Submitted by Adam Johnson / Harvey Mudd College on Fri, 01/25/2008 - 18:19
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Teaching
Has anybody ever done the liquid oxygen demo?

(to prove its paramagnetic and that simple Lewis structures don't accurately describe the bonding in O2?)

I'm doing it monday.  wish me luck!
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liquid oxygen
demo
demos
Joanne Stewart / Hope College
Adam,
I've done it a bunch. Here's what I've learned: 1) use a powerful magnet that has the poles close together (we borrow one from physics, it's huge), 2) cool the magnet down very well with liquid nitrogen first (very important...the oxygen won't stick otherwise, and it shows that nitrogen is diamagnetic), 3) before or after the demo pour some oxygen in a beaker to show that it's blue, and 4) pour a little oxygen on a piece of cotton wadding and light it with a splint attached to a yard stick (stand back!). The last one is just because I need to blow something up every few days.
 
We "make" our oxygen by using a coil of copper refrigerator tubing immersed in a Dewar of liquid nitrogen. We blow oxygen from a gas cylinder through the coil and liquid O2 comes out the other end (and we collect it in another Dewar).
 
Have fun,
Joanne
Sat, 01/26/2008 - 19:07 Permalink
Maggie Geselbracht / Reed College

I do liquid oxygen every year.  It's always a real hit.  And now I will have to add Lori's video as a finale!

Don't be so quick to misjudge Lewis...Take a look at one of his original papers (JACS, 1916, 38, 762-785), especially Fig 4 on page 778.  He shows 2 "tautomeric forms" for O2 represented in his cubical atom approach.  Form B sure looks like it could be paramagnetic to me!  I have always wondered how the history coincided.  Lewis does not mention the paramagnetism of O2, and I am not sure when it was discovered.
 

Sun, 01/27/2008 - 16:17 Permalink
Maggie Geselbracht / Reed College

In reply to by Joanne Stewart / Hope College

Here's an alternative method...
I have a long, large test tube-like piece of glassware (about 3.5 cm diameter) that fits inside a Dewar.  I slowly blow O2 from a gas cylinder through a long piece of glass tubing stuck inside this test tube.  Cool the outside part of the Dewar with L-N2 and voila.  It takes about 15-20 minutes to condense a bunch of O2 (maybe 100-200 mL?).  I can show them it is blue by lifting the test tube out of the Dewar.  And it is a nice review to get them to think about the relative boiling points of N2 and O2 and how you can use this to your advantage.  I also usually say something about the Air Products empire and air fractionation.

Sun, 01/27/2008 - 16:19 Permalink
Nancy Williams / Scripps College, Pitzer College, Claremont McKenna College

In reply to by Maggie Geselbracht / Reed College

Lewis *does* mention that O2 invariably forms peroxides first in all known reactions, suggesting that it is in an "odd state" at least to some degree (we might say, has some diradical character).  Keep in mind that orbitals don't exist yet-this paper preceeds Schroedinger's solution by 11 years.  In 1924, Lewis discusses the paramagnetism of O2 directly, and argues for the single bonded structure with two unparied electrons.  In the "Lewis O2" paper, he goes into mre detail, showing that O4 is diamagnetic.  Pretty cool...he talks about the fact that the physicists' models for the atom yet can't explain pairing (Pauli's Exclusion principle came 1 year later).  These two papers are Chem Rev, 1924, 24, 231-248, and JACS 1924, p 2027-2032, respectively.



Sun, 01/27/2008 - 21:20 Permalink