What Is a Reaction Rate?

Experimentally, the rates of reactant consumption or rates of product formation are measured. These rates are related to, but different from, reaction rates. A reaction rate is a property of a given reaction, not of chemical species. The relationships between these two kinds of rates are not always simple. In a homogeneous closed reactor, a correct definition of a reaction rate is (dξ/d t )/ V, where V is the reactor volume and ξ the extent of reaction. Open systems require a different approach to the concept of reaction rate and there is no simple equation that can be used as a general definition in all kinds of reactors. We propose a definition leading always and simply to the correct equations.