Kinetics of the reaction between Manganese Dioxide and Ferrous Ion

The kinetics of the reaction between manganese dioxide and ferrous ion in acid solution have been investigated by using the potential of the ferrous-ferric couple as a measure of the extent of reaction. The experimental conditions were such that the reaction rate was independent of ferrous, ferric, manganous ions, and acid concentrations and the agitation was sufficient to prevent bulk diffusion in the solution from being a rate-determining factor. The reaction rate of sized samples of pyrolusite and γ-MnO2 in ferrous sulphate solution was proportional to the surface area of the solid and was constant (i.e. " zero-order ") until 50 per cent. of the solid was consumed. γ-MnO2 reacted about twice as rapidly as the pyrolusite. The reaction occurred most readily at certain active sites on the particles and appeared to proceed along crystal boundaries in such a manner that the active surface area was not significantly changed during the first half of the reaction. In ferrous perchlorate the reaction rate of 10 μ diameter pyrolusite was about one-hundredth of that in sulphate and the reaction appeared to occur at a more even rate over the whole surface of the particle so that the zero-order law was no longer obeyed. Activation energies of 7.4 and 5 kcal in sulphate and perchlorate respectively, for the temperature range 18 to 40 �C, suggest that the difference in rate is a result of a change in the entropy factor of the Arrhenius equation. It is suggested that this difference in rate may result from the activation, by sulphate ions, of a less reactive lower oxide of manganese which is formed on the surface.