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Ortho- and Para-Hydrogen 13
The electrons must spin in opposite directions (antiparallel), or the molecule
would not hold together; but the spins of the nuclei may either be
in the same (parallel) or in opposite directions, and certain properties of
the molecule, especially its specific heat, will be different in the two
arrangements. In 1927 Dennison pointed out 4 that the observed specific
heats could be made to agree with the theory only if it was assumed that
the time of transition was very long in comparison with the time in which
the specific heats were measured, so that the gas was in effect a mixture
of the two forms in fixed proportions. Subsequent work has entirely confirmed
this view. The two differ especially in the fact that of the rotational
energy (which is practically all the heat energy other than translational
that the molecules have up to high temperatures) the ortho (parallel)
molecules have the odd quantum states 1, 3, 5, etc., and the para (antiparallel)
the even, 0, 2, 4, etc., which results in the ortho having, especially
at low temperatures, a smaller specific heat than the para. We thus have:
Ortho Para
(Oj (OJ (OJ ^)
Spins parallel Spins antiparallel
Rotational quanta odd Rotational quanta even
The proportions can be determined from the heat conductivity (which
depends on the specific heat) by observing the rate of cooling of a heated
wire in the gas,
At equilibrium at the ordinary temperature and above it the gas contains
25 per cent, of para and 75 per cent, of ortho, but on cooling the
percentage of para rises, because more of the gas can thus go into the
(even) zero-quantum state, and at the boiling-point of hydrogen (20*4° K.)
only a fraction of 1 per cent, of ortho remains. The proportions at equilibrium
and the specific heats are:
Temp.
Per cent, para
Hp. ht. Pure para
Pure ortho
Normal* H2
2O 0 K.
99-8
50° K.
76-9
0040
0000
0010
100° K.
38-5
1-504
0-073
0-431
200° K.
26-0 .
2-767
1151
1-555
298° K.
251
2-186
1-838
1-925
Inf.
2500
* i.e. the equilibrium mixture at the ordinary temperature, which has 25 per
umit. para.
In order to make para-hydrogen (it is of course impossible to get the
ortho with less than 25 per cent, of para) the gas must be cooled (if possible
with liquid hydrogen, though liquid air under reduced pressure will give
a gas with 45 per cent, of para), and then brought to equilibrium by means
of a catalyst. This is most easily done by absorbing the gas on charcoal,
and then after some minutes or hours, according to the activity of the
the charcoal, pumping it off.
* D. M. Dennison, Proo. Boy, Soo, 1927» 115, 483,