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One way of determining if a process/reaction is spontaneous in the direction we wrote/want, is to determine the entropy change of the universe, deltaSuniv.
The equation is deltaSuniv = deltaSosys + {(-deltaHosys)/Tsys)}
So you can start picking different temperatures (Tsys) and calculating what the deltaSuniv will be for that temperature. If you do this several time and pick the correct temperatures, you can see where the deltaSuniv does from positive to negative or visa versa. Whichever way, it is when
deltaSuniv=0 is where the system is at Equilibrium
This physically means that the system is going through a phase change and that system temperature is Temperature of Phase Change. For example, the boiling (condensation)point, melting freezing)point, etc.
Example : H2O(l)==> H2O(g)