How much may the equilibrium be shifted in a chemical relaxation experiment?

Abstract

A simple method for determining the largest permitted displacement of an equilibrium in a chemical relaxation experiment is described. The rate equation can always be put in the form –d Δ[Xi]//_dt=τ^–1Δ[Xi](1 +r), where r is a dimensionless parameter which can be positive, negative or zero. This parameter depends on the extent of the perturbation, on the equilibrium constant and on the concentrations of the species involved in the reaction. It is related to Δ[X_i] by the fundamental equation r=v_irΔ[X_i] where r depends only on the equilibrium constant and the concentrations, v_i being a stoichiometric coefficient. Convenient expressions for r are given for the most common unimolecular and bimolecular reactions. The new linearization condition is given by |r| 1. This condition is generally satisfied, whatever the equilibrium, whenever the displacement of the equilibrium is small compared to the equilibrium concentrations. However, in many cases, the above condition is also fulfilled even if the shift in the equilibrium is large (low r values). The case of the dimerization reaction 2A ⇌ A₂ is considered in detail.