Rational definition of rate of reaction. General method of expressing rate which leads to no anomalies in systems of changing volume

Abstract

The conventional way of expressing rate of reaction in terms of rate of change of concentration (e.g. –d[A]/dt) is shown to be inappropriate for systems in which any volume change accompanies the conversion. Thus for such liquid phase reactions, with or without inert solvent, it is demonstrated that the rate of decrease of reactant concentration is not equal to the rate of increase of product concentration, that the apparent external order of reaction depends on whether the rate is expressed in terms of reactant or product, and that the apparent internal order of reaction can not reflect the molecularity of the reaction process. A revised expression for rate of reaction is proposed; this defines the rate at any time as the product of the time derivative of the number of moles of a reaction component and the reciprocal volume of the system at that time, e.g.–(1/V_t)(dA/dt)_t. The above anomalies do not arise when rates are expressed in this way; it is therefore recommended that this revised definition of rate be generally employed in all kinetic studies.