This reaction illustrates 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 offlight 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-mLbeaker .Buehner funnel (8-10 cm across the mouth, 1-L vacuum flask *filter paper fur Buchnrr funnel tweezers .scissors
.Mg ribbon propane tarch .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 vou are readv to perform the demonstration, turn off the" lights in thk classroodecture hall and lieht a piece of Me ribbon with the propane torch. With the iheez&, hold th; hurning Mg rihhon close I10 cm] LO thc silver bromide. When the ribbon has been consumed, turn the lights back on and remove the paper from the surface of the disk. The art of the silver brimide that was exposed to the light frdm 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 demonstratiou 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 s o u r c e of chlorine gas *source of hydrogen gaq 'stnppprs w t ~ g h t l yfit ends oftuhmg ring stand with test tube clamp .high intensity electronic flash with test button .MSDS for hydrogen, chlorine, and hydrogen chloride gas
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.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 h e 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
