Change in Gibb's free energy of system (ΔG) = ΔH - TΔS.........(Eq. 1) Now, if magnitude of ΔG <0, then reaction is spontaneous. if magnitude of ΔG > 0, then reaction is non-spontaneous. At equilibrium, ΔG = 0 When at boiling point, liquid state is in equilibrium with vapour state. Hence, it present case ΔG = 0