OLD AND NEW STUDIES OF THE THERMAL-DECOMPOSITION OF POTASSIUM-PERMANGANATE

The overall chemical equation representing the thermal decomposition o f potassium permanganate up to almost-equal-to 300-degrees-C is given approximately by: 10 KMnO4 --> 2.65 K2MnO4 + [2.35 K2O. 7.35 MnO2.05] + 6O2, the bracketed material being delta-MnO2. The experimental mass loss in air is almost-equal-to 12% and the enthalpy of decomposition i s almost-equal-to 10 kJ/mol of KMnO4. Analysis of published kinetic st udies of the decomposition show that most of the results can be repres ented by the Prout-Tompkins equation In (x/(1-x)) = k(T)t + constant, and insertion of the rate constants into the Arrhenius equation gives an activation energy for decomposition of almost-equal-to 150 kJ/mol o f KMnO4. Although the kinetic studies have always been interpreted in terms of a single type of chemical decomposition, with the different r ates encountered during the course of the decomposition ascribed to ph ysical effects, X-ray diffraction studies by Boldyrev and co-workers h ave shown that the reaction actually occurs in two stages, with essent ially all the KMnO4 transformed into K3(MnO4)2, delta-MnO2 and O2 in t he first stage, and the K3(MnO4)2 then decomposing into K2MnO4 and mor e delta-MnO2 and O2 in the second stage. We have confirmed the Boldyre v diffraction results and extended them by measuring the kinetics of t he appearance and disappearance of K3(MnO4)2 by an X-ray diffraction m ethod. Our earlier isotope studies have shown that the oxygen molecule s come from oxygen atoms produced by breaking Mn-O bonds in different permanganate ions i.e. the decomposition mechanism is interionic. We c onclude by summarising what is, and is not, currently known about the thermal decomposition of potassium permanganate up to almost-equal-to 300-degrees-C.