An Experiment on Homogeneous Catalysis Juan A. Coch F w o n l . Marllene Zeoka. . .Alvaro Rocha FlawIra, - and h a S a b i C~ a m p Universidade de Rio Grande, Rio Grande, R. Brasil -
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The experiment described here shows, by color changes, the chemical participation of the catalyst and its regen& tion when the reaction is complete. I t also provides proceduresfor measurine the reaction rate both f& secondGv and tertiary levels. If d h t e d KzCrzO, and H202are mixei in a test tube, the orauae color chanees to hrown and oxven evolutionstarts. once the reaction is complete, the colo; of the solution changes back t o oranee. If we add more HIO1 the same are observed once again. All these facts show that KzCr207 decomposes Hz02 catalytically. The brown color, nicely indicates a chemical interaction between the catalyst and the HzOz.The experiment can be presented to secondary school pupils with a very simple equipment shown in Figure 1. A plastic tray filled with water and a thermometer serve as a thermostatic bath and a 10-ml glass syringe, without needle, as a reactor. In a twical ex~erimentoerformed a t E°C. 2 ml ~ with 0 2~1;11 of~ r e of 0.1 N ~ ~ ~ rmixed 3% HzOz and 6 ml of Hz0 in a small beaker partially immersed in the bath; 2 ml of the mixture and then 6 ml of air were transferred t o the syringe, which is then placed horizontally a t the bottom of the tray. After five minutes, when bubbling becomes regular, successive time intervals corresponding to five bubbles are counted. When the number of bubbles is plotted against time, a straight line is obtained, from which the rate of the reaction, expr&ed as bubhles per minute was found. A new run was ~ e r f o r m e ddoubline the concentration of the catalyst by miking 2 ml of the pe;oxide, 4 ml of dichromate, and 4 ml of water. The results were dottad as before. comparing the slopes it was concluded that the rate is proportional to the concentration of the catalyst. When working a t higher temperatures, bubbling must be adjusted to about 10 bubbles per minute by reducing concentrations; otherwise, the volume of the solution transferred to the syringe will be less than 2 ml due to the presence of much hubhles of oxygen. As a connequence~the oxygen evolution rate, which is proportional to rhat volume. will he lowered introducinea hir error. In the ohvsical chem~~~~istry course at our university, the experiment is performed with the atmaratus described bv McGarvev and KniDel.. shown sche~aticallyin Figure 2. Chanaine and H101 concentrations in different - - K0Cr707 runs a t constant iroim) temperature, i t is found that the reaction follows the rate law
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respect t o Hz02 with temperature, from second order a t 56 O C to zero order a t 0 "C. All these facts give support to the following mechanism described in detail in a textbook on chemical kinetics and catalysisz, faat
K,Cr,O, orange
+ 2H20, K
2Hz0 + KzCr20, brown
In order to justify zero order a t lower temperatures, 0 OC in our case, the first step must be exothermic.
Figure 1. Assembly far measuring oxygen evolution rate farsecondary school courses. T, plastic hay; S, syringe: B, beaker containing the reaction mixhire: Th, thermometer.
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from which k may be calculated. The quantitative regeneration of the catalvst a t the end of the reaction is studiedby meawring absorbances at 350 nm. For best results, the H202 solution mu$t he DreDared from chemically pure Hz02 free from stabilizers Gesent in commercial preparations, which will reduce the dichromate. Merck'ii Pcihidrol was used in our experiments. C a r e must be taken with Perhidrol becauseit iscorrosive. An interesting property of this reaction is the change of the order
' McGarvey, J. E. 8.: Knipe A. C. J. Chem. Educ. 1980.57.155,
Panchenkov, G. M.; Lebedev. V. P. "Chemical Kinetics and Catalysis"; MiR Publishers: Moscow. 1976; pp 380-383.
U Scontin~o~~ mnitwina of the volume of oas evolved in Fiaure 2. A . O. O ~ ~ for a chem ca react on ' ST. three-way stopcock: G. 3 bmm-bore glass tube (-80 cm longl: M, mercury bead: T, plast c tray: Th, mermometer. F, reactm flask: S, syringe; R, meter rule (scale in mi). :
Volume 63 Number 6 June 1986
549
