Thermoanalytical and kinetic degrees of conversion in the application of general rate equation

Kinetics of condensed-phase processes are routinely studied by methods based on the general rate equation. It is demonstrated here that, for the kinetic analysis, two degrees of conversion should be employed: (i) the degree of conversion used in classical kinetics, α_kin, conventionally defined as the reacted amount of a reactant normalized to its initial amount; (ii) the thermoanalytical degree of conversion, α, defined as the thermoanalytical effect observed at temperature T (or at time t for isothermal measurements) divided by the total thermoanalytical effect. For elementary reactions, α_kin = α so that the general rate equation is a true rate equation describing the mechanism of the reaction. For complex processes, α_kin and α differ considerably in general; they are equivalent for some special cases only. In this case, the general rate equation represents the single-step approximation. The values of α thus describe the kinetics of heat exchange (for DSC) or mass loss (for TG) and so do the kinetic parameters obtained from the treatment of experimental data. Even though no mechanistic conclusions should be drawn from such kinetic parameters, they still enable us to model the kinetics of complex processes from the point of view of the quantity measured.