be poured on a desktop to demonstrate this behavior.) The gas produced burns above the drop, resulting in an object with a temperature range from -164 "C to +I300 'C (for a poorly oxygenated natural gas flame). The liquid also can be poured straight downward, again from no more than a foot above the floor, and allowed to scatter along radii from its point of impact. In this case you should be prepared to back away. An interesting thing to notice about this phenomenon is that as long as the drop is moving, the flame is fairly quiet, but if it comes to rest it flares up. Whenever it is in one position, the drop cools the surface underneath to the point that it touches it, because it is not vaporizing as rapidly. It adheres, and the much more rapid heat transfer causes increased boiling and the resulting flareup.
Poly(acry1amide)is not merely a laboratory curiosity In addition to its extensive use as a matrix for electrophoresis, it is used commerciallv as a thickening - agent - and as a flocculant (4). Literature Cited l.Rdriguez,F.:Mathiaa,L.J.:Kmseharite,J.;Caneher,C.E.,Jr J Chem.Educ 1387,
~ ~ . ~ - ~
84 R R C R-R R~ - ~ 2. Hiemem, P. C. Pdymrr Ckamr*L'y; Mawel Dekker New Yo*, 1984, p 351.
3.Billmeyer. F. W, Jr,'RdbmkofPolymorSciance, 3rded..Wiley: NewYork, 1984.p 12. 4. Billmeyer, F W ,Jr.,p 390.
A Demonstration of the Molar Volume of Nitrogen Gas Submitted by
FreeRadical Polymerizationof Acrylamide Submined by
Checked by
Ernest F. Silversmith Morgan State University Banimore, MD 21 239
Daniel T. Haworth Marquene University Milwaukee, WI 53238
Checked by
Wayne C. Wolsey Macalester College St.Paul. M N 551 01
A demonstration of the polymerization of acrylamide,initiated by light and riboflavin, has been described ( I ) . Several minutes' irradiation is needed to start the reaction and, in our hands, the demonstration sometimes fails comoletelv. It seems to be inhibited bv traces of im~uritiesthat ;nay de present. We have rnudifigd the conditions to give a thermal polymerization that i~ rapid and foolproof. Preparation Prepare the following solutions within 2 h of the demonstration. (They will last longer if refrigerated.) A. 5.0 g o f acrylamide, electrophoresis grade, in 5.0 mL of
water. ((faution: Wear gloves when handling acrylamide. Acrylamide is toxic and is a suspected carcinogen.) B. 2% aqueous iron(I1)sulfate heptahydrate C. 2% aqueous ammonium peradfate (Caution:corrosive) The Demonstration Solution Ais placed into a 100-mL beaker and 10 drops of B are added. The mixture is shaken till it is homogeneous. Ten drops of C are added, and the mixture is shaken again. Within seconds, the material becomes solid (the beaker can be inverted to dramatize this), and the lower part of the beaker becomes too hot to handle. (The mixture may even bubble and steam!)At thenext class, the rubbery polymer can be scraped out of the beaker with a spatula. It may be disposed of along with other solid waste of low toxicity. Discussion The reaction in this demonstration is an addition polymerization initiated by anion radicals generated as follows (2):
~ ~ 0+ 2~ -8
Elvin Hughes, Jr. Southeastern Louisiana University Harnrnond, LA70402
+ So4'- + SO4:
% 4 Fe3+
The heat generated is wnsistent with the exothermic nature of vinyl polymerizations (3). The polymerization is rapid a t first; solidification and heat evolution occur within a few seconds of mixing. Complete polymerization to give a nonsticky, rubbery polymer requires somewhat more time.
This demonstration provides the student with a graphic illustration and a calculation of the a ~ ~ r o x i m amolar te volume of nitrogen gas. Place a mole (28 g or 35 mL) of liquid nitrogen in a precooled 100-mL graduated cylinder. Then attach a cylindrical balloon, capable of expanding to a diameter of approximately 20 an and to a length of 120 cm, to the mouth of the graduated cylinder. The liquid nitrogen is then allowed to evaporate and fill the rubber balloon. Measure the length and diameter of the cylindrically shaped balloon and calculate the volume. The volume of the expanded balloon is the molar volume of gaseous uitrogen a t the prevailing pressure and temperature. Procedure Place a 100-mL Pyrex graduated cylinder, without the plastic base, in an expanded polystyrene cup filled with liquid nitrogen to precool the cylinder. The molar volume of liquid nitrogen needed to form one mole of gaseous nitrogen can be comnuted bv dividing the molar mass (28 e)bv ;he density (a~proxim&ely0.81 g/mL). This cal'cul&on yields an approximate volume of 35 mL of liquid nitrogen. This volume of liquid nitrogen is equivalent to 28 g (one mole) of nitrogen. (Alternatively, one could weigh 28 g of liquid nitrogen in a precooled cylinder.) Pour liquid nitrogen, - from another ex~andedwlvstvrene cup, into the ~ r e cooled graduated cylinder, that 'stiil is in the cup containing liquid nitropen. Adjust the level to 35 mL. Place a balloon, which will form an approximate cylinder shape, over the graduated cylinder and remove the assembly from the liquid nitrogen. The liquid nitrogen immediately vaporizes and fills the balloon. Material One Dewar or stainlesssteel thermos bottle full of liquid nitroeen ~ne'ballmn capable of stretching to a 20-cm diameter and 120-cm length Several expanded polystyrene mffee cups One 100-mL Pyrex graduated cylinder without the base Cloth gloves or a hand tawel to ~ r o t e dyour hands Ear pl;gs (optional) One meter stick Safety Considerations Use standard laboratory safety procedures. Safety glasses should be worn by demonstrator and observers. Volume 69 Number 9 September 1992
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