 1 bar and 298.158K
f
and DS 0 in J=8K
298
0
0
H (298.15, 1.000 bar) S (298.15, 1.000 bar)
f f
Compound kJ=mol J= K mol

0 42.55
H 2
O 2 0 205.152
CO 110.53 197.660
CO 2 393.51 213.785
0 5.74
C graphite
HCCH þ227.4 200.9
þ52.40 219.3
H 2 CCH 2
84.0 229.2
H 3 CCH 3
74.6 186.3
CH 4
45.94 192.77
NH 3
HCl 92.31 186.902
0 233.081
Cl 2
H 2 O 285.830 69.95
H 2 CO 108.6 218.8
Hg (liq) 0 75.90
90.79 70.25
HgO (red)
ENTROPY CHANGES AT T > 298.158K
Once again, we need to correct a state variable for temperatures other than the standard state:

prod
reac
qDS 0 X qS 0 X qS 0
i j ,
qT ¼ qT qT
i j
but we know that

ð T ð T
DC P dT
0
d(DS ) ¼
T
298 298
so we can write

ð T
0 0 DC P dT:
rxn
rxn
T
DS (T) ¼ DS (298) þ
298
We know from the previous chapter that we may have to integrate over the various terms of a
polynomial heat capacity, but there is a slight difference in the first term in this case. Once again, we
can calculate the difference in the C P polynomial coefficients according to the n i coefficients in the
balanced reaction.
prod react
P P
Da ¼ i n i a i j n j a j and similar expressions for Db, Dc, Dd, and De are obtained using
0
C P polynomials from Table 4.3. Thus, we need to integrate a slightly different formula for DS (T).
rxn