Studies on the kinetics and mechanism of the complex formation between hexa-aquachromium(III) and phthalic acid

Abstract

Kinetic studies of the interaction of [Cr(OH₂)₆]³⁺ with phthalic acid have been carried out at 30, 35, and 40 °C, the hydrogen-ion concentration ranging from 0.25 × 10^–3 to 3.16 × 10^–3 mol dm^–3. The variation of the pseudo-first-order rate constant (k_obs.) with [H⁺] and ligand concentration indicates a mechanism based on ion-pair formation. The reaction is believed to be of the associative-interchange (I_a) type. Activation parameters calculated by the Eyring equation are ΔH^‡= 93 kJ mol^–1 and ΔS^‡=–5 ± 2 J K^–1 mol^–1 for the formation of [Cr(OH₂)₅(O₂CC₆H₄CO₂)]⁺, and ΔH^‡= 94 kJ mol^–1 and ΔS^‡=–2 ± 1 J K^–1 mol^–1 for the formation of [Cr(OH₂)(O₂CC₆H₄CO₂H)]²⁺.