Does Molecular Mass Greatly Affect Boiling and Melting Points? (author response)
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Chemical Education Today www.JCE.DivCHED.org • Vol. 81 No. 5 May 2004 • Journal of Chemical Education 643 Does Molecular Mass Greatly Affect Boiling and Melting Points? In “A Thermodynamic Analysis to Explain the Boiling- Point Isotope Effect for Molecular Hydrogen” (1) the title is just right, but we need to point out clearly, and perhaps of- ten, 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 boil- ing 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 repeat- edly as a relevant independent variable. This is misleading for melting points too, although the importance of symme- try 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 informa- tion in both of these articles, together with a perspective (3) that incidentally exposes the uniqueness of molecular hydro- gen 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 polariz- ability 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 gen- eralizations 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 in- clude the idols of the tribe, which assume more order than actually exists in nature. As Rich suggests, a presumed de- pendence of boiling point upon molecular mass implies a general dependence that is not observed. With regard to the specifics of the dependence of boil- ing point upon mass in molecular hydrogen, our article points out that several other explanations for the effect were con- sidered. However, none of these are able to explain the ob- served results. The actual working model treats each hydro- gen 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 re- quired to initiate boiling as the molecular mass increases. Such a model, indeed, explains the observed boiling point depen- dence upon mass in molecular hydrogen, but, as Rich points out, the effect is rather small and is not universal through- out 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 mo- lecular 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 consid- erably upon isotopic substitution. 2. It is relevant that while increasing mass by isotopic substi- tution 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 point- ing 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 sim- plified 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 won- der whether it could be used to understand the mass de- pendence 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] Letters
Transcript of Does Molecular Mass Greatly Affect Boiling and Melting Points? (author response)
Chemical Education Today
www.JCE.DivCHED.org • Vol. 81 No. 5 May 2004 • Journal of Chemical Education 643
Does Molecular Mass Greatly AffectBoiling and Melting Points?
In “A Thermodynamic Analysis to Explain the Boiling-Point Isotope Effect for Molecular Hydrogen” (1) the title isjust right, but we need to point out clearly, and perhaps of-ten, that the mass effect, which is mentioned repeatedly, andproperly, throughout the article, is very small in the rest ofchemistry. Too many otherwise well-prepared chemists stillteach and write about a supposed general dependence of boil-ing point on molecular “weight” or mass, and some readersmay 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 repeat-edly as a relevant independent variable. This is misleadingfor melting points too, although the importance of symme-try is well elucidated in that article.
Even for molecular hydrogen, the difference in boilingpoints between 20.4 K for diprotium and 23.5 K fordideuterium, 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 informa-tion in both of these articles, together with a perspective (3)that incidentally exposes the uniqueness of molecular hydro-gen with regard to the importance of mass.
Literature Cited1. 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 polariz-
ability to predict boiling points.
Ronald L. Rich112 S. Spring StreetBluffton, OH [email protected]
D. Blane Baker replies:
Ronald L. Rich’s letter clearly alludes to two commonconcerns for those of us who teach. The first is that generaldependences in science can be assumed mistakenly fromhighly specialized cases. A second is that these misleading gen-eralizations often appear in the classroom. In light of theseconcerns, I would agree with Rich’s proposal for teaching thetopics in question.
As an historical note, the tendency to over-generalize isaddressed by early Enlightenment thinkers such as FrancisBacon who warned of the idols of the mind. Among these in-clude the idols of the tribe, which assume more order thanactually exists in nature. As Rich suggests, a presumed de-pendence of boiling point upon molecular mass implies ageneral dependence that is not observed.
With regard to the specifics of the dependence of boil-ing point upon mass in molecular hydrogen, our article pointsout that several other explanations for the effect were con-
sidered. However, none of these are able to explain the ob-served results. The actual working model treats each hydro-gen molecule as residing within a harmonic potential energywell where more massive isotopes lie deeper within the well.As a result, greater energies (or higher temperatures) are re-quired to initiate boiling as the molecular mass increases. Sucha model, indeed, explains the observed boiling point depen-dence upon mass in molecular hydrogen, but, as Rich pointsout, the effect is rather small and is not universal through-out chemistry.
D. Blane BakerDepartment of PhysicsWilliam Jewell CollegeLiberty, MO [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 willlist in point form.
1. Reference 1 cited by Rich is about the boiling point of mo-lecular hydrogen, which has the smallest mass and smallestmoment of inertia of any molecule. It is such a special casethat it is no surprise that the boiling point changes consid-erably upon isotopic substitution.
2. It is relevant that while increasing mass by isotopic substi-tution increases the boiling point of hydrogen, deuterationdecreases the boiling points of many hydrocarbon liquidssuch as cyclohexane. (I am indebted to a colleague for point-ing this out to me.) The model given in ref 1 should applyto these liquids as well as to hydrogen, but predicts thewrong mass dependence of the boiling point. It would beuseful to collect some data to demonstrate this, for it wouldseem to undermine the arguments of ref 1 as being too sim-plified to be applied generally to a range of liquids.
3. Reference 2 cited by Rich makes only slight use of the massdependence (e.g., Figure 10) within a homologous series,where boiling points generally do increase with molecularmass. However this paper is primarily about symmetry inrelation to melting points, rather than boiling points.
4. Reference 3 cited by Rich is the author’s own impressivecontribution to the prediction of boiling points, and I won-der whether it could be used to understand the mass de-pendence of the boiling point of hydrogen, and the oppositemass dependence of the boiling point of hydrocarbons.
Rich’s letter raises some interesting questions, but it doesnot deal with them adequately in its present form. There is agood case to be made for raising these issues in this Journalin a somewhat expanded paper.
R. J. C. BrownDepartment of ChemistryQueen’s UniversityKingston, Ontario K7L 3N6, [email protected]
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