4
$\begingroup$

I ran into a fascinating little fact I didn't remember hearing before: if H2 is cooled below 60 K, its heat capacity looks like that of a monoatomic gas. It simply doesn't have enough thermal energy to rotate, due to quantum effects.

Inevitably I find myself wondering how that works... in particular, does this mean you could take a container of liquid hydrogen at 14K or so and purify out molecules where the hydrogens are at opposite ends of an arbitrary x, y, or z axis as desired? (And to make that answer meaningful, how long would they actually stay that way once purified?)

Improve this question
$\endgroup$
10
  • 3
    $\begingroup$ There is no liquid hydrogen at 60 K. This temperature is above the critical point. $\endgroup$
    – Oscar Lanzi
    Commented Sep 2 at 18:00
  • 1
    $\begingroup$ Technically I said the liquid H2 cooled below 60K, but OK, I can change that. :) Also, after some consideration, I think the molecule will rotate at exp(-60/14) of its normal rate, still too fast to separate out, but we'll see what the real answer is from someone in a minute. :) $\endgroup$
    – Mike Serfas
    Commented Sep 2 at 18:08
  • 2
    $\begingroup$ Wikipedia blows this up. It reports that the low-pressure phase of solid hydtogen has freely rotating molecules. $\endgroup$
    – Oscar Lanzi
    Commented Sep 2 at 18:23
  • 1
    $\begingroup$ I was having some probable errors during my first attempt, but I'm calculating something like km/s for the rotational speed of the hydrogen atoms. If we cool it down to 1/4 of the thermal energy needed to rotate the molecule, it will still be in a rotating state sometimes, just e^(-4) less often. So I think the answer will be it is moving well beyond a walking pace, which means it flips in some tiny fraction of a second, even though it is almost always not rotating. $\endgroup$
    – Mike Serfas
    Commented Sep 2 at 19:23
  • 2
    $\begingroup$ In nmr you can move the magnetisation around from axis to axis but the individual nuclear spins still precess about the magnetic fields, it is not possible to align a quantum object in the way you suggest. Same thing applies to molecular rotational motion. As H2 is light its rotational quantum is v large $\approx 61$ cm$^{-1}$ so at low temperature it will mostly be in $J=0$ so will be oriented at all angles by uncertainty principle in the absence of an external field. $\endgroup$
    – porphyrin
    Commented Sep 2 at 21:13

0

Reset to default

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Browse other questions tagged or ask your own question.