Spontaneity
The tendency of a process to occur without any continuous external input of energy.
Formula / equation
ΔG < 0
Unit
kJ/mol
In depth
Spontaneity is decided by the balance between enthalpy and entropy at the temperature concerned. It says nothing about rate: a spontaneous reaction may be immeasurably slow if its activation energy is high. A process with ΔG = 0 is at equilibrium, and one with ΔG > 0 requires driving.
Examples in the real world
Rusting is spontaneous but takes years; the diamond-to-graphite conversion is spontaneous but effectively frozen.