MARCH, 1952
0
A NEW EXPERIMENT ON REACTION RATES IN GENERAL CHEMISTRY GORDON G. EVANS Tufts College, Medford, Massachusett.
A SURVEY
of several laboratory manuals in general chemistry shows that three experiments are in use to illustrate the effect of concentration on reaction rate. These are: (1) the rate of evolution of hydrogen by the reaction of a metal and an acid, (2) the rate of liberation of sulfur from an acidified thiosulfate solution, and (3) the "iodine clock." None of these can be considered ideal for the purpose. In the case of the first, only one of the reactant concentrations can be varied; in the case of the second, the kinetic dependence is not a simple bimolecular one,' and the opacity of the resulting sulfur suspension, by which the rate is usually measured, is dependent on certain factors in addition to the concentration of sulfur. The 'icloclc" reaction, though apparently the most popular, is subject to the most serious objections, some of which have been pointed out by Hered.z It should also be noted that the time elapsed is that required for exhaustion of the sulfite ion, and therefore it is not possible to use the reaction to show dependence of the rate on the sulfite concentration; in those instances where the reaction is complete sooner a t a higher initial concentration of sulfite, another factor (acid catalysis) is operating which is concealed from the student. The reaction between persulfate ion and iodide ion appears to be free from all these objections. The reaction is bimole~ular,~ the concentrations of both reactants can be varied, and the effect of depletion of the reactants can be minimized by observing only the first part of the reaction. The time required for the development of a predetermined depth of color serves to establish the reaction rate. Concentrations which have been found
convenient are 0.1 M (N&)28208 and 0.2 M KI. If a solution containing iodine (0.001 M) and potassium iodide (0.03 M) is used as a color standard, about two minutes will elapse from the time when equal volumes of the reagents are mixed until the color of the reacting mixture matches that of the standard. The effect of concentration can of course be shown by diluting one or the other reagent before mixing. In the experience of the author and his colleagues, students' results in this experiment have been generally quite satisfactory. Two drawbacks to the experiment have become evident. The first is the cost of the reagents; unfortnnately, the solutions cannot be much more dilute than is stated above without prolonging the reaction time inordinately. However, the cost may be offset to some extent if the students' mixtures are collected in a large jar; the further progress of the reaction leads in a few days to the exhaustion of the potassium iodide and the deposition of crystals of iodine, which can be recovered. The other drawback is the difficulty of observing the point of exact matching of the colors. Students have a tendency to anticipate this point, and continuous close watch of the test tubes fatigues the eye; it is well to instruct that the tubes be compared every 15 seconds (not oftener) and that observations be continued until the reaction tube is definitely darker than the standard, whereupon the time when the colors matched may be determined by interpolation. Finally, it must be noted that the ammonium persulfate must be pure and the solution freshly prepared; otherwise an immediate liberation of iodine may take place, rendering rate measurements useless.
LA MER, V. K., AND R. H. DINEQAR, J . Am. Chem. Soc., 72,4847 (1950); T A ~ EH., , ibid., 65,527 (1943). HERED,W., J. CAEM.EDUC.,27,542 (1950). a M E R ~ ~ JA,, A ,Ann. A d . Sci. Fennicae, [A] 2 , Chem. No. 24 (1947) (Chem. Absl~a&, 42, 2163 (1948)).
ACKNOWLEDGMENT
The author wishes to express his gratitude to Professor Robert D. Eddy for his cooperation in testing the experiment.
