The sign and magnitude of Gibbs free energy is a criterion of spontaneity for a process. When ∆G>0 or +ve, it means Gproduct >Greactant the reaction will not take place spontaneously,i.e. the reaction should be spontaneous in reverse direction. SnO2→SnO;∆G>0 +4+2 (more favourable ) ∆G<0 or -ve, the reaction or change occurs spontaneously. PbO2→PbO;→∆G<0 +4+2 (more favourable)