Explain entropy and spontaneity of reactions.

(N/A) Entropy $(S)$ is a measure of the degree of randomness or disorder in a system. For a spontaneous process,the total entropy change of the system and surroundings must be positive,i.e.,$\Delta S_{total} = \Delta S_{sys} + \Delta S_{surr} > 0$.
Spontaneity is determined by the Gibbs free energy change $(\Delta G)$. $A$ reaction is spontaneous if $\Delta G < 0$. The relationship is given by $\Delta G = \Delta H - T\Delta S$.
For an exothermic reaction $(\Delta H < 0)$,if $\Delta S > 0$,the reaction is spontaneous at all temperatures. If $\Delta S < 0$,it is spontaneous only at low temperatures.
For an endothermic reaction $(\Delta H > 0)$,if $\Delta S > 0$,it is spontaneous at high temperatures. If $\Delta S < 0$,the reaction is non-spontaneous at all temperatures.