A Summary of Different Methods Used to Measure Vaporization Enthalpies


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A Summary of Different Methods Used to Measure
Vaporization Enthalpies
BG Bourdon gauge C calorimetric
determination GC gas chromatography GCC gas
chromatography-calorimetry CGC correlation
gas chromatography DM diaphram
manometer DSC differential scanning
calorimeter EB ebullometry GS gas
saturation transpiration HG Heise gauge
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A Summary of Different Methods Used to Measure
Vaporization Enthalpies (continued)
HSA head space analysis I
isoteniscope IPM inclined piston
manometry ME Mass effusion-Knudsen
effusion MG McLeod gauge MM
mercury manometer OM oil manometer RG
Rodebush gauge SG spoon gauge STG strain
gauge T tensimeter TE torsion
effusion UV ultraviolet absorption
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TB thermobalance TGA thermal gravimetric
analysis TPTD temperature programmed thermal
desorption particle beam mass
spectrometry TRM thermoradiometric method TSGC
temperature scanning gas chromatography UV u
ltraviolet absorption HSA V viscosity
gauge VG MKS Baratron Vacuum Gauge
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Measurement of Vaporization Enthalpies
1. Measurement of vapor pressure as a function of
temperature - using a manometer 2. Knudsen
effusion P m(2RT/mw)1/2/ t AKc Kc 8r/(3l
8r) where P pressure m mass loss from
cell t period of time A area of
opening mw molecular weight T temperature
(K) r radius of opening l thickness of
opening
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3. Calvet calorimeter 4. Transpiration 5. Head
space analysis 6. Correlation gas chromatography
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Correlation gas chromatography
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What is ta
ta is the adjusted retention time ti - tnrr ti
retention time of ith component tnrr retention
time of a non retained reference What does ta
measure
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For a pure component a plot of ln (vapor
pressure) vs 1/T over a narrow temperature range
results in a straight line. The slope of the line
is equal to - glHm(Tm) the enthalpy of
vaporization. A plot of ln (1/ ta) vs 1/T over a
narrow temperature range results in a straight
line. What does the slope measure
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Enthalpy of Transfer Determination for Tetradecane
ln(1/ta) -gslnHm(Tm)/R intercept gslnHm(Tm)
8.314 J mol-1 53.158 kJ mol-1
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What is slngHm(Tm) What does it measure
Solute on stationary phase of column gas
phase Thermochemical cycle Vapor pure
liquid solution on the capillary
column slngHm(Tm) lgHm(Tm) slnHm(Tm)
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Characteristics of capillary gas chromatographs
with FID detectors
Typical sample sizes microgram
quantities solids or liquids are in solution or
adsorbed concentrations are low and too
dispersed for crystallization temperatures are
also high for crystals to form
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Equations for the temperature dependence of
ln(1/ta) for C14 to C20
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lgHm (298.15 K) (1.4360.019) slngHm(Tm)
(4.540.35) r2 0.9991
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Why does lgHm (298.15 K) correlate with
slngHm(Tm) in a linear fashion
gslnHm(Tm) glHm(Tm) slnHm(Tm) We know
that glHm(298.15 K) 4.69 (nC -nQ)
3.0 However T 298.15 K is an arbitrary
temperature glHm(Tm) AT (nC) BT where A is
some constant and B is a variable but
small in magnitude Lets assume for the moment
that slnHm(Tm) Asln(nC) Bsln where B is a
variable but small in magnitude The slope of the
line from the correlation is given by slope
lgHm (298.15 K) / slngHm(Tm)
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slope lgHm (298.15 K)/slngHm(Tm) slope
A298 (nC) B298/AT (nC) BT Asln(nC)
Bsln slope A298 (nC) B298/(AT Asln)(nC)
(BT Bsln) let A (AT Asln) B (BT
Bsln) slope /A298 (nC) B298/(A)(nC)
(B) if (A)(nC) B and A298 B298 then
slope (A298)/(A) constant
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Ruzicka K. Majer V. Simultaneous treatment of
vapor pressures and related thermal data between
the triple point and normal boiling temperatures
for n-alkanes C5-C20 J. Phys. Chem. Ref. Data
1994 23 1-39.
Table 4. Parameters of the Cox Equation.
Tb Ao 103A1 106A2 tetradecane 526.69
1 3.13624 -2.063853 1.54151 pentadecane 543.797
3.16774 -2.062348 1.48726 hexadecane 559.978
3.18271 -2.002545 1.38448 heptadecane 575.375
3.21826 -2.04 1.38 octadecane 590.023
3.24741 -2.048039 1.36245 nonadecane 603.989
3.27626 -2.06 1.35 eicosane 617.415
3.31181 -1.02218 1.34878 Cox Equation ln
(p/po) (1-Tb/T)exp(Ao A1T A2T 2)
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Hv(298) Hv(lit) (449) Hslnv Hsln tetra
decane 71.7 56.909 53.2 -3.709 pentade
cane 76.8 60.701 56.4 -4.301 hexadecan
e 81.4 64.485 60.3
-4.185 heptadecane 86.5 68.171 63.3
-4.871 octadecane 91.4 72.092 66.6
-5.492 nonadecane 96.4 75.998 70.3
-5.698 eicosane 101.8 79.793
74.2 -5.593
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lgHm(T)/ kJ mol-1 3.816nC3.43 slnHm(T)/ kJ
mol-1 0.34816nC1.08
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lgHm(449 K) / kJ mol-1 3.82nC3.43 slnHm (449
K) / kJ mol-1 -0.35nC1.08 lgHm(298.15 K) / kJ
mol-1 4.98nC1.88 lgHm(298.15 K)/slngHm(449 K)
(4.98nC1.88)/(3.82nC3.43- 0.35nC1.08) lgHm(
298.15 K)/slngHm(449 K) 4.98/(3.47)
1.435 lgHm (298.15 K) / slngHm(Tm)
(1.4360.019)