Kinetics and mechanism of electron-transfer reactions of aqueous and co-ordinated thallium(III). Part X. Kinetics of reduction of hexa-aquathallium(III) by hydrogen peroxide and induction of the reaction by cerium(IV) and iron(II) ions.
Abstract
Thallium(III) and H2O2 react in aqueous perchloric acid solution as in equation (i). The kinetics of reaction have TlIII+ H2O2→ Tl I+ 2H++ O2(i) been followed by estimating TlIII iodometrically and the rate law for [HClO4]= 0·5–2·0M and I= 0·5–2·5M is –d[TlIII]/dt=k′[TlIII][H2O2]2/(Kh+[H+]). Here k′ is the apparent rate constant, which probably includes a formation constant also, and Kh is the acid dissociation constant of Tl3+(aq); k′ has the value 6·7 ± 0·5 l mol–1 s–1 at I= 2·0M and 30 °C, and the energy of activation is 10·3 ± 0·7 kcal mol–1. The reaction is independent of ionic strength in the range 0·5–2·5M(LiClO4). The reaction is induced by cerium(IV) ions through the intermediate radicals H2O2+ and/or HO2˙, and the induction factor is unity. Iron(II) ions act as a strong initiator for the reaction, and hydrogen peroxide can be determined in their presence using thallium(III) as a primary standard.