The esterification reaction of acetic acid with isoamyl alcohol is an equilibrium-limited chemical reaction:
Although the mechanism of such reactions is well known 12, a literature survey shows that different kinetic models have been proposed to correlate kinetic data 13–20. In general, reported rate laws can be written as in Eq. (1):
Based on Eq. (1), several kinetic models with adjustable parameters can be obtained to represent the rate of the considered esterification reaction. In Eq. (1), ϕ denotes a concentration variable, typically the mole fraction, molar density, or activity of each substance involved in the reaction. Owing to the differences in the molar densities of pure substances, the three approaches are not equivalent. Furthermore, if considering only a nonrandom two liquids (NRTL) activity coefficient model (see the Appendix) for the computation of activities in the liquid phase, it is possible to say at first that there are three different choices for ϕ in the kinetic models. Second, exponent α in Eq. (1) represents the autocatalytic effect of acetic acid in the esterification reaction. Theoretically, it can be equal to 0 (no autocatalytic effect), 1.0 (autocatalysis due to nondissociated acid), or 0.5 (autocatalysis due to dissociated acid) 21. As a fourth option, exponent α may be considered as an adjustable parameter in the rate law expression. Typically, the rate constant k1 presents an Arrhenius-type dependent on temperature and can be expressed as in Eq. (2) with two adjustable parameters 11:
where T0 is a reference temperature, in this case 363.15 K and Ea is the activation energy for the reaction. On the other hand, the equilibrium constant Keq in Eq. (1) may be estimated from either a correlation found by Wyczesany from a reinterpretation of the scarce data available in the literature for this reaction 22, or it can be regarded as an adjustable parameter itself. In the last case, it can be either assumed to be temperature independent or expressed as in Eq. (3), which has two adjustable parameters and is derived from Van't Hoff's equation when a constant heat of reaction Δ Hrxn is assumed:
Thus, there are three choices for the equilibrium constant. It must be noticed that, as pointed out by Wyczesany 22, thermochemical data are not reliable to estimate the equilibrium constant in esterification reactions.
Therefore, in Eq. (1), three possibilities for ϕ, four choices for exponent α, and another three independent cases for the equilibrium constant, Keq, were considered. Thus, 36 kinetic models, listed in Table II, will be compared on the basis of experimental data to describe the rate of reaction between acetic acid and isoamyl alcohol.