The heat capacity of the transition is sufficient to release enough heat, as orthohydrogen converts to the lower-energy parahydrogen, to boil the hydrogen liquid to gas again, if this evolved heat is not removed with a catalyst after the gas has been cooled and condensed.
12.
Because of the antisymmetry-imposed restriction on possible rotational states, orthohydrogen has residual rotational energy at low temperature wherein nearly all the molecules are in the J = 1 state ( molecules in the symmetric spin-triplet state cannot fall into the lowest, symmetric rotational state ) and possesses nuclear-spin entropy due to the triplet state's threefold degeneracy.
13.
Since " normal " room-temperature hydrogen is a 3 : 1 ortho : para mixture, its molar residual rotational energy at low temperature is ( 3 / 4 ) x 2R? rot = 1091 J / mol, which is somewhat larger than the enthalpy of vaporization of normal hydrogen, 904 J / mol at the boiling point, T b = 20.369 K . Notably, the boiling points of parahydrogen and normal ( 3 : 1 ) hydrogen are nearly equal; for parahydrogen " H vap = 898 J / mol at T b = 20.277 K, and it follows that nearly all the residual rotational energy of orthohydrogen is retained in the liquid state.
