Why entropy change is not criterion for spontaniety?
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Decrease in enthalpy may be a contributary factor for spontaneity but it is not true for all cases.For example-melting of ice ,evaporation of rain water etc are endothermic reactions but proceed spontaneously.
Thus, there is another concept for the criteria of spontaneity and this is the concept of Gibbs free energy at constant temperature and pressure.
Gibbs free energy is the maximum amount of non expansion work that can be extracted from a thermodynamically closed system.G is related to enthalpy change ,the entropy change and temperature of the system.
ΔG = ΔH - T ΔS
So,if ΔG <0,reaction is spontaneous.
ΔG>0,reaction is non spontaneous.
ΔG=0,reaction is in equilibrium.
Thus, there is another concept for the criteria of spontaneity and this is the concept of Gibbs free energy at constant temperature and pressure.
Gibbs free energy is the maximum amount of non expansion work that can be extracted from a thermodynamically closed system.G is related to enthalpy change ,the entropy change and temperature of the system.
ΔG = ΔH - T ΔS
So,if ΔG <0,reaction is spontaneous.
ΔG>0,reaction is non spontaneous.
ΔG=0,reaction is in equilibrium.
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