GEORGEL. GILBERT Denison University Granville. Ohio 43023
Demonstrating the Chemistry of Air Pollution S U B M ~ ~ BY EO
J. Leland Hollenberg Unlreslly M Redlands Redlands. CA 92373 Edgar R. Stephens and J a m e s N. PHs, Jr. Slateulde All Pollullon Research Center Unwersllv of Calltoma Riverside, CA 92502 CHECKED BY
Luther K. Brice, Jr. Virginia Tech Blacksbum. -. VA 24061 For many years we have performed several demonstrations t o illustrate some of t h e most imoortant chemical reactions involved in air pollution. These relate t o t h e three essential conditions or inaredients for t h e formation of phot, oxides, a n d hytochemical smog, n a m e ~ y s u n l i ~ hnitrogen drocarbons. Our demonstrations involve N O or NOz, unsaturated hydrocarbons, a n d 0 3 . Those not familiar with t h e various processes involved in air pollution should consult a good reference' or a n annotated h i h l i ~ g r a p h y . ~ , ~ Here are three of our favorite demonstrations t h a t have proven simple a n d ahle t o hold audience interest. Hold onto Your Socks An unpleasant side effect of t h e ozone accompanying photochemical smoe is t h e raoid deterioration of rubber oroducts. As a resuit, elastic-in socks loses its elasticity a n d hreaks, a n d sidewalls of tires develop cracks. Sufficient 0 3 for this demonstration a n d t h e following one can h e generated while t h e audience watches. Directions Flush the air out of a 1000-mL Erlenmeyer flask with Oz gas, and, cap it with a stopper. This can be done hours before the demonstration if neeessarv. When readv" to eenerate OX. . the lower half of the nask is wrapped with aluminum foil, which is grounded t o a cold water pipe. Remove the stopper and quickly insert a copper wire, the end of which is formed into a spiral for greater surface area, and replace the stopper. The portion of the wire formed into a spiral should be at least 6 in. in length to increase the rate of productionof ozone. Adjust a Tesla coil toproduce a strong spark. Touch the tipof theTesla coil to the upper end of thecopper wire to moduce a silent diseharee for about 30 s. Undue. the Tesla coil. ~ u k k l yremove the copper wire and cap the flask with another stopper wrapped in aluminum foil. To show the effect of 0 3 on rubber, stretch a rubber band on a frame made of brazing rod or other stiff wire. Drop this stretched rubber hand into the flask containing the 03/02 mixture, and quickly replace the stopper. The rubber band is seen to deteriorate rapidly and usually breaks in less than 1min. Warning: Keep the flask containing O3 a t arm's length at all times.. and avoid breathing.. the eas. .. After the demonstrations are finiihd, fill Ih* flask with ThRtCT in a fume hood. Inhaling smnll quantilies 01 O1 rat, irritnre the lungs and can make brcnrhing painful for several Itourr. The Federal Air Qunlity Standard is 0.12 ppm (by volume). Instant Smog One of t h e processes contributing t o t h e haze t h a t accompanies photochemical smog is t h e reaction of ozone with hydrocarbons. If t h e hydrocarbon molecules a r e unsaturated, t h e reaction with 0 3 goes roughly 100 times a s rapidly a s
with saturated hydrocarbons. Lemon peel provides a convenient source of a n unsaturated hydrocarbon, limonene. Directions With a knife, peel a segment of the rind from a lemon, as if you lwere preparing a martini. Remove the stopper from the flask of 031 Oz used in the rubber band demonstration. Twist the lemon peel sharply, drop it into the flask and replace the stopper. Within seconds, wisps ofwhite cloud form that are easily seen, and soon the entire flask is hazy. The haze is probably the result of O3 attacking the double bonds in the limonene, converting it to earboxylic acids, which due to their low vapor pressures undergo spontaneous nucleation and condense to visible liquid aerosol particles. Be careful to avoid leaving your audience with the impression that this is the sole or even the most important source of outdoor haze. But it daes make the important point that visihle aerosol haze can be generated from invisible pollutants ( 0 3 and limonene vapor). Warning: Stay at arm's length and avoid breathing the 0 3 / 0 2 mixture. Donot addany substances toosother than thosedescribed here to avoid the risk of an explosion. Note: If Oz is not readily available, a flask of air produces sufficient 0 3 to produce "instant smog" with limonene, provided the Tesla coil use is extended to about 3 min. This method daes not produce sufficient O3 to break a rubber band, even after 20-min exposure. NO, Colorless NO gas is produced i n small quantities by t h e reaction,
N,
+0 2
-
ZNO
which accompanies any combustion process. For preparing this demonstration, a lecture bottle of N O is very convenient.
,.
~ireciions In'a fume hood, flow NO gas through a 250-mL sampling bulb (for example Kimax #30040-250). The NO should first he passed through an absorption tube packed with Ascarite or 1SX Molecular Sieve to remove any NOz. (See the description hy Shakhashiri for other procedures that demonstrate the properties of NO. An alternative method of removing NOzfrom the NO is to hubble it through water.) Warning: NOz is quite toxic. A fume hood must be used for filling the gas bulb with NO. Duringsubsequent demonstrations, keep the apparatus at arm's length and avoid breathing any NOn. After all the air has been flushed out, shut off the NO supply, and quickly close both stopcocks on the sampling bulb. When ready to perform the demonstration, connect a large syringe containing about 50 mL of air to one end of the bulb of NO. Open the connecting stopcock and inject the air, keeping the other stopcock closed. Brown NO2 forms from the reaction, NO
+ %02
-
NO2
Under tvoical .. atmosoheric conditions, this reaction requires houri instead of s e w n d ~hrenuse ofthe murh lower NO concrntrarim. Autucrhausl, for rxnmple, contains up ton feu thuusand ppm of NO 2nd unly a frw pprrpnt d mde~ularoxygen. During thr short
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Shakhashiri, 6. 2. Chemical Demonstrations; University of Wisconsin: 1985: Vol. 2, p 163. Moore, J. W.; Moore. E. A. Environmental Chemistry; Academic: New York, 1976. Moore J. W.: Moore, E. A. J. Chem. Edoc. 1978, 53, 167. Volume 6 4
Number 1 0 October 1987
893
