RON
edited by: DELORENZO
Middle Georgia College Cochran. Georgia 31014
Auto Analogies
With this institution's location in Car Country, the use of analogies between chemical concepts and automohiles seems appropriate. Since most students have direct experience with automobiles, such analogies should facilitate the understanding of chemical concepts. Four such analogies are presented here.
significant decrease in probability of such a collision occurring as the number of particles involved in the collision increases. The analogy t o assist in understanding this decrease in probability is the automohile accident. The prohahility is rather high for an automohile accident involving one automobile, uniauto, or two automohiles, biauto. The prohability of three or more automobiles colliding simultaneously, terauto, etc.. -~~~ is verv small. Therefore. almost all accidents are uniauto or biauto. Although the numbers of multiple automobile accidents are sienificant..thev. are most likelv a series of uniauto and/or biautg collisions.
Electron Density
Rate-Determining Step
The understanding of where electrons are in an atom is often difficult for thestudent since their locations cannot be specified exactly if their energy is to be measured with some accuracy, which is essentially a statement of Heisenberg's Uncertaintv Princinle. The best descri~tionof these locations is the prodability bf finding electrons a t given points. This electron nrobabilitv is then translated into a three-dimensional electron-density map or the electron rluud. To assist students in undt:rsrandinr"the orobahilitvof finding moving. objects a t some location a t some point in time, the following automobile analoev can he used. At night - a camera is set uo on a freeway overpass, and the shutter is opened for some period of time. The resulting picture shows streams of lights on the freeway. The intensity of light can be correlated with the probability of finding automohiles at various locations a t some point in time; automohile density. The greatest density would he in the lanes, hut there is significant density in between lanes due to lane changing. 1f the shutter were left open long enough, the picture could show an automobile off the freeway as the result of a breakdown, accident, or an intoxicated driver; a very small but non-zero probability. This analogy may help a student understand that there are certain regions in space around the nucleus where the electron is m a t likely to he found, others where the probability is less, hut even a t significant distances the prohahility is not zero.
For a multinle-sten reaction mechanism. the rate of the overall reaction is dependent on the rate of the slowest step, the rate-determining step. This dependence is due to most chemical reactions consisting of steps that are very different (powers of 10) in their rates: fast or slow. The analogy to help the student understand this concept of rate dependence is an automobile trio from the suburbs to the center of the city during rush hour. This trip can be divided into a number of s t e ~ s drivewav, : suburban street, local road, service drive, expressway, ser&e drive, city street, parking lot. How fast one makes this trip is probably dependent on how had the expressway is jammed up, the rate-determining step. Times for each step can be exaggerated to make the dependence clearer and to more closely approximate the large differences in individual step rates of chemical reactions. For example, if the trip of 50 miles requires 2 hours and if those steps other than the expressway are less than a mile in length and require only a few minutes each. then the overall s ~ e e is d about the same as the speed on theexpressway.
Richard A. Potts University of Michigan-Dearbarn Dearborn. MI 48128
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Reaction Mechanisms
How chemical reactions actuallv occur is ex~lainedhv describing the reaction mechanism. ~ a c step h in ihe mech&ism involves only a few species: unimolecular, bimolecular, termolecular, etc. Almost all reaction steps are unimolecular or bimolecular, with a few being termolecular, because of the
This feature presents a collection of descriptive applications and analogies designedto help students understand some of the difficult concepts frequently encountered in chemistry. Contributionsthat will pr* duce a greater application and knowledge of politicel, religious, e m nomic, historical, and scientific aspects of life are encouraged.
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Kinetic and Thermodynamic Factors of a Process
T o better understand any chemical or ohvsical, the . Drocess, . . . student must realiee that two factors control that process: thr kinetics and the thermodvnamic factors. The rhrrmodynamic factor depends only on the initial and final states of the process to determine its driving force while the kinetic factor involves the mechanism of the process. T o illustrate these factors, the analogy of an automohile trip from one place to another can he used. A very significant part of any trip is where it begins and where it ends: initial and final states. If there is a need for the trip, then the arrival at the destination satisfies that need, the driving force. However, the time required to reach the destination and what happens along the way depend on the route, the rate and mechanism. The several possible combinations of kinetic and thermodynamic factors can be pictured in this analogy. For example, a trip to hear a rock concert, a spontaneous process, is never completed because of a traffic jam, a very slow step. This analogy will hopefully aid students in understanding that both factors, kinetic and thermodynamic, must he considered for all processes. Volume 62
Number 7
July 1985
579
