Each solution is then diluted with 40 mL of a 1:l methanol-ethanol mixture delivered by syringe, and the vent needles are removed. The flasks are now ready for use and may be removed from the hood. To conduct the demonstration, 5.5 mmol of a n appropriate substrate is injected into a flask, and the time required for the disappearance of the red of the Se," ion is noted. A set of experiments may be conducted in series or in parallel. The wlor change is easier to see when viewed against a white background. Demonstration of Solvent Effect The dramatic rate enhancement of the SN2reaction by the use of dipolar aprotic solvents may be demonstrated a s follows. A flask is prepared with NaBH4 and Se a s usual except that 0.5 mL of water and 9.5 mL of N,N-dimethylacetamide (DMA) are added to the flask in place of the ethanol-water mixture. After 30 min, the solution is diluted with 40 mL of DMA, and the flask is allowed to stand vented for a n additional 30min before the demonstration. Cleanup is effected as described helow. (DMA is specified instead of N,N-dimethylformamide because NaBH4-DMF mixtures are unstable and can decompose in a n unpredictable manner.) Cleanup Since organic selenides and diselenides have notoriously offensive odors, the reaction inixtures remaining from the demonstration are decomposed by oxidation before opening the flasks. I n this manner virtually no odor is detected. After the demonstration, the flasks are removed to the hood and are placed in a glass baking dish to catch any spillage that might occur. While working with the sash lowered as much a s uracticable. each seutum is uierced and vented with four i8-gauge needles. i hen 2 m i o f 1 M aaueous ontassium iodide is added bv svrlnee to each flask a s a n oxikation catalyst and indicatht"~he>5 mL of 30% hydrogen peroxide is added to each flask. After a brief period, elemental iodine is formed, and the excess hydrogen peroxide decomposes to oxygen and water. When the reaction has subsided, the flasks may be ouened and the deodorized contents disposed of in a n aDpropriate manner in accordance with the institution's policy f i r reaction wastes. The septa will oRen retain diselmide residues. These may be iendered less offensive by soaking overnight in a beaker of household bleach before disposal. The syringe needles may likewise he cleaned with bleach. If a n odor is encountered during the quenching process despite working in a closed hood, then the quenching process may he conducted outdoors.
The Old Nassau Demonstration: Educational and Entertaining Variations Submined by:
John J. Fortrnan Wright State University Dayton, Ohio 45435 Checked by:
Jesse Blnford University of Southern florida Tampa, FL 33620 The double-color-change clock reaction that has come to be known a s the "Old Nassau" reaction is probably one of Hubert Alvea's most imitated demonstrations used for sheer entenamment. Instruct~onscan be found in many sources (1-10, for t h ~ reactlon s wh~chfirst turns yellow-or-
236
Journal of Chemical Education
ange-gold and then black-blue. There are many variations in its use and in the colors that can be generated to salute different schools. However, the Old Nassau reaction can also be used to illustrate the effects of concentration and temuerature on rates in a fun way. Sto~chiometricvariations, which are accomolished t)\. halving or doutrline the \ d u m r of the three standard sol;tions used, also lea2 to different end results that are interesting and educational. Shakhashiri's Chemical Demonstrations has a n excellent discussion of clock reactions in Volume 4 with a n explanation on why we see colors appear suddenly after a time delay, instead of seeing a gradual darkening. Therefore these points will not be discussed here. Standard Procedure (1-10) Reagents Solution A. Dissolve 4 g of soluble starch in 500 m L of boilinr! water. After coolina. -. add 13.0 e ofsodium bisultite. and dilute to 1L. This concentration is sliehtlv lower than that used in all of the recommended proce~u& (1-91, except one (10). This is necessary so that the concentration ratios for the stoichiometric variations will not be too close to the limits (11). Otherwise there will be difficulties with changes not stopping a t the correct color. Solution B. Dissolve 3.0 g of mercuric chloride (HgCl,) in 1L of water. Solution C. Dissolve 15.0 g of potassium iodate in 1L of water. Procedure Mix, in order, equal volumes of A, B, and C. The solution will turn bright orange and then flash blue-black. The color changes result from the following reactions.
HP+ 21-
+ HgI,L (orange)
XI, + I + starch + palyiodide ion on starch (blue-black) Educational Variations Kinetic ll/ustrations(The Effect of Dilutions and Cooling) Method I. Carry out the standard procedure, and ask the class to wunt the number of seconds that elapse before each color change. For fresh solutions approximately 3-4 s will elapse before the appearance of the orange, and 6-10 s before the black. Method 2. Dilute samples of each of the three solutions to 50:50 with water, and then mix equal volumes. The times required to see a color change will increase by a factor of approximately 3 4 . One does not get the exact 4-fold increase predicted for the reaction whose incubation times are directly proportional to the starting concentrations of the iodate and bisulfite ions. The reaction is actually first-order in iodate, but it has been questioned (12) whether it is actually first-order in bisulfate as proposed (4, 13) or a n unusual second-order proces. Regardless of the order and the mechanism, the Tnese moodlcatlons were presented at the Sympost~rnn honor of Hmen A yea at tne 197tn Nat ona ACS Meeltng In Da las. Texas on Apr 11. 1989
total times that were measured before the color changes occurred were directly proportional to each concentration raised to the first power (13). The variation from a 4-fold increase is probabiy at least partially due to the catalytic effect of the acid concentrations and the relative changes that occur with dilution. Using different dilution factors for the three solutions often leads to confusing results that can be understood in terms of the stoichiometricvariations discussed in the next section. Method 3. Cool equal-volume samples of the three original solutions in an ice bath, or to be more dramatic, add small oieces of drv ice to each. and let them stand lone enougl; to cool. MLand observe' the increased times. ~ a c 6 time should be increased by the same factor. The added acidity due to the carbonic acid formed from the dry ice can shorten the times, so the solutions cannot be allowed to sit very long before mixing. Stoichiometric Nlustrations
The involvement of three concepts can make the discussion of stoichiometricvariations too wmplicated for simple treatment in an elementarv chemistrv wurse: the exact stoichiometry as given in the reaction equations listed above; the reaction mechanisms (4); and the general nature of clock reactions (11).However, if one uses the indicated concentrations for the three solutions and then discusses the results in terms of variances from the 1:l:l volume ratio, the students can learn a great deal about the results of stoichiometric adjustments. Shakhashiri's Chemical Demomtmtwns wntains an excellent detailed treatment of the actual stoichiometricconcentration limits in Volume 4, Section 10.3, Procedure C. Method 1. Mix equal volumes of solutions A, B, and C (orange-black). Give the class handouts of the three reactions above, and ask them to predict the results for each of the following variations one by one. Then carry out the reactions, and explain the results. Method 2. Mix equal volumes of solutions Aand C, with no solution B (black only). Also ask the students why pairing solutions Aand B or solutions B and C would not show anything. Method 3. Mix equal volumes of solutions B and C with half the volume of solution A (orange only). All of the Iformed is precipitated as HgIz, so none is left to react with iodate to form 12. Method 4. Mix equal volumes of solutions B and C with two times the volume of solution A. The solution turns orange, and then becomes clear (10). All of the iodate is converted to iodide by the excess hydrogen sulfite, so no iodine can form. The excess iodide ion forms a colorless wmplex with the orange Hg12.
Method 6. Mix equal volumes of solutions A and C with double the volume of solution B (yellow-orange). The excess Hga precipitates all the iodide. This combination is closest to a crossover ooint for the blue-starch reaction. If the HS03-MgZI ratio fs too high by about 2%,the reaction will continue to a chocolate brown or off-black. With careful adjustments this could be used for school colors of yellow or gold with brown.) Different Entertaining Variations To Salute a School with Blue and Gold Colors
Line uo seven to nine 600-mL beakers. In front of the audience'fdl the beakers from the same three bottles, but when you first add solution A (sodium bisulfite), alternate between adding 100 and 50 mL. The beakers should be at least twice the size of the total volume of the three solutions t o be added so that the fmal differences in volumes are not noticeable. It may take a little practice to pour the correct amounts using only the calibrations on the beakers. Then add 100 ml of solution B (mercuric chloride) to each. Finally add 100 mL of solution C (potassium iodate) to each while singing an appropriate school song such as the "Notre Dame Victory March". The beakers will alternate between the gold-blue double change and the gold alone to leave you with alternating blue and gold colors. To Salute a School Play~nga ainst a School Whose Colors Are Blue and &Id
While singing your school's fight song, do the 'Old Nassau" demonstration in the usual manner. Pretend that you have become perplexed that your opponents colors hive suddenly appeared. Then show the class how easily the opposing team will be vanquished in the game by adding more solution Auntil the solution turns clear. Be careful to use a beaker that is large enough for the total volume. Disposal (14, 15)
Mercury that remains in solution can be precipitated as the sulfide by adding sodium sulfide solution. This solution is made basic bv addine ammonium hvdroxide. f i r washm e t i sulfide, it should be ing and dryingthe disoosed of in a landfill desienated for toxic chemicals. The remaining solutions can be gushed down the drain. Literature Cited 1. Alyea. H. N. in %(pd Jkmonatmtionr in Chemlatq, 6th ed.; Dutton, F. B., Ed.; Jovmal of Chemical Education: Eaaton, PA 1965:p 19. Chpm.Educ. 1953.32.9 3. Alyea, H.N. J ChpmEduc 1911,54, 166161. 4. Lambert, J. L: Fins,G.T J Chcm.Educ. 1984,61.1031-1038. 6. Kemo M. ~ K: Houee. ~ J ChpmtmA , ~ 1979.21CL215. ~. ~ ~ ~ , ~ 6. Anon. Chpm 13Npws.Nowmber 1976. -ACoilection of Chemical Demanatrstions'.
2. Alyea, H.N.J
~
~
~~
~~
~
HgI, + 1- + Hg13-or H ~ I ? or both (clear)
(4)
If the solution remains orange, add a little more of the iodate solution, solution C. The problem is not that the excess of hydrogen sulfite is too small. Rather, there is not enough iodate present to be converted to give enough iodide. Thus, the 1- concentration is not high enough to convert all the HeL to the clear com~lex. Method 5. -&equal volumesbfsolutions Aand B with half the volume of solution C (oranee onlv). There is no .excess iodide formed to produceeithG I2 or"~g1~-.
9. Suwnerh, L.; Ealy, J. L.Chpmlml J k m * t i m ; American Chemical Saieh.: Washingtrm,DC, 1985:Vol. 1,p 77. 10. Mos8,A.J ChpmEduc. l978,55,2&245. 11.Shslthashui, B. C h i d Domonsfmtiwrs; Universityaf Wiamnain Reas: Madison. WI;Vd. 4, in press. 12. Haight, G.,U.IUincis, pemndeommvnicatim, 1989.
13. Chureh, J. k: Dreakin, S. A. J Phys. Chpm. lW. 72,1387. 14. ShaWlashiri,B. C h i d Lkmomtmtlons; Univplsity ofWiamnain h e : Madiean, WI, 1983:VoL1,p 274. 15. Summerlin, L. a,;Bomfard, C. L.; Esly, J. B.Chmicci &monsfmtiom; American Chemical Society: Waahhgbn, D.C.,1981;Vol.2, pp 220-221.
Volume 69 Number 3
March 1992
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