Gibbs Free Energy (ΔG) predicts reaction spontaneity using the equation: ΔG = ΔH - TΔS. A negative ΔG means the reaction is spontaneous (happens on its own). This value combines enthalpy (ΔH), which is heat change, and entropy (ΔS), which is disorder. The reaction is spontaneous only at certain temperatures if ΔH and ΔS have the same sign.