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 Tf in the case of the exothermic decomposition of KMnO4 in a high vacuum and Tf≥490 K reaches a few tens of degrees.