Low-Temperature Heat Capacity of Hydrogen Molecules
| |||||||||||
Hydrogen 
is the lowest boiling molecular species, remaining a gas down to 20K. At and above room temperature, 
,
the rotational degree of freedom is fully excited; thus the rotational
contribution to heat capacity approaches its equipartition value, 
per mole. Owing to the exceptionally small moment of inertia of ,
rotation becomes inactive at temperatures below about 50K. However, the
heat capacity behaves anomalously as the temperature is lowered. This
anomaly was first explained by Dennison in 1927. Since
is a homonuclear molecule, only half of its rotational states are
accessible. In the singlet nuclear-spin state, known as parahydrogen (p-) only even-
rotational states are accessible; in the triplet nuclear-spin state, known as orthohydrogen (o-) only odd-
rotational states are accessible. The molecular partition functions
representing the rotational and nuclear spin degrees of freedom are
given by
Contributed by: S. M. Blinder (March 2011)
Open content licensed under CC BY-NC-SA
Snapshots
Details
Snapshot 1: ortho and para 
rotational heat capacities as functions of temperature
Snapshot 2: 3:1 mixture and equilibrium mixture with catalyst
Snapshot 2: rotational heat capacity of heteronuclear HD molecule
Reference: S. M. Blinder, Advanced Physical Chemistry; A Survey of Modern Theoretical Principles, New York: Macmillan, 1969 pp. 475–478.
Permanent Citation
https://demonstrations.wolfram.com/LowTemperatureHeatCapacityOfHydrogenMolecules/
No comments:
Post a Comment