Benson

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Bensons Group additivity of Cp is available for
gases what about for solid and liquid?
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references A Group Additivity Approach for the
Estimation of Heat Capacities of Organic Liquids
and Solids at 298 K, Structural Chemistry 1993,
4, 261-269. Heat Capacity Corrections to a
Standard State A Comparison of Some New and
Literature Methods for Organic Liquids and
Solids, Structural Chemistry 1993, 4, 261-269.
available on the web at www.umsl.edu/jscumsl/.
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Application of Group Additivity to Estimate Cp(l)
and Cp(c) Protocol 1. Group values are defined
in terms of their hybridization and substitution
pattern. 2. Primary, secondary, tertiary and
quaternary carbons are defined in terms of the
number of hydrogens attached, 3, 2, 1, 0. 3.
Aromatic, aliphatic and cyclic carbons are
treated separately. 4. Functional groups are
treated separately.
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Calculate the heat capacity of 2.2 spiropentane
at T 298.15 K. Cp quat sp3 C 4 sec sp3 Cp(l)
18 4(25.9) 121.6 J mol-1K-1 Cp(c)
6.1 4(24.6) 104.5 J mol-1K-1 Experimental
value Cp(l) 134.5 J mol-1K-1
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When are Cp(l) and Cp(c) values useful?
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Suppose you measured ?Hv(325 K) of spiropentane
and you wanted to correct the number to T
298.15 K. How would you do it?
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What do you do in the case where the group value
or correction is not available? 1. Find data to
identify a group value. 2. Look for another
approach.
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At T 298.15 K Cp(l) - Cp(g)expt 10.26
0.26 Cp(l)calc(298)
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At T 298.15 K Cp(c) - Cp(g)expt 0.71 0.15
Cp(c)calc(298)
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?Hfus(298) Cp(c) Tfus-298 ?Hfus(Tfus)
Cp(l)298-T ?Hfus(298) ?Hfus(Tfus) Cp(c) -
Cp(l)Tfus-298
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Cp(l) - Cp(g)expt 10.26 0.26
Cp(l)calc (1) Cp(c) - Cp(g)expt 0.71 0.15
Cp(c)calc (2) Eq (2) - Eq (1) Cp(c) - Cp(l)
0.15 Cp(c) - 0.26 Cp(l) - 9.83 (3) ?Hfus(298)
?Hfus(Tfus) Cp(c) - Cp(l)Tfus-298 ?Hfus(2
98)?Hfus(Tfus)0.15Cp(c) - 0.26Cp(l)-9.83Tfus-
298
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How accurate are eq. (1), (2), and (3)? 95
confidence level 2 standard deviations For eq.
(1) ?30 J mol-1 K-1 For eq. (2) ? 1/3 the
correction
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Suppose you measured ?Hv(325 K) of spiropentane
and you wanted to correct the number to T
298.15 K. How would you do it?
?Hv(298 K) ?Hv(325 K) 10.58
.26Cp(l)325-298 Cp(l) 14 4(31.9)
141.6 J mol-1K-1 ?Hv(298 K) ?Hv(325 K) 4100
J mol-1