Edward M. ArneH1 and Dumifru O a n c e a z University of Pittsburgh and Mellon Institute Pennsylvania, 15213
1I
I
A Simple, Isothermal Calorimeter for Determining Accurate Heats of Vaporhation
We wish to report here a simple inexpensive system which may he used to determine heats of vaporization with high accuracy and precision. Ancillary to its development we have had to devise a means for operating a conventional adiabatic (more correctly isoperihol) calorimeter in the isothermal mode. We helieve that the apparatus and techniques are so simple and inexpensive that they are readily applicable to experiments in the undergraduate laboratory. This may be used to demonstrate not only calorimetry and vaporization thermochemistry but the use of the null point method in a flow system a t the steady state. Thermodynamic properties for liquid to vapor transitions are of fundamental importance in many areas of theoretical and applied chemistry. Recently, we have drawn attention to the current significance of heats (and free energies) of vaporization to the classical problem of relating ionization processes in the gas phase to those in solution ( I ) . Thanks to several new mass spectrometric methods it is now poss~hleto completely analyze an exchange process such as
RNHl
(gas)
+
NH,+
2
RNH,+
+
NH,
The solvation energy of the ion RNH3+ relative to NH4+ can be obtained exactly from ion cyclotron resonance (ICR) data and heats of ionization in solution if the heats of solution AHs(B) of the neutrals RNHz and NH3 are known. As the range of the ICR data expands we have found that such an analysis in terms of ionic solvation energies is often limited hv a dearth of reliable heat of vaporization data which c o h d be combined with heats of solution of licluid solutes to afford AHsiB) values. Heats of vaporization can be measured either by direct calorimetric methods, or by indirect vapor pressure method (2). In the calorimetric methods, the amount of electrical energy needed to vaporize a measured quantity of liquid isothermally is determined. Among the calorimetric methods, those involving vaporization into a stream of carrier gas give good results, without using complicated equipment. The carrier gas method has been used to measure the heat of vaporization of water 13) and thermodynamic properties of aqueous salt solutions (4). Recently, Wads6 (5) improved the method and used it to obtain the heats of vaporization for many organic liquids. The liquid to he vaporized was weighed before and after vaporization. Evaporation occurred into a nitrogen gas stream at, reduced pressure, the conditions being appropriate to liquids having vapor pressures between 0.5 and 200 torr. The simple system described here was assembled from readily available commercial equipment with simple, in-
'Communications should he directed to this author. k c h a n e e visitor. Deoartment of Phvsical Chemistrv. ., Universit! of Bucharest, domania. We are &efd to the N. S. F. for Fellowshipsupport of D. 0 .
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Table 1. Comparison of Heats of Vaporization (AH., kcal/mole) at 25°C by this Method with Literature Values -Compound
Liquid Flow Rate (mole/ mi") X lor
(AH")
P r e m t Study
+ 0.05
CHIOH
0.2078
CHaOH
0.1441
10.11 1 0 . 0 2
CHa(CHd8-OH
0.0919
12.27 f 0.02
Pyridine
0.1044
9.61 f 0.03
8.91
(AH
