THE SELF-HEATING EFFECT IN THE PROCESS OF KMnO4 DECOMPOSITION IN VACUUM

It has been shown that the exothermic decomposition of KMnO4 in vacuum is accompanied by a significant self-heating effect. It manifests itself in the reduction of the molar enthalpy of the reaction, determined by the third-law method. In comparison with the value 138.3 +/- 0.6 kJ mol(-1) that is valid for the decomposition of KMnO4 under atmospheric pressure at 493-553 K, the molar enthalpy in vacuum (10(-4)-10(-7) bar) at 484-511 K varies in the range of 136 to 126 kJ mol(-1). This is related to the reduction of the thermal conductivity of residual air in the furnace and, as a result, to the self-heating of the reactant, that accelerates the exothermic reaction. A simple method of evaluation of the self-heating effect has been developed. By analogy with the known method for evaluation of the self-cooling effect, it is based on the determination of the molar enthalpy by the third-law method at two different temperatures. The increase of sample temperature above the furnace temperature T-f in the case of the exothermic decomposition of KMnO4 in a high vacuum and T-f >= 490 K reaches a few tens of degrees.