A reaction is spontaneous if the Gibb's Free Energy of the reaction is negative. If, the enthalpy, and, the entropy, are both negative, then the reaction will be spontaneous if and only if the magnitude of the enthalpy is greater than the magnitude of the entropy times the temperature. When the reaction is exothermic (negative ΔH) but undergoes a decrease in entropy (negative ΔS), it is the enthalpy term that favors the reaction. In this case, a spontaneous reaction is dependent upon the TΔS term being small relative to the ΔH term, so that ΔG is negative. If the total entropy change is negative, the process is a non-spontaneous process. A process will be spontaneous when the randomness of the molecules present in the system increases. For a spontaneous process entropy change of the system plus surrounding must be positive.