The physics and chemistry of a self-sustained reaction between iron and potassium permanganate

Abstract

Self-sustained reaction between reduced iron powder and finely powdered potassium permanganate in the proportions 80/20 by weight has been investigated by recording temperature changes as the reaction front passes a point. Velocities of propagation calculated from these records by means of the thermal theory agree well with directly observed velocities. It is therefore considered that this theory is a good representation of the behaviour of this composition, and that it may be used to calculate reaction rates from temperature records. Reaction rates so obtained have been expressed as functions of the absolute temperature and the extent of reaction. It is found that the kinetics at low density (loose powder) are very different from those at high density (hard pellets); and that the difference can be explained on the hypothesis that at low density the reaction takes place by way of the gas phase, and at high density, where contact is better, by diffusion across solid-solid interfaces. It is thought that the mobile species at low density is O 2 , at high density Fe2 + . Support for this view is obtained by measuring the cooling curves of the products and by calculating the atomic heats of the products from observed final temperatures and heats of reaction. Thermal and X-ray data are used in an attempt to formulate the chemical change. In the experimental section, methods of measuring temperatures, thermal conductivities (three methods) and heats of reaction are described.