Second Law of Thermodynamics

According to the second law of thermodynamics, all spontaneous or natural processes produce an increase in entropy of the universe.

ΔS_total = ΔS_univ = ΔS_system + ΔS_surroundings > 0

Thus, for a spontaneous process when a system is at equilibrium, the entropy is at maximum, and the change in entropy is zero.

ΔS = 0 (at equilibrium)