edited bv GEORGE L. GILBERT Denison University Granville, OH 43023
tested demonstrations Keeping Chemistry "Light"
White phosphorus is readily soluble in carbon disulfide. Caution:Phosphorus always should be handledunder
L. ~hillip~ilverman' and Barbara B. Bunn V rg n a Po ytechn c nsrit~teand State Un versinl Blacksburg, VA24061 Checked by
Sarah Davis Denison University Granville, OH 43023
Durine mv tenure as the Lecture Demonstrator for the
VPI & sIU ~ k e m i s t r yDepartment, I was able see and hear
some of the rationale the facultv used to explain the demonstrations that were employedin their classes. All of the demonstrations that will be discussed here have some basic photochemical orientation; some emit light, while others usc liaht to cause a reaction. As a lead-in to wntlng about photo~hemicallyoriented demonstrations, I want share with you one of the standard departmental vignettes about a particular demonstration, the reactivity of white phosphorus in air. The First "Famillar" Element "White phosphorus is the first element about which we know the time of the discovery and the discoverer. We don't know who discovered gold, silver, iron, mercury, or sulfur, but we know who discovered phosphorus. Phosphorus was first isolated by H. Brandt in Germany in 1669. Brandt was a n alchemist who sought the elixir of life, a mythical compound that would render anyone who ingested it immortal and give him eternal youth. His studies led him to examine possibilities as to where he might find the secret of the elixir, so he collected his urine in a vat for several months. He added sand to the vat and cooked the mixture down until i t was a thick paste. He then reduced the paste over coke. At the final stage he obtained small waxy yellow beads that glowed when they were exposed to air. He gave this element the name that it has today: phosphorus, from the Greek word phosphoros, meaning "light bearing". However, the most important feature of this element is that i t is the only element that was "first derived from its own svmbol." At this point the professor points to the symbol for phosohorus. clement 15 on the periodic table. Lauzhter is the Iesult when the studentsArealizethe full extent of attempted professorial humor. Materials for Phosphorus Light Emission 0 5 g white phosphorus 10 mL carbon diiuliidr forceps .knife 50-mL bottle paper towels fire extinguisher MSDS for white phosphorus (P4)
'As of 8/19/91, L. Phillip Silverman has been employed by Program Resources, Inc. DynCorp NCI-FCRDC, Frederick, MD 21702.
water and should never be touched by hand;the MSDS
should be consulted. Cut a small piece of white phospho(0.25 to 0.5 g ) and dissolve i t in 5-10 mL carbon disulfide. Pour a small amount of the solution onto a paper towel and turn off the lights. Gently wave the towel in the air to help evaporate the carbon disulfide. Have a fire extinguisher nearby. As the carbon disulfide evaporates, the white ~ h o s ~ h o r u s glows as i t is exposed to a& As mist of the white phosphorus is exposed, the paper towel bursts into flame ( I ) . This redox reactioncan be seen from the formula in eq 1 P, + 50,- P4010,, (1) Disposal of the excess carbon disulfiddphosphorus mixture is best accomplished by simply pouring the excess mixture onto a couple of paper towels and letting the reaction take place. Long-term storage of the solution isn't recommended because the carbon disulfide may eventually evaporate. The Luminol or "Firefly" Reaction Another demonstration t h a t is light-emitting is the luminol reaction. This reaction is also known as the Firefly Reaction because, like a firefly, it gives off light without heat. Chemiluminescence is dramatic, and this reaction always has been one of the most popular demonstrations. The chemiluminescent properties of luminol have been well established for over 60 years. Acitation in Chemical Abstracts from 1929 mentions the chemiluminescence of luminol. while a thoroueh examination of the lieht-emittin(: properties of luminol appeared in the Journn/ ofthe Americnn Chernical Socretv in October 1935 (21.While Iobviously can't claim to have discovered this reaction, I feel that no discussion of chemiluminescence would be complete without mentioning the oxidation of luminol. Please note that the chemicals that I have listed are usuallv more readily available and less hazardous than those usually suggested for use i n t h i s demonstration (Hz02 or %Fe(CN)d (3).
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Materials for the Luminol Reaction
two l-L Erlenmeyer flasks -4.0 e NaOH -2L water 2-Lbeaker
Solution A: Add 100 mL bleach to an Erlenmeyer flask. Add water to the l-L level. Stopper the solution. Solution B: Put 0.4 g Luminol into an Erlenmeyer flask. Add water to the l-L level. Add 4.0 g NaOH to the solution and let the pellets dissolve. Swirl the solution to mix the chemicals in the water. Stopper the solution. When ready to demonstrate this reaction, place the 2-L beaker on the table, have someone turn out the lights, unstopper both solutions and prepare to pour them together so that they mix together as they are poured into the beaker. Volume 70 Number 5
May 1993
405
This reactionillustrates not only the point ofchemiluminescence, but also of quantum mechanics (a photonis emitted as the complex experiences a n energy change), thermodynamics (not all reactions give off heat as energy), and The Reduction of Silver with Light Reactions that use or require light are a little more exotic than those that give off light and heat. Strike a match and you get light and heat, but how many reactions use light? Materials for the Reduction of Silver 100 mL 0.5 M silver nitrate solution 100 mL 0.5 M sodium bromide solution 250-mL beaker Buehner funnel (8-10 cm across the rnouthi l - L vacuum flask tiller paper far Buchnrr funnel tweezers .scissors
Mg ribbon propane torch matches Mix the silver nitrate solution and the sodium bromide solution in the 250-mL beaker. Filter out the precipitate with the funnel and the vacuum flask. Carefully remove the filter paper from the bottom of the funnel so the disk of silver bromide doesn't break apart. Place a small paper cutout on top of the disk of silver bromide. At Virginia Tech, we would cut a small W logo and have that placed on the still damp disk of silver bromide. The lines of the cutout should be a t least 7 mm wide. When you are ready to perform the demonstration, turn off the lights in the classroodecture hall and lieht a ~ i e c of e Me ribbon with the propane torch. With the iheez&, hold th; burning MI: rihbon close I10 cm] LO thc silvcr hromide. When the ribbon has been consumed, turn the lights back on and remove the paper from the surface of the disk. The part of the silver bromide that was exposed to the light from the burning Mg ribbon has been "tanned" (i.e., reduced), while the covered part of the disk remains light ( still the Agi ion). The photons from the burning magnesium ribbon have sufficient energy to reduce a small fraction of the exposed Ag ions to Ag metal (4). Photoinitiated Reactions There are lots ofreactions that are photoinitiated; for example, leaves change color in the fall not by a change in temperature but by a decrease in the number of daylight hours, and fabrics fade partly because the energy from ordinary light breaks down the pigment molecules. The last demonstration of this paper concerns a reaction that is completely photoinitiated. When a student asks "What do you mean by hv starting a reaction?", this demonstration shows how a flash of light can cause a rapid reaction to take place (5). Materials for Photoinitiation 2-em x 120-cm piece glass tubing source of chlorine gas source of hydrogen gas stoppers to tightly fit ends of tubing r i n g stand with test tube clamp high intensity electronic flash with test button MSDS for hydrogen, chlorine, and hydrogen chloride gas
Caution: Read the MSDS for the gases that will be used or generated in this demonstration. Chlorine gas is toxic, and hydrogen gas is dangerous. AU of the preparative p-edures must be carried out in a fume hood. 406
Journal of Chemical Education
Using the glass tube, stopper one end and fill the tube with chlorine eas. Because chlorine eas is heavier than air. the tube mus
