Kinetics in Solution

Abstract

The main equations used to extract thermochemical data from rate constants of reactions in solution were presented in section 3.2. Here, we illustrate the application of those equations with several examples quoted from the literature. First, however, recall that the rate constant for any elementary reaction in solution, defined in terms of concentrations, is related to the activation parameters through equations 15.1 or 15.2. Equation 15.1 yields the enthalpy and the entropy of activation respectively from the slope and the intercept of a ln(k/T) versus 1/T plot (an Eyring plot). Equation 15.2 leads to the Arrhenius activation energy and the frequency factor, respectively, from the slope and the intercept of a ln k versus 1/T plot (an Arrhenius plot). All the parameters refer to the mean temperature of the plot, and Δ‡Ho is related to Ea by equation 15.3. Finally, recall that if the activation parameters are available for the forward (subscript 1) and the reverse (subscript −1) reaction, the enthalpy of this reaction is calculated by equation 15.4. In the preceding chapter on equilibrium in solution, it was pointed out that any analytical method suitable for determining equilibrium compositions of a reaction mixture at several temperatures can be used to obtain the enthalpy and entropy of that reaction. A similar statement can be made here: Any analytical method suitable for monitoring concentration changes with time at several temperatures can be used to derive the activation parameters of a reaction. Therefore, the analytical techniques used in equilibrium experiments are also applied in nonequilibrium (kinetics) studies. However, in this case, the choice of the analytical method will have an additional and important restriction, for it must consider the reaction rate. An instrumental technique suitable for determining the concentration of a given species under equilibrium conditions may be inappropriate for determining a fast concentration change of the same species.