The Molecular Volumes and Expansivities of Liquid Normal Hydrogen and Parahydrogen

The molecular volumes at saturation of liquid normal hydrogen and parahydrogen between 14° and 20.4°K were measured with a fused quartz dilatometer, the amount of hydrogen being determined from the pressure of the gas in a calibrated flask at a measured temperature. The results are represented by the equations: V(n−H2) cm3mole−1=24.747–0.08005T+0.012716T2,V(p−H2) cm3mole−1=24.902–0.0888T+0.013104T2.At the normal boiling point of n-H2(20.38°K), V(n-H2)=28.397±0.010 and ΔV(p-n)=0.138±0.010. The expansivity of p-H2 is only slightly greater than that of n-H2. The change observed in the molecular volumes of H2 in passing from the state in which the molecules rotate (o-H2) to the state in which they do not rotate (p-H2) is opposite in direction to the change observed in other substances when passing between these two states of molecular rotation and nonrotation. The changes in ``lattice'' energies are also of opposite sign. It is shown that the random state of orientation of the axes of p-H2 molecules makes the state of nonrotating p-H2 fundamentally like that of rotation in other substances at high temperatures. The differences in the molecular volumes and ``lattice'' energies of the condensed phases of o- and p-H2 arise from a difference in the magnitude and symmetry of the intermolecular forces of repulsion. These differences in the forces of repulsion result from a difference in the distributions of the average electron density of o-H2 and p-H2 molecules arising from the difference in the rotational motion of the two varieties at low temperatures.

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