The
Second Law of Thermodynamics states that a process is spontaneous (product-favoured)
if the overall change in entropy (system + surroundings) is positive.
The overall entropy change is directly related to the change in free energy (ΔG).
ΔG = ΔH - TΔS
The free energy change can be calculated from the changes in enthalpy and entropy of the
system.
- If ΔG is negative, the process is spontaneous in the forward direction and is referred to as product-favoured.
- If ΔG is zero, the system is at equilibrium, and there is no tendency for the composition of the mixture to change.
- If ΔG is positive, the process is NOT spontaneous in the forward direction and is referred to as reactant-favoured.
The standard free energy change (ΔrG°) for a reaction can be related to the equilibrium constant for the reaction.
- If ΔG° is negative, ln K is positive, the equilibrium constant is greater than one, and the reaction is product-favoured.
- If ΔG° is zero, ln K equals 0, and the equilibrium constant equals 1.
- If ΔG° is positive, ln K is negative, the equilibrium constant is smaller than one, and the reaction is reactant-favoured.