VOLUMES OF ACTVATION FOR THE REDUCTION OF CHLOROPENTAAMMINE-COBALT (III) BY IRON (II) FROM HIGH PRESSURE SOLUTION KINETICS: THE INFLUENCE OF AN IRON (D)-SULPHATE ION PREEQUILIBRIUM

SURAPONG SUVACHITTANONT

Summary: The reduction of [Co(NH₃)₅Cl]⁺² by ferrous ammonium sulphate has been studied at ionic strengths (I) of 1.0, 0.6 and 0.3 mol dm^-3. The rates of reaction at room pressure are greater than those reported previously in the reduction by iron (II) perchlorate.

The volumes of activation (v^T) are +35.3, +24.7 and -8.9 cm³ mol^-1 at I=1.0, 0.6 and 0.3 mol dm^-3 respectively. These are interpreted in terms of a rapid pre-equilibrium between iron (II) and sulphate ion to form the iron (II) sulphato complex followed by a rate determining step in forming the bridged intermediate with the [Co(NH₃)₅Cl]²⁺ via either the iron (II) sulphato complex or the hexaaquoiron (II) complex. The v^T of +35.3 and +24.7 cm3 mol^-1 are consistent with the reactions proceeding predominantly via the iron (II) sulphato complex while the reaction whose v^T =-8.9 cm³ mol^-1 tends to proceed via the hexaaquoiron (II) species.

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Department of Chemistry, Prince of Songkhla University, Haadyai, Thailand.

Received 25 August 1977