GEORGEL. GILBERT Denison University Granville, Ohio 43023
Temperature Effect on Reaction Rates Susulrrto sv:
Robert Eliason and Terence McMahon Southwest State University Marshall, MN 56258 CHECKEDBY:
Allan M. Schoffstall University of Colorado at Colorado Springs Colorado Springs. GO
Durinn the first two years of a student's exposure to college chemistry, the subjects of reaction rates, temperature eCfects on rates, and activation eneray come up. The student is usually told that increasing the temperature of a reacting system causes the system to react faster, and, in fact, the general rule of thumb that the reaction rate doubles for every ten degree rise in temperature is most likely gwen at the same time. The nhenomenon is exolained theoreticallv with the concept of activation energy. A demonstration has been developed which nicely illustrates the temperature effect on reaction rates and the general rule relating temperature increases with rate increases. Also from the demonstration students can gather data from which an activation energy can be calculated. The demonstration involves the reaction of benzoyl peroxide and aniline.' The system is heterogeneous and decomposes suddenly to give a puff of white smoke. Procedure Into 3-in. test tubes place about 1cm of benzoyl peroxide. Place the test tubes in expanded polystyrene cup temperature baths to which water of the desired temperature has been added. The temperature range should be between 5" and 50"C.2 Allow the test tubes to equilibrate for about 10 min. Add two drops aniline; stand back and wait for the smoke pufL3 At 50°C the reaction is very fast; so, one must move quickly. If the demonstration is to be used quantitatively, the instructor should record the temperature of the water bath just before beginning the aniline addition. The students, using any convenient timepiece which shows seconds, record the starting time (in seconds) at the addition of aniline and the finished time (in seconds) when the smoke appears. A 10' rise in the temperature decreases the reaction time by about half the previous value. The reaction times range from -2 seconds a t 50°C to -40 seconds a t 5". An Arrhenius plot of ln(1ltime) versus lltemperature may be constructed. We have found that the Arrhenius activation energy, E,, is 11-12 kcallmole which is close to E, = 10.6 kcallmole predicted theoretically from using the above rule of thumb.
'Bailey, P. S., Bailey, C. A,, Anderson, J., Koski, P. G., andRechsteiner, C., J. CHEMEDUC.52,524 (1975). ZWehave performed the demonstration at ODC(icewater mixture) and have found that the reaction occurs very slowly and sometimes fizzles instead of decomposing instantaneously. Also, points at O°C or below fall badly below an Arrhenius plot. 3When we have used the demonstration qualitatively, the aniline was added to all the test tubes sequentially starting with the coldest first. 4Steere,N. V., (Editor)"Handbook of Laboratory Safety," 2nd Ed., The Chemical Rubber Co., Cleveland, OH.
354
Journal of Chemical Education
Safety Precautions The decomposition is mildly explosive and, therefore, the test tubes should never he corked a f t e r t h e aniline addition. The smoke has a rather distinctive, unpleasant odor so the demonstration should be performed in a well ventilated room. Benzoyl peroxide is a hazardous substance t h a t should he handled with care; it explodes a t -68' when heated, and it may decompose violently when handled with a metal ~ p a t u l a . ~
Spectacular Gas Density Demonstration Using Methane Bubbles SUBMITTED BY
Robert Snipp Bruce Mattson Winters Hardy Creighton University Omaha. NE 68178 CHECKEDBY
Kenneth W. Watkins Colorado State University FORCollins. CO 80521
An- unforeettahle demonstration of the relative densities ~ ~~-~ - ~ "~~ of gases can be performed by creating giant methane bubbles with the aid of a small funnel and tov"soar,.bubble solution. A rubber tube is used to connect the stem of a small (35 mm ID) general purpose filtering funnel to a natural gas outlet. Giant soap bubbles can be formed by momentarily immersing the inverted funnel into a beaker of the soap solution. As soon as the mouth of the funnel is covered by a soap film, the funnel is removed, and the methane is allowed to inflate the bubble to 10 cm in diameter. The bubble is dislodged by turning the funnel upright and applying a gentle shake. Once free of the funnel, the bubble will slowly rise and can he ignited by means of a candle. The bubhles burn producing a blue flame in mid-air. The effect is somewhat eerie and most impressive. ~
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submitting or checking demonstrations should contact the column editor. An outline of format requirements was given on p. 166 of the March 1976 issue of THIS JOURNAL. George Gilbert's interest in chemistry demon~trationstraces from Hanover High School (Mass.) through undergraduate work at Antioch College (BS 1958)and his thesis at Michigan State University (PhD 1963)Additional areas of interest in chemical education include seif-pacing. TIPS, and project~stylelaboratories. He consults with science museums and is active in fostering science-awareness in oublic arenas. Recreational interests include canoe-tripping, backpacking and outdoor activit~es.He suffers from frequent anacks of paranomasla which is best treated by studied forbearance.
