applications and analoqies Pictorial Analogies XIII: Kinetics and Mechanisms John J. Fortman Wright State University Dayton, OH 45435 The Analogies to kinetics and mechanisms abound (1-7). illustration of the analogies in this article help fix the ideas in the students' minds a s well a s helping them to understand the abstract concepts. I n a first-order process the half-life is a constant. Students have difficulty under-standing how one half of a ton of something can react in the same time a s one half gram of it. The trick is to get them to focus on the fact that since there is more to start with, there is more t h a t can be changing simultaneously. Figure 1 compares the nine months that it takes one couple to have one baby to the nine months that it takes five couples to have five babies. First Order Kinetics HOW can one hail of a n n dsomething react in the same time as one hail Of a gram of it?
edited by
RON DELORENZO Middle Georgia College Cochran. GA31014
The time is the same, but since you started with five times as many couples you can produce five times as many babies. Auto collisions have often been compared to the bimolecular collisions in mechanisms (8).The probability of collisions increases with the number of cars in a n area (Figs. 2 and 31, just like the effect on reaction rates by increasing concentrations. Increasing speeds of the cars will increase the probability of collisions for the same concentration (Figs. 4 and 51, just like increasing the temperature increases reaction rates because it increases the speeds of the molecules, decreasing the time between collisions. Part of presentations at the 5th Biennial Conference on Chemical Education, Fort Collins. CO, July 25, 1978, Proceedings, pp 6 7 ;the 9th Biennial Conference on Chemical Education. Bozeman. MT Julv 26, 1986; and the 1951" Nal onal ACS Meel ng an; 3ro &emid Congress of honn Amer~ca,Toronlo. Canada Jme 8 . 1988
Higher Concentration, Faster Rate
\
OrnonlhS
& 5-
c
4.. 7 baby
More cars, collisions more likely
If there are more la start, more can res
Figure 1. Analogy for the constant value of the half life of a first order reaction regardless of starting amount.
Effect of Concentration on Rates Cars in a parking lot
, Few can, collisions less likely Figure 2. Analogy to low reactant concentrations.
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Journal of Chemical Education
Figure3.Analogyto highreactantconcentrations.
Effect of Temperature on Rates Slow Cars in Lot
Longer time for collisions fewercollisions per time Figure 4. Analogy to low reaction temperatures.
Higher Temperature, Molecules Move Faster
Molecules Must Collide with Proper Orientation for Reaction
Fast Cars in Lot
Shorter time till collisions, more collisions per time Figure 5. Analogy to high reaction temperatures
Higher Temperature, F a s t e r Molecules, More Energy
Collisions more likely to cause more damage - bumper transfer Figure 6. Analogy to high temperature collisions being more likely to cause reactions. Faster speeds will also increase the damage to cars in collisions that do occur (Fig. 6) a s for instance making a ''bumper transfer" more likely just a s atom transfers may be more favored in more energetic collisions. I n other words, high temperatures in reactions also supply more
Bumper transferrequires head on collisions. not front to side Figure 7. Analogy to the effect of the orientations of molecules on reactions. energy to overcome activation harriers increasing the likelihood of the contact resulting in a reaction, such a s a n atom transfer. The orientation of the cars in a collision will also affect the resulting damage, just a s the orientations of collidina molecules affect the ~ossibilitvof a reaction a s shown Figure 7. Thc! condition of I he cars ma" also affect the outcome, iust hke the condition of honds in rrnctina molecules may affect the reaction. Copies of these illustrations will be sent on request. Acknowledgment I wish to thank the artist..Bruce Stiver. of Media Production Services at Wr~ghtState for his hard work and talent in producing all rhe illustrations in this series and the chemistry department ibr the financ~alsupport rcquircd. I also wish to thank Jackie Ciever ibr t". w i n e a11 of the manuscripts.
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Literature Cited 1. White,A. D. J Chem.Educ. 1981.58, 645. 2. Bonneau,M. C. J. Chem Educ 1986.62. 519. 3. Bal1.D. W J. Chem. E d w 1985.6% 579. 4. Last. A M . J. Cham. Edue 1983,60, 748. 5 . La8t.A.M. J. Chem. Edue 1985,62,1015. 6. Krug, L.A. J. Ckem. Edue 1987, M, 1000. 7. O l n e ~0. J . J Cham Educ 1988,65. 696697. 8. Coley W. Presented st the 9th Bienmal conference on Chemical Education, Bozeman. MT, July 26.1936.
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Number 10 October 1994
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