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A Thermodynamic Analysis to Explain the Boiling-Point Isotope Effect for Molecular Hydrogen
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Abstract
The analysis reported provides an explanation for the boiling-point isotope effect observed for molecular hydrogen isotopes (H2, D2, and T2); that is, an explanation for the increase in normal boiling point with increasing molecular mass. The standard enthalpy of vaporization 
is shown to depend on molecular mass, and itself is related to temperature via a standard solution to the Clausius-Clapeyron equation. Simplification of the solution, at standard pressure, yields an expression for the normal boiling point that depends on molecular mass. An evaluation of the expression, using constants obtained from two of the boiling points in the series, allows a prediction of the third boiling point. As an example, the predicted boiling point of ditritium is 24.9 K, in close agreement with the observed value of 25.0 K.
Keywords (Audience):
Upper-Division UndergraduateKeywords (Domain):
Physical ChemistryKeywords (Subject):
IsotopesCiting Articles
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This article has been cited by 4 ACS Journal articles (4 most recent appear below).
Does Molecular Mass Greatly Affect Boiling and Melting Points? (author response)
R. J. C. BrownJournal of Chemical Education2004 81 (5), 643Does Molecular Mass Greatly Affect Boiling and Melting Points? (author response)
R. J. C. BrownJournal of Chemical Education2004 81 (5), 643I have read the letter by Ronald L. Rich with interest. There are some interesting points to be made.
Does Molecular Mass Greatly Affect Boiling and Melting Points? (author response)
D. Blane BakerJournal of Chemical Education2004 81 (5), 643Does Molecular Mass Greatly Affect Boiling and Melting Points? (author response)
D. Blane BakerJournal of Chemical Education2004 81 (5), 643Ronald 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 ...
Does Molecular Mass Greatly Affect Boiling and Melting Points?
Ronald L. RichJournal of Chemical Education2004 81 (5), 643Does Molecular Mass Greatly Affect Boiling and Melting Points?
Ronald L. RichJournal of Chemical Education2004 81 (5), 643Too 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.
A Note on "A Thermodynamic Analysis to Explain the Boiling-Point Isotope Effect for Molecular Hydrogen"
Lilia Meza-Montes , Peter Hoffmann-PintherJournal of Chemical Education2001 78 (3), 416A Note on "A Thermodynamic Analysis to Explain the Boiling-Point Isotope Effect for Molecular Hydrogen"
Lilia Meza-Montes , Peter Hoffmann-PintherJournal of Chemical Education2001 78 (3), 416A simple one-dimensional harmonic oscillator potential well has been used to provide a pedagogical quantitative explanation for college chemistry students for the difference in the boiling points of the three forms of molecular hydrogen. In that scheme ...
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- Received: August 03, 2009
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