Free energy functions and criteria for equilibrium

The application of the second law to chemical equilibrium is facilitated by the introduction of two more state functions. These are defined as (a) the Helmholtz free energy 

Arguments based on the second law which are given in standard text books [3,4] lead to the conclusion that, on its own, a chemical reaction will proceed in the direction in which the Gibbs free energy decreases and, when equilibrium is reached, this quantity has a minimum value. More precisely, for a system undergoing some small change 

At this stage it is worth re-emphasizing that, although thermodynamics can furnish information about the direction of a chemical change, by its very nature it can say nothing about the rate at which the change will take place. 

---