edited by: RON DELORENZO Middle Georgia College Cochran. Georgia 31014
Doing the Dishes: An Analogy for Use in Teaching Reaction Kinetics Arthur M. Last Sir Wilfred Grenfeli College Corner Brook Newfoundland,Canada A2H 0P9
Analogies can be of considerable use when trying to explain chemical phenomena t o students, and the area of reaction kinetics is no exception to this generali~ation.'.~ One published analogy1 uses a crowd hurriedly leaving a large stadium through a single turnstile to explain the rate-determining step of a reaction. While useful, this analogy is of limited appeal to me, as many of my students come from small, isolated communities3 and have never experienced the large crowds that we associate with football and baseball stadia. T o help these students with the concept of the ratedetermining step, I have developed an analogy between an everyday situation, doing the dishes, and a two-step reaction. Having introduced students to the idea that a large number of reactions occur in a series of steps taking place at different rates rather than in one single step, I then ask the class t o consider a two-step reaction of the type
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Reactants Intermediate Intermediate Products
Reaction coordinate
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Figure 1. Reaction In which the second step has the higher activation energy.
(1) (2)
Clearly, two possibilities exist. In case (l), the formation of the intermediate is a rapid step with a low activation energy. The second step, in which the intermediate is converted into products, has a higher activation energy and is, therefore, the rate-determining step. An energy diagram for such a reaction is shown in Figure 1. The second possibility, case (21, is one in which the first step has the higher activation energy and is, consequently, slow and rate-determining. Once formed, the intermediate is rapidly converted into products in a step having a low energy of activation. Figure 2 shows the energy diagram for a reaction of this type. Grasping the idea that a reaction may occur in a series of steps, and that the overall rate of the reaction is controlled by the slowest of these steps, can he difficult for some students. I try to alleviate the prohlem by encouraging students to think of the above two-step reaction as being analogous to two people tackling the after-dinner chore of doing the dishes. This task can he envisaged as being a two-step process, washing and drying,
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Dirty dishes Clean, wet dishes Clean, wet dishes Clean, dry dishes
I
Reactants
Reaction coordinate
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Figure 2. Reaction in which the first step has the higher activation energy.
The dirty dishes are analogous to reactants; the clean, wet dishes represent the intermediate; and the clean, dry dishes represent the product. If the person washing the dishes is very quick and the person drying is slow, intermediate (clean, wet dishes) will accumulate and the drying process will he the rate-determining step. That is, the drying step determines the rate a t
The overall process then is Dirty dishes
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Clean, dry dishes
' White, A. D., J. CHEM. EDUC.,58, 6 4 5 (1981).
Last. A. M., J. CHEM. EDUC..6 0 , 7 4 8 (1983). Webb, M. J., Last. A. M.. Perkins, R. R., and Rayner-Canham, G. W., J. Coil. Sci. Teach., 12, 4 3 7 (1983).
This feature presents a collection of descrlptlve applications and aneIOgieS designed to help students understand some of M e difficult concepts frequently encountered in chemistry. Contributionsthat will prduce a greater appreciation and knowledge of political, religious, economic, historical. and scientific aspects of life are encouraged.
Volume 62
Number 11
November 1985
1015
which the complete job gets done. This is analogous to case (1)above. However, if the person washing the dishes is slow, as might he the case if he or she has to wash several foodencrusted casserole dishes, the person who is drying will have no difficulty in keeping pace and the dishes are dried as rapidly as they are washed. In chemical terms, the intermediate has a very short lifetime and is converted to product almost as soon as i t is formed. This is analogous to case (2) above. If the washing process is rate-determining, as in the second situation described above, the overall process can be accelerated if the person doing the washing uses a piece of
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Journal of Chemical Education
steel wool to scour the dishes. This is analogous to the use of a catalyst to lower the activation energy of the first step of the reaction described in case (2). The rate of the ratedetermining step is increased and the overall rate of reaction therefore increases. The use of a steel-wool scourer (catalyst) in case (1)would also cause an increase in the rate of the first step, but here no increase in overall rate would be observed since the rate-determining step is not affected. The analogy can he made more appealing by introducing a personal e1ement"When my spouse and I do the dishes .. ."-and can provide an enjoyable note on which to conclude a lecture on reaction kinetics.
