Jan., 1952
KINETICSO F EXCHANGE OF WATER BETWEEN SOLVENT h N D Cr(HzO)B+++
it can be assumed that the slow step is removal of the chelating group from ferric ion. This removal is facilitated by Parallel processes take ions which associate with Y". place Kith various types of cations-H +,Fe+++, FeOH++-
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functioning. -4test of the role which the metal ions play would be to study the rate of exchange in the presence of an ion such as Al+++ which is not involved in the net exchange process.
OBSERVATIONS OF THE KINETICS OF THE EXCHANGE OF WATER BETWEEN Cr(HzO)s+++AND SOLVEXT BY ROBERTA. PLANE' AND HENRY TAGBE The George Herbert Jones Laboratory, University of Chicago, Chicago, Illinois Received August SO, 1361
The rate of the eschange Cr(Hz0)6+++-H~0 has been found to be first order in Cr(HnO)r+++,and to increase with the concentration of anion. For solutions at varying high salt concentration, the rate varies directly with concentration of anion. The anions decrease in effectiveness for promoting the exchange in the order NOS-, Clod-, C1-, Br-. With CI- as the only anion present, the rate of exchange greatly exceeds the rate of formation of the ion Cr(H20)&1++. The activation energy for the exchange in the perchlorate system is 24 f 2 lical./mole and the p z factor ea. 6 X 1013 1. mole-' min.-l. The rate of exchange is markedly increased by Cr++, slightly by Cr20,; and is induced by the reaction of Cr+++with Ce+4. The effect of Cr++ is greatly enhanced by C1-.
Some observations on the rate of exchange of water between hydrated cations and solvent have already been p ~ b l i s h e d . ~ ~The 3 . ~ ions investigated were Al+++, Cr+++,Fe+++,CO+++,CO++,Ga+++ and Th(1V). Of these, only Cr+++ showed a measurably slow rate of exchange of the bound water with the solvent. A few results were obtained' on the kinetics of exchange in the chromic ion-water system, but the investigation was by no means complete. In the present work, a study in some detail of the kinetics of water exchange for the hydrated chromic ion was undertaken. The results of the work already referred to proved that chromic ion in the presence of acid and perchlorate ion exists as the species c r ( H @ ) ~ + + + .It seemed of interest to learn whether the exchange of water with the solvent resembles that of other complex ion substitution reactions in its kinetic properties such as rate law and activation energy. The hope that the results of the investigation would have a direct bearing on the problem of the mechanism of substitution in hexacoordinated complex ions provided an additional stimulus for undertaking the work. Besides the aspects of general interest referred to, the system Cr(HzO)e+++-HzO presents some of specific interest. The system can serve as a tool in the study of the rate of electron transfer between Cr(HaO)e+++and chromium in other oxidation states. If the rate of exchange of water between Cr++ aq. (for example) and solvent is rapid, Cr++ aq. must increase the rate of exchange between Cr(&o)~,+++ and H20a t least to the extent fixed by the rate of electron transfer between Cr++ aq. and Cr(H20)6+++. The catalytic effect exerted in the Cr(H20)6+++-Hz0 exchange mill therefore set an upper limit on the rate of electron transfer. The investigations of this type can be extended also t o unstable oxidation states of chromium, by studying the Cr(H20)b+++-Hr0exchange induced by oxidis ing agents acting on Cr+++aq. or of reducing agents acting on CrOd-. (1) A.E.C. Predoctoral Fellow, 1950-1951. (2) J. P. Hunt and H. Taube. J . Cham. Phya., 111, 757 (1950). (3) H.L. Friedman. H. Taube and J. P. Hunt,
