Classes of Free Energy Relationship

A free energy relationship is defined by Equation (2) where the parameter a is called the similarity coefficient. 

The term AG is the free energy of a process such as a rate or equilibrium and AGs is the free energy of a standard process, often an equilibrium, which could be the process under investigation or some other standard reaction. 

Equation (2) holds for many processes over relatively large changes in free energy and is usually written in terms more directly related to 

Class I free energy relationships compare a rate constant with the equilibrium constant of the same process. The Bronsted and Leffler equations (see Chapter 2) are examples of this class of free energy correlation. 

The rate or equilibrium constant in Class II is related to the rate or equilibrium constant of an unconnected but (often) similar process. Class II free energy relationships are in general more common than those of Class I because equilibrium constants are more difficult to measure than rate constants (except in certain cases such as dissociation constants). The Hammett equation is the best-known Class II free energy relationship. 

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