Isokinetic analysis of reaction onsets

Abstract

Kinetic studies of chemical reactions typically assume a constant fractional conversion of reactants to products, α, at a particular time, t, or temperature, T, during the reaction. This constant (iso) conversion approach cannot provide both Ea and A of the Arrhenius rate constant, k = A exp[-Ea/RT], without specifying an algebraic model or value for the reaction mechanism, f(α), or its integral F(α), which are rarely known a priori. A new isoconversion rate approach eliminates the reaction model from consideration by specifying f(α) =1 at the reaction onset, so that the rate of conversion, dα/dt = k, is the same in isothermal and nonisothermal experiments. This equal (iso) kinetic approach reduces the kinetic problem to the relationship between time and temperature at the reaction onset, and allows for the determination of Ea and A for the initial step of a potentially complex chemical reaction by measuring the change in the onset temperature Tonset with heating rate, β = dT/dt, in nonisothermal experiments, or the change in onset time tonset with temperature in isothermal experiments. In a test of the new theory, oxidation onset times of the reaction of hydrocarbons with oxygen at constant temperature (tonset) were calculated for a wide range of materials and experimental conditions using isothermal and nonisothermal values for Ea and A, and found to be in quantitative agreement with experimental values of the oxidation induction (onset) time (OIT).