Chemical Education Today
Letters Does Molecular Mass Greatly Affect Boiling and Melting Points? In “A Thermodynamic Analysis to Explain the BoilingPoint Isotope Effect for Molecular Hydrogen” (1) the title is just right, but we need to point out clearly, and perhaps often, that the mass effect, which is mentioned repeatedly, and properly, throughout the article, is very small in the rest of chemistry. Too many otherwise well-prepared chemists still teach and write about a supposed general dependence of boiling point on molecular “weight” or mass, and some readers may take this article as supporting that. For the related variable of melting point, for example, the molecular mass as such is cited currently (2) and repeatedly as a relevant independent variable. This is misleading for melting points too, although the importance of symmetry is well elucidated in that article. Even for molecular hydrogen, the difference in boiling points between 20.4 K for diprotium and 23.5 K for dideuterium, although important at these low temperatures, is perhaps not striking for a mass ratio of 1 to 2. Let’s give students the useful and interesting information in both of these articles, together with a perspective (3) that incidentally exposes the uniqueness of molecular hydrogen with regard to the importance of mass. Literature Cited 1. Baker, D. B.; Christmas, B. K. J. Chem. Educ. 2000, 77, 732– 734. 2. Brown, R. J. C.; Brown, R. F. C. J. Chem. Educ. 2000, 77, 724–731. 3. Rich, R. L. J. Chem. Educ. 1995, 72, 9–12, on using polarizability to predict boiling points. Ronald L. Rich 112 S. Spring Street Bluffton, OH 45817-1112
[email protected] D. Blane Baker replies: Ronald L. Rich’s letter clearly alludes to two common concerns for those of us who teach. The first is that general dependences in science can be assumed mistakenly from highly specialized cases. A second is that these misleading generalizations often appear in the classroom. In light of these concerns, I would agree with Rich’s proposal for teaching the topics in question. As an historical note, the tendency to over-generalize is addressed by early Enlightenment thinkers such as Francis Bacon who warned of the idols of the mind. Among these include the idols of the tribe, which assume more order than actually exists in nature. As Rich suggests, a presumed dependence of boiling point upon molecular mass implies a general dependence that is not observed. With regard to the specifics of the dependence of boiling point upon mass in molecular hydrogen, our article points out that several other explanations for the effect were conwww.JCE.DivCHED.org
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sidered. However, none of these are able to explain the observed results. The actual working model treats each hydrogen molecule as residing within a harmonic potential energy well where more massive isotopes lie deeper within the well. As a result, greater energies (or higher temperatures) are required to initiate boiling as the molecular mass increases. Such a model, indeed, explains the observed boiling point dependence upon mass in molecular hydrogen, but, as Rich points out, the effect is rather small and is not universal throughout chemistry. D. Blane Baker Department of Physics William Jewell College Liberty, MO 64068
[email protected] R. J. C. Brown replies: I have read the letter by Ronald L. Rich with interest. There are some interesting points to be made, which I will list in point form. 1. Reference 1 cited by Rich is about the boiling point of molecular hydrogen, which has the smallest mass and smallest moment of inertia of any molecule. It is such a special case that it is no surprise that the boiling point changes considerably upon isotopic substitution. 2. It is relevant that while increasing mass by isotopic substitution increases the boiling point of hydrogen, deuteration decreases the boiling points of many hydrocarbon liquids such as cyclohexane. (I am indebted to a colleague for pointing this out to me.) The model given in ref 1 should apply to these liquids as well as to hydrogen, but predicts the wrong mass dependence of the boiling point. It would be useful to collect some data to demonstrate this, for it would seem to undermine the arguments of ref 1 as being too simplified to be applied generally to a range of liquids. 3. Reference 2 cited by Rich makes only slight use of the mass dependence (e.g., Figure 10) within a homologous series, where boiling points generally do increase with molecular mass. However this paper is primarily about symmetry in relation to melting points, rather than boiling points. 4. Reference 3 cited by Rich is the author’s own impressive contribution to the prediction of boiling points, and I wonder whether it could be used to understand the mass dependence of the boiling point of hydrogen, and the opposite mass dependence of the boiling point of hydrocarbons.
Rich’s letter raises some interesting questions, but it does not deal with them adequately in its present form. There is a good case to be made for raising these issues in this Journal in a somewhat expanded paper. R. J. C. Brown Department of Chemistry Queen’s University Kingston, Ontario K7L 3N6, Canada
[email protected] Vol. 81 No. 5 May 2004
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Journal of Chemical Education
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