Thermal decomposition of sodium carbonate perhydrate in the presence of liquid water

Abstract

A kinetic study has been made of the decomposition of sodium carbonate perhydrate, Na2CO3· 1½ H2O2, in the presence of small quantities of added water at 323–343 K. Reactions were deceleratory throughout and rates in the later stages were further reduced when the quantity of water available was insufficient to permit complete initial dissolution of the reactant solid. Rate coefficients measured for these reactions were compared with similarly determined data for the probable contributory processes. These were the decompositions, in saturated aqueous Na2CO3, of (i) H2O2 and (ii) Na2CO3· 1½ H2O2. From the pattern of behaviour observed it was concluded that the reaction of Na2CO3· 1½ H2O2 in water proceeds in two stages: heterogeneous dissolution of the reactant crystallites is followed by the homogenous breakdown of H2O2 in solution. This mechanism is distinct and different from the vacuum decomposition of the solid. It is concluded that the rate of the homogenous breakdown of H2O2 is probably controlled by catalytic processes involving transition-metal ions present in solution as impurities. This conclusion is supported by the observation that the present reaction was inhibited by added sodium silicate. The kinetics and mechanisms of these reactions are discussed.The heterogeneous reaction investigated involved both a solid reactant and intermediates dissolved in the added liquid water. This combination of reactants has hitherto been the subject of relatively few detailed kinetic studies. Separate investigations of the individual steps which contribute to the overall change, in this particularly favourable system, has led to the identification of a simple reaction mechanism that is entirely consistent with the observations. The approach demonstrates the value of using complementary rate measurements to characterize the kinetics and mechanism of this decomposition involving both solid and dissolved participants.