Spectrophotometric kinetic study demonstrates dual pathway oxidation of iron(II) by chlorine with copper(II) catalysis, suggesting iron(IV) and copper(I) reactive intermediates.
The kinetics of the oxidation of iron (II) by aqueous chloring has been investigated spectrophotometrically at an ionic strength of 1.00 M. In solutions with [H+] = 0.05 - 1.00 M, [Cl-] = 0.10 - 1.00 M, [Fe++] = 10-5 - 10-4, [Cl2] = 10-5 - 10-4 M, [Fe+++] 0 = 0.00 - 10-4 F, the rate law is d [Fe+++] / dt = k1 [Fe++] [Cl3-] + k3 [Fe++] [Cl2] ; k1 = 1510 (± 150) M-1sec.-1, k3 = 316 (± 30) M-1sec.-1, and R25/ [Fe++] [Cl2] = 109.47 exp (-9.320/RT). There is no apparent ferric inhibition of the reaction and the data suggest an iron (IV) intermediate. Small amounts of copper (II) catalyse the reaction and the data suggest a mechanism involving a copper (I) intermediate, since ferric ion inhibits the reaction. In solutions with [H+] = [Cl-] = 1.00 M, [Fe+++]0 = 0.00 - 4.40 X 10-5 M, and [Cu++]0 = 10-6 - 10-4 M, the data are consistent with the additional rate term: k11[Fe++][Cu++] (1/1 + k12[Fe+++] /k14[Cl2] k11 = 370 (± 20) M-1sec.-1, and k12/k14 = 0.14 (± 0.02) at 30.0°.
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James Hansen Crabtree (2026) studied this question.
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