Adventures in Thermochemistry
James S. Chickos*
Department of Chemistry and Biochemistry
University of Missouri-St. Louis
Louis MO 63121
9
McDonnell Planetarium
Applications of the Correlation-Gas Chromatographic Method
Objectives: To go where no one else has gone
1. Evaluation of the vaporization enthalpies of large molecules
2. Application of Correlation-Gas Chromatography to a Tautomeric
Mixture –Acetylacetone
3. Evaluation of the Vaporization Enthalpy of Complex
Hydrocarbon Mixtures
VAPORIZATION ENTHALPIES OF
COMPLEX MIXTURES
The use of gas-chromatography to measure the
vaporization enthalpy of complex hydrocarbon
mixtures
Vaporization Enthalpies of High Energy Density
Fuels for Aerospace Propulsion RP-1, JP-7, JP-8
Why is it important to know the ∆lgHm(298.15 K) of complex
mixtures as found in aviation fuels?
The most obvious role for aviation fuel in advanced aircraft is for
propulsion.
A second and increasingly important role is as an airframe coolant
in supersonic aircraft.
Recently there has been an interest in finding endothermic fuels
which initially undergoes an endothermic reaction to form
secondary products that are subsequently used for propulsion.
• RP-1 (Rocket Propellant 1)
Refined petroleum, a mixture of complex hydrocarbons
A GC plot of RP-1 without standards
7000000
6000000
Area
5000000
4000000
3000000
2000000
1000000
0
1
10 19 28
37 46
55 64 73
82 91 100 109 118
Com pound Num ber
Compound number distribution for RP-1 without standards
• Physical properties of RP-1
•
•
•
•
•
•
•
•
•
•
•
•
•
•
Approx. formula
Boiling range (F)
Freezing point (F)
Flash point (F)
Net heating value (btu/lb)
Specific gravity (70F)
Critical T (F)
Critical P (psia)
Preliminary composition
n-paraffins (wt%)
i-paraffins
naphthenes
aromatics
C12H23.4
350-525
-56
155
18,650
0.806
770
315
2.1
27.1
62.4
8.4
Application of the GC method to a complex mixture
For a mixture of i structurally related components, the following relationship applies:
ln(to/t1) = ln(A1)- slngHm(Tm)1 /RT
ln(to/t2) = ln(A2)- slngHm (Tm)2 /RT
…
ln(to/ti) = ln(Ai)- slngHm(Tm)i /RT
Multiplying each component by its mole fraction, ni and summing over all i components
result in the following equation:
∑ni ln(to/ti) = ∑ni ln(Ai)- ∑ni slngHm(Tm)i/RT



A plot of ∑ ni ln(to/ti) versus 1/T should result in a straight line with a slope of

- slngHm(Tm)mix.
When several structurally related standards are included in the mixture, a plot of
ln(to/ti) versus 1/T for each standard should also result in a linear plot. The
slngHm(Tm) term for each standard can be correlated to its respective vaporization
enthalpy. From the correlation equation and slngHm(Tm)mix of the mixture, the
vaporization enthalpy of the ensemble, lgHm(Tm)mix, can be determined. This
assumes that the enthalpy of mixing is small
A GC Plot of RP-1 with 6 Standards
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5
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3
Area
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1 2
2
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3
6
4
4
5
6
2000000
1000000
0
1
9
17 25
33 41 49 57 65 73 81
Compound Number
89 97 105 113 121
RP-1 with standards: 1. n-octane 2. nonene 3. n-decane 4.
naphthalene 5. n-dodecane 6. n-tridecane
2
1
Ln(1/tc)
0
-1
-2
-3
-4
-5
0.00255
0.00260
0.00265
0.00270
0.00275
0.00280
0.00285
1/T, K-1
A plot of natural logarithm of the reciprocal adjusted retention times for
(top to bottom): ,n- octane;  , nonene; , n-decane;  , naphthalene;
 , n-dodecane; , n-tridecane.
Equations resulting from a linear regression
of ln(to/ta) versus (1/T)K-1
ln(to/ta)= - slngHm/RT + ln(Ai)
Compound
n-octane
1-nonene
n-decane
naphthalene
n-dodecane
n-tridecane
ln(to/ta)= (-3887.5/T) + (11.064 ± 0.008)
ln(to/ta)= (-4222.9/T) + (11.159 ± 0.010)
ln(to/ta)= (-4687.9/T) + (11.655 ± 0.010)
ln(to/ta)= (-4965.5/T) + (11.176 ± 0.008)
ln(to/ta)= (-5566.1/T) + (12.685 ± 0.010)
ln(to/ta)= (-6018.6/T) + (13.232 ± 0.010)
r2=0.9995
r2=0.9993
r2=0.9994
r2=0.9997
r2=0.9996
r2=0.9997
slngHm(Tm) = lgHm(Tm) + slnHm(Tm)
to = 1 min Tm = 368 K
A demonstration of the application of the method for a 1:1
molar mixture of n-Octane and n-Tridecane
Vaporization enthalpy of n-Octane
= 41560J/mol
Vaporization enthalpy of n-Tridecane = 67062J/mol
Vaporization enthalpy of 1:1 Mixture = 54120J/mol
(assume ideal mixing)
[0.5×41560+0.5×67062]
For a 1:1 mixture of n-Octane and n Tridecane
∑niln(to/ti)= ∑niln(Ai)- ∑nislngHm(Tm)i/RT
T/K
354.0
358.9
363.9
369.0
374.1
379.2
384.2
(1/T) K-1
0.002825
0.002786
0.002748
0.002710
0.002673
0.002637
0.002603
ln(to/ta)
n-octane
n-decane
0.0761
0.2278
0.3756
0.5234
0.6673
0.8073
0.9396
-3.7705
-3.5357
-3.3070
-3.0783
-2.8556
-2.6390
-2.4343
niln(1/ti) (ni = 0.5)
n-octane/n- tridecane
-1.847
-1.654
-1.466
-1.277
-1.094
-0.9158
-0.7474
∑niln(to/ti)= 12.1498 ± 0.003 – 4954/T (1:1 octane: tridecane)
A plot of lgHm(298.15 K) vs slngHm(368 K) for the remaining standards
vaporization enthalpy(literature); J mol-1
64000
62000
dodecane
60000
58000
56000
naphthalene
54000
52000
decane
50000
48000
46000
44000
34000
nonene
36000
38000
40000
42000
44000
46000
48000
enthalpies of transfer from solution to the vapor; J mol-1
lgHm(298.15 K) = (1.444  0.092)slngHm(368 K) – (4818  746); r2 = 0.9919
Vaporization enthalpies calculated for
the standards and for 1:1 mixture of
n-Octane/n-Tridecanea
slngHm(368 K)
lgHm(298.15 K)
lit
lgHm(298.15 K)
Calcd [eq (2)]
nonene
35.108
45.50
45.9±5.0
decane
38.973
51.42
51.5±5.2
naphthalene
41.281
55.65
54.8±5.3
dodecane
46.274
61.52
62.0±5.7
1:1 mixture
of n-octane/
n-tridecane
41.188
54.6±5.3
54.1b
lgHm(298.15 K) = (1.444  0.092)slngHm(368 K) – (4.82 3.7); r2 = 0.9919 (2)
aenthalpies
in kJ /mol
bcalculated for a 1:1 mixture of n-octane/n-tridecane
Approximation of the Mol Fraction
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8C
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Area
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13C
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0
1
9
17 25
33 41 49 57 65 73 81
Compound Number
89 97 105 113 121
FID detector response is proportional to the number of carbon atoms
DETECTOR BIAS
The area of each peak was
adjusted for carbon number
based on its retention time.
14
13
N, the number of carbon atoms
The observed correlation
between the number of carbon
atoms present in the standards
and the natural logarithm of
their adjusted retention time at
T = 364 K. The point that falls
off the line is naphthalene, all
others are n-alkanes/alkenes.
12
11
10
9
8
mol fraction =
area(i)/[Nc(i)/Σiareai/Nc(i)
where
Nc = -1.218.ln(1/ta) + 8.39
7
-4
-3
-2
-1
ln (1/ta )
0
1
The slopes, intercepts, enthalpies of transfer, and enthalpies of vaporization
of the standards and those calculated for RP-1; enthalpies in kJ.mol-1
Slope
Intercept
slngHm(368 K)
lgHm(298.15 K) lgHm(298.15K)
lit
calcd
41.56
41.8
octane
-383878
10.880.01
31.91
nonene
-416284
11.050.01
34.60
45.5
45.8
decane
-461584
11.490.01
38.37
51.42
51.3
naphthalene -488448
10.960.01
40.60
55.65
54.8
dodecane
-546458
12.410.01
45.42
61.52
61.7
tridecane
-589743
12.910.01
49.03
66.68
67.0
RP-1
-4640100
10.580.03
38.57
51.61.2
RP-1a
-462694
11.580.03
38.45
51.51.2
lgHm(298.15 K)/kJmol-1 = (1.4720.041) slngHm(368 K) –(5.1450.59); r2
=0.9970
a
adjusted for detector bias
STANDARDS CHOSEN FOR JP-7
Samples of JP-7 and JP-8
already contain substantial
amounts of n-alkanes as
identified by GCMS and
retention time studies.
0.10
0.08
n-Undecane, n-dodecane, ntridecane, and n-tetradecane
were identified and used as
internal standards for JP-7
Mol Fraction
0.06
C11 C12 C13
C14
0.04
0.02
0.00
0
5
10
15
20
25
30
Adjusted retention time (min)
35
40
STANDARDS CHOSEN FOR JP-8
0.06
C11 C12
C15
0.05
C13
C14
0.04
Mol Fraction
n-decane through to
n-pentadecane were
similarly identified and
used as standards in
JP-8. Similar in
composition to Jet A used
in commercial aviation
0.03
0.02
0.01
0.00
0
5
10
15
20
25
30
Adjusted retention time (min)
35
40
A comparison of vaporization enthalpies of RP-1, JP-7, and JP-8 with literature values
lgHm(298.15 K)
kJ.mol-1
lgHm(298.15 K)
kJ.kg-1
calcd
lgHm(298.15 K)
kJ.kg-1
(lit)
51.5
C12H23.4
167.4
308
291, 246b
55.9
C12H25
169
331
330c
65.4
C11H21
153
428
RP-1
JP-7
JP-8
Approximate
Formula
Massa
g .mol-1
reference Edwards, T. “Kerosene Fuels for Aerospace Propulsion-Composition and Properties”
b reference CPIA Liquid Propellant Manual
c reference “Aviation Fuel Properties” CRC Report No 530, Society of Automotive Engineers, Inc.
a
The vaporization enthalpy of JP-10, A High Energy Density Rocket Fuel
∆glHm (298.15 K)
exo-THDCPD
endo-THDCPD
49.1 ± 2.3
50.2 ± 2.3
The enthalpies of vaporization and
sublimation of exo- and endotetrahydrodicyclopentadienes
at T = 298:15K
Chickos,J. S.; Hillesheim, D.; Nichols,
G. J. Chem. Thermodyn. 2002, 34,
1647–1658.
RJ-4 A High Energy Density
Rocket Fuel
Standards Used
decane
exo-tetrahydrodicyclopentadiene
endo-tetrahydrodicyclopentadiene
n-tetradecane
lgHm(298.15 K) = 55.3 ± 1.0 kJ/mol
Chickos, J.S. Wentz, A. E.;
Hillesheim-Cox, D. Zehe, M. J. Ind.
Eng. Chem. 2003, 42, 2874-7
Acknowledgments
Tim Edwards, Wright Patterson Air Force Base
W. Hanshaw, P. Umnahanant, and D. Hillesheim-Cox
Solutia STARS program support for A. E. Wentz.
Fundacāo para a Ciệncis e a Tecnologia (Portugal) support for D. Hillesheim-Cox
NASA