Adventures in Thermochemistry
James S. Chickos*
Department of Chemistry and Biochemistry
University of Missouri-St. Louis
Louis MO 63121
E-mail: [email protected]
A portion of the Science Complex UMSL
Adventures in Thermochemistry
Research Interests
Phase Transitions and Their PropertiesFusion Enthalpies Vaporization Enthalpies Sublimation Enthalpies
Estimating vaporization enthalpiesEstimating fusion enthalpiesEstimating melting temperaturesEstimating boiling temperaturesEvaluating vapor pressures (liquid and solid)Estimating heat capacities (solid and liquid)
Hypothetical Thermodynamic Properties
Phase change properties have been measured and studied for over 200 years. Many methods have been developed for measuring and estimated these properties. One of the major interests in my research group was focused on developing methods both experimental and computational that would allow studies of the properties of materials that can not measured directly because of their characteristics. For example, they decompose at the temperatures needed for measurement, they exist naturally as mixtures, or the property is experimentally inaccessible.
Hypothetical thermodynamic properties: useful in constructing thermochemical cycles
Vapor pressures of many solid polutants are relatively non-volatile and present in very low concentrations on particulate matter. The vapor pressures of these materials can be modeled by the vapor pressure of the sub-cooled liquid.
Estimation of Thermodynamic Properties
A measurement always results in a number. Estimations though only approximate offer a rational way of deciding on whether the result is reasonable or not. This is particularly useful to those of us that are training students.
Estimation of Fusion Enthalpies
The direct estimation of fusion enthalpies is problematic for the following :
The DSC heating curve of CCl4:
T/°C
-80 -60 -40 -20 0
Hea
t Flo
w E
ndo
UP
mW
20
25
30
35
40
45
∆Hfus(251 K)
2.52 kJmol-1
∆Htrns(226 K)
4.58 kJmol-1
Fusion enthalpy does not seem amenable to a group additivity approach; neither does fusion entropy
Waldon’s Rule: ∆Hfus(Tfus)/Tfus ≈ 54.4 Jmol-1K-1
∆Hfus(Tfus) Tfus ∆Sfus(Tfus) ∆Stpce
dodecane 36820 263.6 139.7 139.7 tridecane 28490 267.8 106.3 7660 255 30.0 136.3 tetradecane 45070 279 161.5 165.5 pentadecane 34600 283.1 122.2
9170 270.9 33.9 156.1
Consider however the total phase change entropy from T = (0 to Tfus) K.
Is total phase change entropy (∆Stpce) treatable by group additivity?
Our Philosophy Regarding Estimation Methods
There seem to be two philosophies regarding estimation methods including group additivity
1. Devise a simple and consequently approximate method
2. Devise a method that is as precise as possible
Our experience has been that the simplest method wins out. It is used the most and misused the least. Methods that have numerous parameters are often improperly used and sometimes these parameters are simply parameterizing experimental error.
Alkanes ∆Stpce = 1.31*7.1*nCH2 + 17.6*nCH3
∆Hfus(Tfus) Tfus/K ∆Stpce (exp) ∆Stpce (est) ∆Htpce(Tfus)est
Jmol-1 Jmol-1K-1 Jmol-
1
dodecane 36820 263.6 139.7 128 33740 tridecane 28490 267.8 136.4 138 36960 tetradecane 45070 279 161.5 147 41000 pentadecane 34600 283.1 158.0 156 44160
tridecane ∆Stpce (exp) = 28490/267.8 + 7660/255 = 136.4 Jmol-
1K-1
∆Htpce (exp) = 28490 + 7660 = 36150 Jmol-1
pentadecane ∆Stpce (exp) = 34600/283.1 + 9170/270.9 = 158.0 Jmol-1K-
1
∆Htpce(exp) = 34600 + 9170 = 43770 Jmol-1
Some Simple Estimations
Aromatics ∆Stpce = [7.4]·n=CH- + [- 7.5] ·n=CR- + [-9.6]·n=CR’- + [17.6]·nCH3
R = sp2 atom; R’ =sp3 atom
1-methylnaphthalene [7.4]·7+[-7.5]·2 + [-9.6] +[17.6] = 44.9 (49.3)expt
2-methylnaphthalene = 44.9 (58.9)expt
1-methylnaphthalene ∆Htpce(Tfus)exp = 11930; ∆Htpce(Tfus)calc = 10900 2-methylnaphthalene ∆Htpce(Tfus)exp = 17740; ∆Htpce(Tfus)calc = 13800
Some Simple Estimations / Jmol-1
∆Hfus(Tfus) = 11.0 kJmol-1 exp∆Htpce(Tfus) = 13.6 calc
Tt = 116, 363, 262 K∆Ht = 1.4, 7.0, 11.0∆Htpce = 12.36∆Stpce = 37.8
∆Stpce = [33.4] + [3.7]·2 –[12.3]·3 – [1.6]·2
+ [7.4]·6 – [7.5] = 37.8
Exp∆Sfus(Tfus) = 84.7; 100.9∆Hfus(484.2; 485.8 K) = 41000; 49000two polymorphs
Calc∆Sfus(Tfus) = 80.5; ∆Hfus(484.2; 485.8 K) = 38980; 39100
C14H12ClNO2
Cl NH
CO2HCH3
Tolenamic Acid
7(=CH-)a + 4(=CR'-)a + 1(=CR-)a + 1(CH 3-) + 1(Cl-) + 1(-CO2H) + 1(-NH-)
7(7.4) + 4(-7.5) + 1(-9.6) + 1(17.6)
+ (10.8)(1.) + 1(13.4)(2.25) + 1(-5.3)
Values in [ ] are tentative assignments
Calc∆Sfus(Tfus) = 85.6∆Hfus(499.2 K) = 42.7
Exp∆Sfus(Tfus) = 71.9∆Hfus(499.2 K) = 35.9
OOH
OCH3
OH
HO
O
C16H14O6 Hespiritin
[33.4] + {3.7][n-3] + [O]c + [C=O]c + 2[=C-R]c
+ 5(=CH-)a + 4(=C-R')a+ 3(HO-) + (-O-) + (CH3-)
+
[33.4] + 3.7[3] + (1.2) + (-1.4) + 2*(-34.6) + (-1.6)(1.92)
+ 5(7.4) + 4(-7.5) + 3(1.7)(13.1) + (4.71) + (17.5)
[-C(H)(C)(O)]c
[33.4] +3.7[3] + [1.2] +[-1.4] + [-14.7] + 2[-12.3] + [17.6] + 5[7.4] + [-9.6] + 4[-7.5] +3[20.3] +[4.7]
A series of compounds
forming liquid crystals
∆Stpce = Σ ∆Hi/Ti
On of the questions left to be answered is why do liquid crystals behave in this way?
The total phase change entropy is not very useful unless the fusion temperature is available. Our next adventure into trying to predict melting temperatures resulted in some surprises.
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