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: $$\begin{array}{c}\mathrm{V(n}\mathrm{-}{\mathrm{H}}_2){\mathrm{cm}}^3{\mathrm{mole}}^{\text{−1}}{\text{=24.747–0.08005T+0.012716T}}^2,\\ \mathrm{V(p}{\mathrm{-H}}_2){\mathrm{cm}}^3{\mathrm{mole}}^{\text{−1}}{\text{=24.902–0.0888T+0.013104T}}^2.\end{array}$$ 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‐H<sub>2</sub> molecules makes the state of nonrotating p‐H<sub>2</sub> 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.