For spontaneous reaction ΔH=−ve, ΔS=+ve Related Theory When the reaction is exothermic (negative △H ) but undergoes a decrease in entropy (negative ΔS), it is the enthalpy term which favours the reaction. In this case, a spontaneous reaction is dependent upon the TAS term being small relative to the ΔH term, so that △G is negative. The freezing of water is an example of this type of process. It is spontaneous only at a relatively low temperature. Above 273 K, the larger TΔS value causes the sign of △G to be positive, and freezing does not occur. Caution Students usually confuse the term spontaneous with the notion that a reaction occurs rapidly. A spontaneous reaction is one in which product formation is favoured, even if the reaction is extremely slow.