Using group additivity values to estimate Δvap
H°(298) and the Kistiakowsky equation to estimate both
Δvap
S°(T
b) and Δvap
H°(T
b) at the boiling point (T
b), it is shown how an average value of Δvap
C
p° can be
obtained which can then be used to calculate values of Δvap
S°(298). This latter can then be used in conjunction
with S°(g,298) to calculate S°(l,298). Alternatively, where values of S°(l,298) are available but not S°(g,298)
the latter can be calculated. The method applies to regular liquids, even those with relatively large dipoles
but not to H-bonded liquids. The accuracy of estimated values of S°(l,298) are 0.45 ± 0.16 cal/(mol K) with
a maximum deviation of 1.0 cal/(mol K) for an assortment of 14 selected compounds and 0.3 ± 0.12 for
another 17 liquids for which groups are not available but Δvap
H°(298) and Δvap
S°(298) are. Here the largest
deviation is 1.9 cal/(mol K). Calculated values of C
p°(l,298) are much less accurate, ±3 cal/(mol K) with a
maximum deviation of 9.0 cal/(mol K). It is also shown that the best average value of C
p° to use in calculating
changes in ΔH° and ΔS° in a specified temperature interval, T
1 to T
2, is the arithmetic mean of the initial and
final values, [C
p°(T
1) + C
p°(T
2)]/2. Changes are recommended in some of the group values for calculating
Δvap
H°(298) and also in the group O−(C)2 for calculating gas-phase entropies of ethers and for N−(C)2(H)
for calculating entropies of secondary amines.