Free Energy and Equilibrium

Both the equilibrium constant and the standard free energy change can be used to determine whether a reaction is product or reactant favored.

For any reaction mixture composition, the ΔG for the reaction is the sum of the standard free energy and RT times the natural log of the reaction quotient.

When the reactants and products are at equilibrium, the free energy change is zero, and the reaction quotient equals the equilibrium constant. So the standard free energy change equals −RT ln K.

If ΔG naught is less than zero, ln K is positive, meaning K is greater than 1. In this case, product formation is favored at equilibrium. The larger the equilibrium constant, the more negative the free energy.

Take, for example, the breakdown of dinitrogen tetroxide at 298 kelvin, in which K is 1.34 × 10¹⁷.

Substituting the known values into the equation, the standard free energy for the reaction equals −98 kJ/mole, and product formation is favored.

Conversely, if ΔG naught is greater than zero, ln K is negative, meaning K is less than 1 and the reverse direction of the reaction is favored.

Consider the breakdown of sulfur trioxide gas at 298 kelvin, which has a ΔG naught of 141.6 kJ/mole.

The equa