Pulsed-accelerated-flow studies of the temperature dependence of fast reactions

Abstract

A pulsed-accelerated-flow (PAF) spectrometer (model V) capable of non-ambient temperature studies of fast reaction kinetics is described. The PAF method uses accelerated flow mixing of reactants during short time periods to enable the resolution of mixing and reaction rate constants. A new mixing/observation cell and cell supports are designed to permit measurement of reaction kinetics from 40 °C to below 0 °C. The cell consists of two machined PEEK [(–OC₆H₄OC₆H₄COC₆H₄–)_n] pieces joined together to give an internal solution distribution system, which greatly reduces the number of connections needed compared to previous instruments to bring the reactants together. The reaction between W(CN)₈⁴⁻ and IrCl₆²⁻ in 0.50 M H₂SO₄ is studied at 0.0, 25.0, and 40.0 °C. Second-order rate constants of 0.650 × 10⁸ M⁻¹ s⁻¹, 1.05 × 10⁸ M⁻¹ s⁻¹, and 1.29 × 10⁸ M⁻¹ s⁻¹ are obtained, respectively. These data give activation parameters of ΔH^‡ = 10.0 ± 0.8 kJ mol⁻¹ and ΔS^‡ = −58 ± 3 J mol⁻¹ K⁻¹. Activation parameters for reverse bromine hydrolysis (HOBr + Br⁻ + H⁺ → Br₂ + H₂O) were determined from rate constants measured from 0.0 to 40.0 °C. These were used to calculate the activation parameters for the forward bromine hydrolysis (ΔH^‡ = 66 ± 1 kJ mol⁻¹ and ΔS^‡ = 10 ± 20 J mol⁻¹ K⁻¹). The temperature dependence of the extremely rapid BrCl hydrolysis reaction (in equilibrium with BrCl₂⁻) is determined as well. For reactions at temperatures of 25.0 °C, 10.0 °C, and 0.0 °C the values are 3.3 × 10⁶ s⁻¹, 2.06 × 10⁶ s⁻¹, and 1.75 × 10⁶ s⁻¹, respectively. These values correspond to activation parameters of ΔH^‡ = 15 ± 7 kJ mol⁻¹ and ΔS^± = −71 ± 24 J mol⁻¹ K⁻¹ for BrCl hydrolysis.