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).
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. Reaction occurs until equilibrium is reached.
- 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 spontaneous in the reverse direction. Reaction occurs until equilibrium is reached.
Standard free energies of formation (Δ
fG°) and
free energies of reaction (Δ
rG) are defined similarly to Δ
fH° and Δ
rH with Δ
fG of elements in their standard state being zero.
Δ
rG° can be calculated using Δ
rG° = ΣΔ
fG°(products) - ΣΔ
fG°(reactants)
Like enthalpy, free energy is a state property, thus
if reactions add to give an overall reaction, their
free energy changes also sum to give the overall free energy change.