This article develops a modified version of the shrinking-core model accounting for the surface heterogeneity of solid particles. This model can be primarily used as a shortcut method for estimating the dependence of the kinetic rates on the concentration of fluid reactants and specifically the reaction order in the presence of a polydisperse solid mixture. This approach is tested numerically for model reactions involving polydisperse systems and is applied to the dissolution of MnO2 in sulphuric acid solutions containing glucose as the reductant agent.