Chemical Education Today
Letters Boiling Point versus Mass In the article “Boiling Points of the Family of Small Molecules, CHwFxClyBrz: How Are They Related to Molecular Mass?”, Laing gave a useful examination of the boiling points of small molecules versus molecular mass (1). However, a molecule escaping from a liquid is not closely analogous to “a satellite breaking free from the earth’s gravitational field” with the requirement of “a minimum escape velocity”, such that the required kinetic energy is proportional to the mass of the satellite at that escape velocity. The difference is that all of the mass of the satellite is acted upon by the restraining force (or curved space–time) of gravity, while the mass of the boiling molecule is practically irrelevant to the crucial van der Waals, London, or mutual-polarization forces in the liquid. Thus germanium tetrachloride, hexafluorobenzene, pentacarbonylruthenium, and tetrapropyltin do not boil at appreciably higher temperatures than carbon tetrachloride, benzene, pentacarbonyliron, or tetrapropylmethane, respectively, where, in most cases, the additional potentially polarizing and polarizable electrons of the heavier molecules are buried in the center. Much more information is available elsewhere (2) and in references therein. The boiling points of a great variety of substances are predicted there. Note that highly polar molecules are explicitly excluded, and that transitional-element compounds, for example, fit the published correlation quite well, excepting only WCl6.
2. It is the polarizability of the electron clouds of the surfaces of the outermost atoms that dominates the attractive London forces.
The following series of “simple” tetrahedral tetrachlorides shows that boiling point is not directly related to molecular mass nor is it related in a simple way to the sum of polarizabilities of the outermost atoms, which is identical throughout this series. Boiling Point /°C
Compound CCl4
76.5
SiCl4
57.5
GeCl4
84
SnCl4
114
TiCl4
136
Similarly, the following series of methane derivatives poses worse problems. Compound
Boiling Point /°C
CH4
–164
µ/D 0
CH3F
–78
1.85
CH2F2
–52
1.97
CHF3
–82
1.65
CF4
–129
0
Literature Cited 1. Laing, M. J. Chem. Educ. 2001, 78, 1544. 2. Rich, R. L. J. Chem. Educ. 1995, 72, 9–12; on using polarizabilty to predict boiling points. Ronald Rich 112 S. Spring Street Bluffton, OH 45817
[email protected] Literature Cited
The author replies: I am very pleased that Ronald Rich has written making these comments, because he is pre-eminent in this field, beginning with his early book, Periodic Correlations (1). He rightly emphasizes two important points: 1. The attractive forces within a liquid are not gravitational in nature.
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Journal of Chemical Education
The molar refraction decreases regularly from CH4 to CF4, clearly not giving a good measure of the boiling point. It appears that now the dipole moment is somehow involved. If these two series of simple tetrahedral molecules pose problems, it must be evident that the situation is indeed complex. For me, this is a case of: “There are more things in heaven and earth, Horatio, than are dreamt of in your philosophy.”
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1. Rich, R. Periodic Correlations; Benjamin: New York, 1965. Michael Laing School of Pure and Applied Chemistry University of Natal Durban 4041, South Africa
[email protected] Vol. 81 No. 5 May 2004
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