1、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 saturatio
2、n, transpiration HG Heise gauge,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,A
3、Summary of Different Methods Used to Measure Vaporization Enthalpies (continued),TB thermobalance TGA thermal gravimetric analysis TPTD temperature programmed thermal desorption particle beam mass spectrometry TRM thermoradiometric method TSGC temperature scanning gas chromatography UV ultraviolet a
4、bsorption HSA V viscosity gauge VG MKS Baratron Vacuum Gauge,1. Measurement of vapor pressure as a function of temperature - using a manometer 2. Knudsen effusionP = m(2RT/mw)1/2/ t AKcKc = 8r/(3l +8r) where: P = pressure; m = mass loss from cell;t = period of time; A = area of openingmw = molecular
5、 weight; T = temperature (K)r = radius of opening; l = thickness of opening,Measurement of Vaporization Enthalpies,3. Calvet calorimeter 4. Transpiration 5. Head space analysis 6. Correlation gas chromatography,Correlation gas chromatography,What is ta?,ta is the adjusted retention time ti - tnrr ti
6、 = retention time of ith component tnrr = retention time of a non retained referenceWhat does ta measure?,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.
7、 A plot of ln (1/ ta) vs 1/T over a narrow temperature range results in a straight line. What does the slope measure?,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,What is slngHm(Tm) ? What does it measure?Solute
8、on stationary phase of column gas phase Thermochemical cycle:Vapor pure liquid solution on the capillary columnslngHm(Tm) = lgHm(Tm) + slnHm(Tm),Characteristics of capillary gas chromatographs with FID detectors,Typical sample sizes microgram quantities solids or liquids are in “solution” or adsorbe
9、d; concentrations are low and too dispersed for crystallization temperatures are also high for crystals to form,Equations for the temperature dependence of ln(1/ta) for C14 to C20:,lgHm (298.15 K) = (1.4360.019) slngHm(Tm) (4.540.35); r2 = 0.9991,Why does lgHm (298.15 K) correlate with slngHm(Tm) in
10、 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) + Bsl
11、n 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),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 + A
12、sln); B= (BT + Bsln)slope =/A298 (nC) + B298/(A)(nC) + (B)if = (A)(nC) B and A298 B298then slope = (A298)/(A) = constant,Table 4. Parameters of the Cox Equation.Tb Ao 103A1 106A2 tetradecane 526.691 3.13624 -2.063853 1.54151 pentadecane 543.797 3.16774 -2.062348 1.48726 hexadecane 559.978 3.18271 -2
13、.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 1.34878Cox Equationln (p/po) = (1-Tb/T)exp(Ao +A1T +A2T 2),Ruzicka, K.; Majer, V. “Simultaneous treatment of vapor pressures and
14、 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.,Hv(298) Hv(lit) (449) Hslnv Hsln tetradecane 71.7 56.909 53.2 -3.709 pentadecane 76.8 60.701 56.4 -4.301 hexadecane 81.4 64.485 60.3 -4.185 heptadecane 86.5
15、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,lgHm(T)/ kJ mol-1= 3.816nC+3.43slnHm(T)/ kJ mol-1= 0.34816nC+1.08,lgHm(449 K) / kJ mol-1= 3.82nC+3.43 slnHm (449 K) / kJ mol-1= -0.35nC+1.08 lgHm(298.15 K) / kJ mol-1= 4.98nC+1.88 lgHm(298.15 K)/slngHm(449 K) = (4.98nC+1.88)/(3.82nC+3.43- 0.35nC+1.08)lgHm(298.15 K)/slngHm(449 K)= 4.98/(3.47) = 1.435lgHm (298.15 K) / slngHm(Tm) = (1.4360.019),
