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JOURNAL OF CHEMICAL EDUCATION
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RATES OF REACTION 1. A. CAMPBELL Oberlin College, Oberlin, Ohio
BEGINNING chemistry ordinarily includes two references to rates of reaction. The external factors such as concentration, temperature, catalyst, surface area, and intimacy of contact are discussed in terms of the kinetic theory, and these concepts are then applied to the discussion of equilibrium states and the methods which can be used to "shift" an equilibrium. Occasionally, a reaction may he pointed out as being slow, as are the common "clock" reactions, but seldom is any attempt made to study with any system some very common slow reactions and to attempt to ascertain the reasons for this slowness. This is, perhaps, just as well in most cases since the study of reaction kinetics, or rates, involves a study of the mechanism of reactions, a very complicated science. It might be noted in passing that probably 90 per cent of the mechanisms suggested in
books on beginning chemistry are either wrong by actual test, or represent guesses based on no actual kinetic experiments. The whole idea of presenting equilibrium constants in terms of competitive rates is open to attack a t this point since the mechanisms assumed in deriving the equilibrium constant are almost invariably incorrect. A typical example would be the equilibrium constant for the reaction of hydrogen with oxygen to form gaseous water. This equilibrium constant may be derived from the usual equation for the net reaction assuming a mechanism which supposes that the reaction of two hydrogen molecules with one oxygen molecule is being opposed by the reaction of two water molecules. The fact that the first of these mechanisms is known to be wiong is never mentioned, and the fact that the actual mechanism is extremely compli-
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ric task requiring both a high-energy collision and a favorable geometrical arrangement on collision and therefore a slow reaction. This may be demonstrated very readily by preparing two cylinders of water to which sufficient sodium hydroxide has been added t o give a deep pink with phenolphthalein. When carbon dioxide and sulfur dioxide are bubbled into the cylinders separately (preferably from tanks of the compressed gases) the cylinder into which the SO2 is bubbled decolorizes almost immediately, whereas the decolorization of the other is a much slower process. In fact, if the bubbling tube is removed after the first decolorization in the carbon dioxide cylinder becomes apparent a steady fading of the color mill be noticed. This, in spite of the fact that no additional COz is being added. Thus, the slow rate of reaction, as opposed t o solution, of the COz with the water is made very apparent and may be contrasted with the rapid rate of hoth solution and reaction between SO2 and water. The great influence of geometry on rate is readily apparent and the general factors may be summarized. A slow rate of reaction may be attributed t o (1) the necessity of breaking a strong bond or bonds, (2) the necessity of considerable atomic rearrangment, (3) the necessity of a very particular geometrical arrangement if collision is to produce reaction due to the lack of symmetry in a reagent, and (4) charge effects such as the repulsion of two negative ions or of the similarly charged ends of two dipoles.
cated involving such exotic molecules as OH and HOz is generally unknown. It is possible t o study a few common reactions and to make some correlations concerning rates without -becoming too involved in a discussion of detailed mechanisms, but rather with the general principles which differentiate between slow and fast reactions. The effectsof concentration, temperature, catalysts, surface area, and stirring are so commonly known that we shall omit them from discussion and try t o choose reactions in which they are constant. Reaction rate is now interpreted in terms of four effects: (1) the number of times the reacting particles collide per unit of time determined primarily by concentration and translational speed; (2) the force of each collision determined by temperature; (3) the probability of a geometric arrangement upon collision which favors the reaction; and (4) the probability that this favorable geometric arrangement (or activated complex) will decompose into the reagents again rather than continue the reaction. The reactions of COz,andSO2 with water may be compared to illustrate the effect of geometry on rate. COz hydrolyzes slowly and SOz hydrolyzes rapidly. I n terms of molecular geometry these two molecules differ primarily in that C02 is linear whereas SOz is angular. In hoth cases the hydrolysis products are triangulara plane triangle in carbonic acid and a triangular pyramid in sulfurous acid. The rearrangement of the atomic center in the case of the COz is a major geomet-
Periodic Chart
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The accompanying illustration shows an electrical periodic chart constnkted by James R. Irving and Tom Smith, Maine Township High School, Des Plaines and Park Ridge, Illinois. The main panel consists of a self-supporting frame in which is mounted a sheet of 25 X 28-inch plexiglas. The actual periodic table is painted on this transparent material with a translucent paint that glows when illuminated from behind. To accomplish this a plywood panel is mounted in the same frame a few inches behind this plexiglas and parallel to it. Individual 6-volt, 0.5-amp. flashlight bulbs are arranged on the plywood in jeweled pilot light assemblies directly in hack of each chemiral symbol and title heading painted on the front. Thus, by lighting these small lamps, any corresponding element desired may be made visible in the color of the light that is lit behind it. A compact remote control box serves this main panel with flexible connecting cables from a step-down transformer operated on 110 volts, a. c.
