Kinetic studies on the reaction of ethylenediaminetetraacetatochromium(III) complex with hydrogen peroxide |
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Authors: | Emil N Rizkalla Samy S Anis Maged S Antonious |
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Institution: | Faculty of Science, Ain Shams University, Abbassia, Cairo, Egypt |
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Abstract: | The rate of reaction of Cr(III)Y]aq (Y is EDTA anion) with hydrogen peroxide was studied in aqueous nitrate media μ = 0.10 M (KNO3)] at various temperatures. The general rate equation, Rate = Cr(III)Y]aqH2O2] holds over the pH range 5–9. The decomposition reaction of H2O2 is believed to proceed via two pathways where both the aquo and hydroxo-quinquedentate EDTA complexes are acting as the catalyst centres. Substitution-controlled mechanisms are suggested and the values of the second-order rate constants k1 and k2 were found to be 1.75 × 10?2 M?1 s?1 and 0.174 M?1 s?1 at 303 K respectively, where k2 is the rate constant for the aquo species and k2 is that for the hydroxo complex. The respective activation enthalpies (ΔH*1 = 58.9 and ΔH*2 = 66.5 KJ mol?1) and activation entropies (ΔS*1 = ?85 and ΔS*2 = ?40 J mol?1 deg?1) were calculated from a least-squares fit to the Eyring plot. The ionisation constant pK1, was inferred from the kinetic data at 303 K to be 7.22. Beyond pH 9, the reaction is markedly retarded and ceases completely at pH ? 11. This inhibition was attributed in part to the continuous loss of the catalyst as a result of the simultaneous oxidation of Cr(III) to Cr(VI). |
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Keywords: | Author to whom correspondence should be addressed |
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