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Title: Kinetics of Oxidation of N, N'-donor Chelating Complexes of Iron (II) by Bromate
Authors: Summer, Shazia
Keywords: Physical Sciences
Issue Date: 2017
Publisher: Jinnah University for Women, Karachi
Abstract: Kinetic investigation of oxidation of N, N’-donor chelating complexes of Fe (II) i.e., 2, 2-bipyridine and 4, 4-dimethyl, 2, 2-bipyridine by bromate has been undertaken. The aforementioned complexes were selected for comparative study to investigate the influence of molecular (reactants and activated complex) complexity, increasing size and the substituted group on the chemical reactivity of redox reaction. Thus, the study helped to establish the validity of Marcus theory and Electron Tunneling Theory for the selected redox systems. Both of the redox reactions were found to follow comparable kinetic behavior under the fixed pseudo-first order circumstances and accordingly an identical rate equation and mechanism is proposed. Both reactions are suggested to follow an outer-sphere mechanism. The kinetics of the oxidation reaction of [Fe (2, 2′-bipy)3] 2+/ [Fe (dmbpy)3] 2+ by bromate ion (BrO3 – ) in aqueous medium was probed spectrophotometrically in the pH range 3.5-5.0. For both reaction systems, pH of the reaction system was maintained by NaCH3COO- CH3COOH buffer system. Whereas, Na2SO4 was used to maintain the required ionic strength from 0.01 to 0.1 mol/dm3 for [Fe (2, 2′-bipy)3] 2+ - BrO3 – reaction system and from 0.003 to 0.1 mol/dm3 for [Fe (dmbpy)3] 2+ - BrO3 – redox system. Abstract Page II The pseudo first order rate constant was found independent of the concentration of [Fe (2, 2′-bipy)3] 2+/ [Fe (dmbpy)3] 2+. However, the rate increases with increasing concentration of bromate ion in both reaction mixtures upto the saturation point at high concentration. This behavior is indicative of a precursor complex formation and also an outer-sphere mechanism. Protonation of BrO3 – in the acidic medium is suggested which yields monoprotonated (HBrO3) and diprotonated (H2BrO3 + ) species. The rising figure of the rate constant upon lowering the pH refers to the involvement of protonated species in the rate-determining step. In addition to that, a rise in the value of the rate constant with increasing ionic strength implies, however, the diprotonated species, H2BrO3 + , as the reactive species. Involvement of the other Br(V) species results in a complex reaction pathway. Thus, the rate data demonstrates that a general rate law is followed by the two reaction systems and that the rate equation emerges is reproduced below:  ( )    (1 [ ] min [ ] min [ ] 2 3 2 3 2 2 3 2 1 3 + + + + + = + K H BrO k K dii e H BrO Rate k Fe dii e HBrO i p i p Redox reaction between [Fe ( 2, 2′-bipy) 3] 2+ and BrO3 - The values of rate constants k2 are calculated to be 0.182 and 0.112 dm3 mol-1 s -1 respectively while, the value of Kip is calculated to be 3.55×106 and Abstract Page III 2.93×106 dm3 mol-1 respectively . The values of activation energy and thermodynamic parameters are estimated by using Arrhenius and Eyring plots respectively and established to be; Ea (20.54 kJ mol-1 ), A (149.28 s -1 ), ΔH≠ (18.29 kJ mol-1 ), ΔS≠ (-210.84 J mol-1 K-1 ), ΔG≠ (81.54 kJ mol-1 at 300 K). Redox reaction between [Fe (dmbpy) 3] 2+ and BrO3 - The values of k2 are calculated to be 5.59×10-3 and 5.77×10-3 dm3 mol-1 s -1 respectively. Whereas, the values of Kip are calculated to be 1.37×106 and 1.46×106 dm3 mol-1 respectively at 300K. The activation parameters are established as; Ea (18.23 kJ mol-1 ), A (7.45 s -1 ), ΔH≠ (15.69 kJ mol-1 ), ΔS≠ (-236.65 J mol-1 K-1 ), ΔG≠ (86.68 kJ mol-1 at 300 K).
Gov't Doc #: 22161
Appears in Collections:PhD Thesis of All Public / Private Sector Universities / DAIs.

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