David M. Goodall,' Paul W. Harrison, Michael J. Hardy, and C. Jane Kirk The University of York Heslington,
York,
England
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I
Relaxation Kinetics of Ferric Thiocyanate A temperature-jump and f l a s h - p h o f ~ / ~study ~k
The blood red color of ferric thiocyanate is commonly used in qualitative demonstrations of complex formation (1, 2) and an experiment is described in THIS JOURNAL using Job's method to find the formula of the complex (3). The complex is produced as soon as ferric and thiocyanate solutions are mixed, and it is not possible by reducing the concentrations of the components to reduce the rate sufficiently to bring it into the range for kinetic study using conventional colorimeters or spectrophotometers. This article describes two perturbation methods for determining the kinetics and mechanism of the reaction. A non-equilibrium situation is generated by (1) a concentration perturbation achieved by Bash photolysis of the equilibrium mixture (2) a perturbation in the equilibrium position, brought about by a temperature jump.
The flash-photolysis apparatus is described in the accompanying article (4),while details of the temperaturejump accessory are given here. The simplicity of these experiments makes them attractive for teaching the techniques of fast-reaction kinetics to students in their physical chemistry practical course. Also, the comparison between two experimental approaches to the same kinetic problem is very instructive. Ferric thiocyanate has not previously been studied using flash-photolytic techniques, but the results of studies by temperature jump (5), pressure jump (6), and rapid mixing methods (7) are available to check the performance of this teaching apparatus. Theory
Reaction (1) is the simplest representation of the equilibrium between ferric and thiocyanate ions and their complex. The ratio kl& is the equilibrium constant K,, which has the value 139 1mole-' at 29X°K and ionic strength 0.5 mole kg-' (8)
The euthalpy change in the reaction is -5 kJ mole-'. Since
we can calculate that under the conditions of our expcriments (ferric and thiocyanate ions in great excess over complex) a 2'K rise in temperature reduces the concentration of the complex by l'%. Figure l a shows the
Pertvbmiar
Time
appled here Figure 1. Response of ferric-thiocyanatr rolution to various perturbations: lo) Temperature jump from temperotvre h to T*. lbl Flash photolysi$ a t T1 under conditions where [Feai] and [SCN-] [FeSCNX+].Icl Flarh [FeSCN2']. photalyrir ot Ta under conditions where [Feat] or [SCN]
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