edited by
GEORGE L. GILBERT Denison University
Paramagnetism and Color of Liquid Oxygen: A Lecture Demonstration Submitted by: Bassam Z. Shakhashiri, Glen E. Dirreen and Lloyd G. Williams' University of Wisconsin-Madison Madison, WI 53706 Checked by:
S. Ruuen Smith Uniuersity of Connecticut, Storrs, CT 06268 When liquid nitrogen is poured between the poles of a powerful magnet, the liquid is not held in the pole gap. When liquid oxygen is poured between the poles of the magnet, some of the liquid is held in the pole gap until the liquid evaporates. This demonstration also displays the characteristic hlue color of liquid oxygen. Materials Liquid nitrogen (5-8 1) O2gas. (cylinder with pressure regulator) 2 Dewar flasks (1 L capacity) Large test tube (38 mm 0.d. X 300 mm long) Ring stand and clamp to hold the large test tube Rubber tuhinr. (7-8 mm 0.d. X 30 cm long); 2 pieces Glass tuhing (7-8 rnm ad. X 30 cm long) Glove (to protect hand from cold) Permanent magnet Copper coil Unsilvered Dewar, 250-ml capacity (optional) 250 ml beaker cigarette (optional) Several types of magnetsmay he suitable for this demonstration. The permanent magnet used a t the University of Wisconsin-Madison i s a "Horn Gap" magnet with an original field streneth of 1.8 kiloeauss. Conical ole oieces machined from iron &ere added to"the pole faces to reduce the pole gap from 4 to about 1cm. The new magnetic field has been mea. sured to he 9 kiiogauss. Another magnet was obtained from Edmund Scientific Company (Adjustable Gap Magnet No. 70,810; $59.95) and used for this demonstration. The adjustable pole pieces should he set for a gap of about 0.5 cm. Under these conditions, the magnetic field strength is approximately 9 kilogauss. This demonstration may be displayed directly or by using an overhead projector and a 3M "Science Table". This device, which is no loneer available from 3M. consists of a cradle and a mirror and used to orient the overhead projector in a horizontal nosition so that the magnet stands in front of the light source. A modified TOPS prGjector-cradle ( 1 ) may be used for the same purpose. Procedure Liquid oxygen can he prepared by either of two methods. Make sure no flames a r e in t h e vicinity. In the fmt method gaseous 0 2 is passed through a copper .. coil immersed in a liq;id nitrogen ~ e w a r ~. i ~ u oxygen i d (h.p. -183°C) is condensable in liauid nitroeen (h.o. -196°C). A coil is made from abwt 2 m of ri,pper tuting (6 Am o.d.) and should have 10-12 turns with a separation ot'ahout 2 cm between turni. The dinmeter ntthe roil is 6 rrn and its height is 22 cm;'the ends of the coil extend about 9 cm above the last turn as shown in Ficure - 1. Liquid oxvaen .- can he collected in a larae - test tube or in a Dewar flask by attaching a short piece of tuhing to
' Present address: Hampshire College, Amherst; MA 01032.
Granville. Ohio 43023 one end of the coil (a piece uf rubber tubing connects the other end to the 0 2 gas cylinder). The gas flow rate should he high enough to push the liquid oxygen out of the coil. The supply of liquid nitrogen should be replenished as 0 2 gas is passed through the coil. The procedure will enable the preparation of about 100 ml of liquid oxygen in less than two minutes. When sufficient liquid is collected, close off the cylinder valve, disconnect the ruhher tuhing and remove the coil from the liquid nitrogen Dewar. Alternatively, liquid oxygen can he prepared as follows. Fill one of the Dewar flasks with ~ i g u r e1. Copper coil tor preparing liquid nitrogen. Clamp the liquid oxygen. large test tube and immerse i t in the Dewar flask. Connect one end of the ruhher tuhing to the oxygen cylinder and the other to the piece of glass tuhing. Place the open end of the glass tuhing in the large test tube and direct a gentle stream of O2 gas from the cylinder into the bottom of the test tuhe. Liquid oxygen will condense in the tube if the gaseous Ozflow rate is not too high. Replenish the liquid nitrogen in the Dewar flask as necessary and allow oxygen to condense until the test tuhe is approximately half-full. This may require 10-15 min depending on the gaseous 0 2 flow rate. Store the test tuhe in the liquid nitrogen Dewar until needed. The hlue color of liauid oxveen is seen when the test tuhe is lifted out of the ~ e w aof r l&id nitrogen. Often, moisture condenses uuicklv on the outside of the test tuhe and obscures the light hlile coior.The color ran be seen if a special unsilvered Dewar flask is used. The unsilverrd Dewar should be protected by tape (Dewars a r e known t o implode a n d shatter), hut vertical windows 2-3 cm in width should be cut to show the characteristic hlue color The fact that a strong magnet is to he used should he shown. Simply place a pen knife or a clamp near the pole gap. Remove this metallic object before proceeding to the next step. Beein the demonstration hv oouriue some liauid nitroeen betwren the poles of the m a i n k ~ o " e01. the iiquid will'he held in the gap. In addition to showing that liquid N, is dinmagnetic this step will help cool the mapet. Nexr, slowly pour several milliliters of liquid oxygen hetween the pdes of the magnet ( 2 ) ;a small amount will he held in the pole gap until the liquid evnporntes. This shows the paramaanetiam of liquid oxygen. The entire procedure should-he repeated to contiast the magnetic properties of both liquids. It is advisable to w e a r a-clove while handline t h e liouid oxveen test t u h e " since t h e cold temperature c a n cause severe frostbite. A different hut less spectacular way of demonstrating the paramagnetism of liquid oxygen has been descrihed (3).
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Hazards The liquified gases used in this demonstration can cause severe frostbite due to their low temperatures. Care should he taken to avoid lettine them come into contact with skin. The high concentration of oxygen encountered in the liquid phase is capable of supporting violent combustion (4) even Volume 57, Number 5, May 1980 / 373
state (3Z,-) oxygen molecules to the first excited state ('A#) in a two-molecule-one-photon process (5)
+
2O Z ( ~ ~ ~ hu(4300A) -)
-
2 O&&)
As the potential energy diagram (Fig. 3) shows, the 'Ag state lies 12,687A (0.98 eV) about the ground state. A single photon a t --6300A thus possesses enough energy to excite two Oz molecules to the 'A, state. The nominally forbidden transition occurs when two ground state molecules collide. The double molecule state thus produced possesses a singlet component. The transition may be represented as
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1~3.5[(32g-)(328-)1 l[(A,)('A,)l This transition is not observed in oxygen gas at low pressure due to the very low probability of the three body process (two 0 2 molecules and a photon) required for absorption to occur. However, in the condensed phase, the collisions between 0 2 molecules are more frequent, making the absorption more likelv. ~ i i demonstration s of triplet state 0 2 can be used along with another demonstration that displays the red chemiluminescence of singlet state 0 2 (6). 1tk noteworthy that the red chemiluminescence is due to the reverse of the blue absorption transition. Both the paramagnetism and color are intrinsic properties of oxygen and are not bulk properties of the liquid phase (5).
Figure 2. Moieeular abital diagram of oxygen.
Literature Cited (1) A1yea.H.N..J.CHEM. EDUC.,55,786(1976). (2) Lefhbridge, J. W., and Davies, M. B.. J. CHEM. EDUC., 50,656 (1973). (3) Ssban, G.H. and Maran, T. F., J. CHEM. EDUC., 50,217 (1973).
( B Wiik.1. J., J. CHEM.EDUC.,15.A547 (1968).
(0 Ogryzl~,E. A,, J.CHEM. EDUC., 12,647 (1965). (6) Shskhaahiri.B. L a n d Willisms,L.G.,J.CHEM.EDUC.,53.358(1976). (7) Henberg. G.."Sprctraaf Diatomic Molecules". D. Van Noatrand Co.,Princeton,N.J.,
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(3) Khan, A. U., and Kaaha, M., J. Arner. Chom. Sor., 92, 3293 (1970), and references therein.
Corridor Demonstration
I
2
PV Work Demonstration
3
r (A) Figwe 3. Potential energy diayam fasome low lying statesof &(a* (7)and Khan and Kasha (8)).
Mberg
in substances which do not burn under ordinary conditions. When the demonstration is completed, liquid oxygen should be poured onto the lecture table and allowed to evaporate. Alternatively, place a lighted cigarette in a 250 ml beaker and very carefully pour in the liquid oxygen! This will show the effect of high concentration of 0 2 on the rate of combustion. Discussion
The observation that liquid oxygen is attracted by a magnet suggests the presence of unpaired electrons in the 0 2 molecule. T o account for the magnetic properties of the oxygen molecule, a molecular orbital description of bonding in 0% can be made since the valence bond descri~tiondoes not indicate the presence of unpaired electrons. A simplified molecular orbital dineram for 01 in i a eround state is shown in Firure 2. The unpaired electrons invthe rzp* molecular orbitals produce a (3X,-). t r i ~ l estate t in the case ofiiquid nitrogen, the lack of interaction with the mametic field indicates that all of rhe electrons are paired. valence bond theory indicates that the electrons in the nitrogen molecule should all be paired. Thus, although the bonding in Nz may be described in terms of molecular orbital theory, the valence bond description is sufficient to account for the ex~erimentalobservation in this case. The bl;e color of liquid oxygen is due to the absorption of lieht in the red reeion of the mectrum. The absor~tionmay be attributed to the simultaneous transition of two ground ~~~~
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374 1 Journal of Chemical Education
E. Koubek U.S. Naual Academy Annapolis, MD 21402 The above ~ i c t u r eis of a hallway demonstration that we have found to be very effective in geherating student interest. (The challenge to test their strength is almost impossible for an 18year-old to resist). The demonstration is designed to give the students an idea of the amount of work done hy a gas in expanding against the atmosphere which leads to a appreciation of AH versus AE. The device was made in our shop and is simply an aluminum cylinder with a piston fitted with a rubber O-ring. The diameter of the piston is 8 cm which we have found is about the ideal size to present a healthy challenge.
