the microscale laboratory 95% range as determined by by titration. For very pure product it is ncessary to distill the material in a micro column although this was not done in the present work. It is possible to do other physical analyses on the products or prepare adducts with amines or ethers and run infrared spectra or NMR of the protons of the adducts. Further chemistry is possible although this is beyond the present synthesis. Literature Cited
The price of a single setup for this experiment, including the reaction chamber, 9-V battery, jumper leads, cork and collection tube, is only $2.95. (Note: The price of our Kmart-brand flash unit was $14.97. We recommend one or two per room.) Furthermore, except for t h e pipets ($0.04/each), all other equipment items can be cleaned easily and are useful for similar laboratory procedures. Theory An electrolysis reaction is employed to initiate Clz gas formation. Although
kzand
2. Szafran. Z., Pike, R. M.; Singh, M. M. Mlcrererele Inogonle Chmistry;Wiley: NewY d .1991.
Photon-Initiated Hydrogen-Chlorine Reaction A Student Experiment at the Microscale Level Leanne M. Egolf and Joseph T. Keiser The Pennsylvania State University 152 Davey Laboratory University Park, PA 16802
The number of student laboratory experiments that integrate the principles of thermodynamics and chemical kinetics are in short supply, as are classroom experiments that effectively illustrate the mechanisms and consequences of phot&hemical processes ( I ) . Here, we introduce a n experiment that addresses both of these needs and also provides an opportunity to investigate three other key chemical concepts: electrochemistry, bond theory, and free radical reactions. The photon-initiated hydrogen-chlorine reaction was presented originally as a dramatic "largescale" lecture demonstration (2).Using microscale technologies, a greatly simplified-yet effective-version of the prototype has been designed. Thus, by capitalizing on both the safety and economic advantages afforded through smallscale techniques, we have been able to bring the chemistry into the hands of the student. The smaller quantities of materials required to demonstrate chemical principles on the microscale level reduce student exposure to, and potential contact with, hazardous materials. In this experiment, for example, a thin-stem plastic micropipette is used to inject the initial reaction solution (approximately 1.5 mL of 8 M HC1) into the reaction chamber. This technique avoids the problems introduced when inexperienced or less than manually adept students attempt to transfer larger amounts of a corrosive chemical by pouring or conventional pipetting. The compact, self-contained apparatus presented here also requires the generation of very little Clz, a useful but poisonous gas. Furthermore, because only one external connection is needed, the potential avenues for escaping vavors are kevt to a minimum and thus the demand on the hood or ventifation system are reduced substantially. t a safe. scaled-down version of the Based on the r e ~ o rof explosive reactiod between 0z'and Hz (31, it appears that the potential danger of a n explosive reaction between Hz and Clz can be controlled in the microscale environment. Because our choice of a glass explosion tube may cause some concern, we have incorporated the use of a plastic safety tube (optional) as a precautionary measure to provide an extra layer of protection to help contain the explosion. However, even after more than 100 consecutive trials, no visible damage to our glass vessel has been noted. A208
Journal of Chemical Education
thermodynamic conditions clearly favor the production of HCI{,, a t this point (see eq 21, Hzw+ Clzcgj + 2HC4,)
AG,
-191 kJ
(2)
the limited kinetic energies of hydrogen and chlorine molecules a t room temperature allow the gases to coexist with no visible change. Only through the application of an exthe ternal energy source-photon energy in this cas-an reaction to form HCk,, be forced to proceed at a n obsew-
11
G ~Tube ~So l l e c t i o q
Exit Tube
Tape
w
Pipette 2
4
Lead Electrode 8MHC14
Pipette Bulb Lip --, Collection Tube Safety Tube
c
Flash Unit
4
The experimental setup: (a)electrolysis; (b) photolysis.
able rate. Free C1 radicals are the initial products of the hoto on-initiated reaction. A series of simple chain reaciions quickly follow, propagating through the gaseous mixture, and finally end with the formation of the deslred HCI gas as summarized below. hv Initiation: C1-CI -> Cl* + CI*
(3)
Propagation: C1. + H-H H. + C1-CI Termination: H- + Cl.
+ HC4g,+H. + HC&, + C1. + HCtg,
(4) (5)
(6)
Reference 2 provides a n excellent detailed discussion of the chemical principles demonstrated in this experiment. Experimental Materials Three Beral thin-stem disposable plastic transfer pipets (Beral Inc., Cat. No. B78-3001, 8 x 43 mm (1.0 mL) clear glass SepCap vial (i.e., collection tube) (National Scientific Company, Cat. No. C4015-961, 12 x 75 mm (6.0 mL) polystyrenelclear disposable culture tube (i.e., safety tube) (VWR Scientific Inc., Cat. No. 60818-496), mini-compact manual flash unit (Kmart, Model M-loo), 9-V battery (fresh), two 14-in. jumper leads, 1.1-mm Scripto pencil lead (4 in.), 00 wrk, 2-mL Pasteur pipet bulb, 2 mL of 8 M HCI solution, deionized or distilled HzO,ringstand, test tube clamp, clamp holder, scissors, Scotch tape, and paper clip. Procedure The experimental design is illustrated in the figure. A stepwise assembly of the apparatus is a s follows. Shorten pipet 1by cutting off approximately 5 cm of its stem. Cut pipet 2 where the bulb begins to taper into its stem (see figure, part a). Discard the stem end. Using a paper clip, carefully punch two small holes on the top of the bulb of pipet 1-one on each side of the stem. Break a 1.1mm diameter pencil lead into two 5-cm pieces. Carefully insert each piece of lead through a separate hole in pipet 1, using the paper clip to widen these holes as needed. The plastic should fit snugly around the leads in order to prevent generated gases from escaping. Position the electrode leads approximately 3 mm above the bottom of the reaction chamber (see figure, part a). Prevent the leads from making physical contact during the reaction by using a piece of tape to fasten the exposed electrode ends to the pipet stem. Secure the reaction chamber in an upright position. Using the paper clip, punch a hole in the bottom of pipet bulb 2. Widen the hole until the stem of pipet 1(gas exit tube) can be inserted aooroximatelv 2.5 cm into bulb 2. , . .\lake a dispensing micropiprt by drawing the thln-.;tern . the mlcrooioet bulb oioet into a microtio diameter ( 4 1 Fill &;proximately 213 h l l of 8 M HC1. Insert the &ropipet into the gas exit tube until the tip emerges in the bulb of pipet 1and dispense the HCl. Connect each of the alligator clips to a separate electrode and the free end of one of the clips to the (+) pole of a fresh 9-V battery. Fill the glass gas collection tube completely with deionized water, ensuring that a convex meniscus rises above the top of the tube. With one continuous motion, invert this tube and slide it over the exit tube of the reaction chamber. (The surface tension of the water should keep the water from spilling during this step.) If any air
bubbles are observed in the collection tube a t this point, remove the tube, pipet the water from bulb 2, and repeat the inversion process. Complete the electric circuit by connecting the second alligator clip to the (-1 battery pole. Within a matter of seconds, gas bubbles should begin to form on both electrodes. The collection tube should be filled completely with gas in approximately 15 s. (Note: The fill-time will vary based on the freshness of the battery, electrodes, etc.) After ap rox imately five "excess" gas bubbles escape into the dispfaced water, the reaction should be stopped by disconnecting one of the alligator clips from the battery. Remove the collection tube from the apparatus and quickly stopper the end snugly but not too tightly with a cork. Make a n o-ring out of a Pasteur pipet bulb by cutting the bulb just below the lip. Slide the lip over the base of the collection tube, moving it up until it rests just below the cork stopper. Mount the plastic safety tube on a ringstand and aim it directly at (i.e., open to) the ceiling or a t a 45 ' upward angle along a wall or empty lab bench. We recommend a 5-ft. vertical or 20-R. horizontal "firing r a n ~ e . " Aflcr wiping the fingerprints off the outside of bo& thccollection und safety tubes, lower the collection tube into the safety tube untilthe bulb lip rests on the rim of the larger tube (see figure, part b). Aim the flash unit directly at the collection tube, ensuring that the face of the flash is flat against and in direct wntact with the outside of the safety tube. Clear the firing range of any students and initiate the final reaction with the flash. The "explosion" should be immediate, causing the cork to eject while creating a clear-but not ear-splitting-"pop." If the explosion fails to occur on the first try, repeat the flash step - re-aligning the flash unit to maximize "photon-to-gas" exposure while trying not to flinch during initiation of the chemical reaction. (Note: After repeated explosions, condensation may form on the inside of the safety tube and will have to be cleaned off so it does not obstruct the photon beams.) Summary This experiment provides an excellent opportunity to exlore and discuss manv kev chemistrv conce~tsresented in introductory chemistry texts. upon testing tLe experiment in the high school classroom setting, the students' response to both the microscale methodologies and the final explosive reaction were found to be very enthusiastic. Furthermore, the apparatus has proven to be easy for students to prepare and assemble, allowing the student to build confidence by working in a more self-sufficient laboratory setting. Finally, because the assembly and run time are kept to a minimum, there is ample opportunity for introductory explanations, encore runs, and summary discussions. Acknowledgment Kenneth L. Egolf and the students in his Carlisle High School physical science (ATIPS) class are gratefully acknowledged for critically evaluating the student utility of our experimental design and associated write-up. Literature Cited 1. P8nn.J. H.. Om,R.D. J . Chzm Edue 1989,66,8688 2. Rarnetfe, R.W. J . Cham. Ed=. 1984,61,722723. 3. Wood. C.G. J . Cham. Educ. 1990.61.586597. . . 4. Thompson,S.Cham~k:Smoll-SeoleE~p~~iiittfo~GP~~mlChmlstry;PrentiHall:New Jersey, 1990,pp 50-51
Volume 70 Number 8 August 1993
A209
