Effect of pressure and temperature on some kinetic and thermodynamic parameters of non-ionic reactions. Volume changes on activation and reaction

Abstract

The formulae currently used to calculate volume changes on activation, ΔV^≠₀ and reaction, ΔV^≠₀, are shown to be unsuitable even at pressures of ≈ 1 kbar, and calculations are shown to be sensitive to errors, owing to empirical coefficients present in the formulae.

The ΔV_p=ΔV₀[1–α ln (1 +βp)] relationship is substantiated and its validity in a broad pressure range shown, as exemplified by 23 non-ionic reactions of various types. The dependence of equilibrium constants on pressure is described by the formula RT ln (K_p/K₀)=–ΔV₀[(1+α)p–(α/β)(1 +βp) ln (1 +βp)], where, as a first approximation, α= 0.170 and β= 3.91 × 10^–3 bar^–1. A second approximation can be obtained with the help of the linear free energy relationship. The validity of these formulae for kinetic values is confirmed. The accuracy of ΔV^≠₀ and ΔV₀ calculations can be increased with the help of both these formulae and some criteria of the value of the computational procedure used. The method of calculation is illustrated by 56 Diels–Alder reactions; the values obtained are α= 0.170 and β= 4.94 × 10^–3 bar^–1.

As exemplified by 34 reactions of eight types, a linear dependence of ΔV on temperature has been shown to exist in the range of temperature variation up to 90°C. The values of κ=(∂ΔV/∂T)_p/ΔV are very close for all the reactions. We have formulated the rule that a 1°C increase in temperature results in |ΔV| being increased by (0.5 ± 0.1)%(at temperatures close to room temperature). The widespread nature of the compensation effect has been demonstrated, as well as the determining role of the entropy in the change of free energy with pressure. Formulae have been obtained to compute the enthalpies and entropies of reaction or activation under pressure from limited data.