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. 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.
Endothermic processes (Δ
H positive) may be
non-spontaneous at room temperature, but
if the entropy change (Δ
S) is positive, the process
may become spontaneous at
higher temperature.
The
temperature above which the reaction
becomes spontaneous in the forward direction may be
calculated from the relationship above by
setting ΔG equal to zero.