Free energy change and the equilibrium constant

Like ΔH° and ΔS°, ΔG° applies to processes where all substances are present in their standard state (solutes in concentrations 1 mol L^–1) or for gases at pressures of 100 kPa).

ΔG can be calculated when reactants and products are present in other concentrations or at other pressures using the relationship shown at the top at the right.
 
Nonstandard concentrations or pressures  are substituted into the reaction quotient Q

At concentrations when ΔG equals 0, the system is at equilibrium and Q = K. Substituting these conditions into the relationship at the top gives a second important relationship that can be used to calculate K for a reaction if ΔG° is known.
Note for reactions having
 

• K > 1, ΔG° is negative
• K = 1, ΔG° = 0
• K < 1, ΔG° is positive