A net chemical reaction proceeds until there is no change in free energy when a molecule reacts.

Imagine a system where two reactants are mixed, and these react to form products. The reason the reaction takes place, expressed in thermodynamic terms, is that the product chemical species have a lower free energy than do the reactant chemical species. Reaction continues so long as this is true.

What about a case where the reactants and products have similar free energies? What happens when the free energy change is small? Concentrations always play a role in determining free energy. The more dilute a species, the lower its free energy. At the outset of a chemical reaction, when no product species are present, there always is a significant free energy contribution from being the first molecule to appear in the neighborhood, so to speak. As reaction proceeds, however, the concentration of the products increases, and this effect becomes ever less favorable. Conversely, as the reactants are consumed and their concentrations are lowered, there is a diminished advantage to lowering the concentration of the reactant side.

These effects often are called entropy effects. Very dilute solutions afford increased entropy for newly arrived molecules as compared to concentrated solutions. A general rule to keep in mind is that, all other things being equal, chemical species strive to lower their concentrations to 0. (The trick is, of course, that all other things usually are not equal!)

Consider a simple imaginary chemical reaction, A B

For A, G_A = G¡_A RT ln a_A

For B, G_B = G¡_B RT ln a_B

ΔG =( G¡_B - G¡_A) RT (ln a_B -ln a_A)

When there is no change in free energy, ΔG = 0,

So, we often see written: ΔG¡ = -RT ln {a_B/a_A}

But, those activities are the equilibrium concentrations of A and B.

So, this usually is written as ΔG¡ = -RT ln K_eq.

When the numerical value of the mass action expression is less than K_eq, there is a thermodynamic driving force for net forward reaction. When it is larger than K_eq, there is a net driving force for the reverse reaction.

This graph is a plot of free energy(G) as function of the extent of the reaction.

If ΔG has a large positive value then the RTlnQ term will require a small amount of product formation.

Salzman has a more complete thermodynamic treatment.