Kinetics of Cu2+– Catalysed Redox Reaction of n-(2-hydroxylethyl) ethylenediaminetriacetatocobalt(III) with Hydrazine Monohydrate in Aqueous Acid

Abstract

Communication in Physical Sciences 2018, 3(1): 10-17

Received 24 November 2017/Accepted 23 July 2018

Authors: A. D. Onu, S. O. Idris & B. Y. Abiti

The kinetics of the Cu²⁺-catalysed redox reaction between n-(2-hydroxylethyl) ethylenediaminetriacetatocobalt(III) (hereafter [Co(HEDTA)OH₂]) with hydrazine monohydrate (hereafter N₂H₅⁺) have been successfully investigated in aqueous acidic medium, under the following conditions T = 298 ± 1 K, I = 0.5 mol dm³ (NaClO4), [H⁺] = 4.0 × 10^-3 mol dm^-3 (HClO₄), [Cu²⁺] = 5.0 × 10^-4 mol dm^-3 and λmax = 550 nm. Stoichiometric study revealed that two moles of [Co(HEDTA)OH₂] reacted with one mole of N₂H₅⁺.The rate of the reaction was first order with respect to [Co(HEDTA)OH₂] and [N₂H₅⁺], and second order overall, with k₂ = (9.81 ± 0.14) × 10-2dm³ mols^-1. The rate of the reaction was inversely dependent on [H⁺] but directly proportional to [Cu²⁺] and both plots of k₂ versus [H⁺]^-1 and k² versus [Cu²⁺] were linear from the origin, indicating a one term rate law with respect to both [H⁺]^-1 and [Cu²⁺]. Changes in ionic strength had no effect on the reaction rate and temperature dependent study gave values of activated enthalpy (ΔH‡) and entropy (ΔS^‡) as 82.89 KJ mol^-1 and 17.13JK^-1 mol^-1respectively. Spectroscopic and kinetic investigations indicated absence of detectable intermediate. Analysis of evidence adduced from the study favour the outer-sphere mechanism and it is proposed for the reaction.