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Free energy, G=HTΔS, is a state function that indicates whether a reaction is spontaneous or non-spontaneous. If you think of TS as the part of the system`s energy that is disordered already, then (HTS) is the part of the system`s energy that is still ordered and therefore free to cause spontaneous change by becoming disordered.
Also, ΔG=ΔHTΔS
From the second law of Thermodynamics, a reaction is spontaneous if ΔtotalS is positive, non-spontaneous if ΔtotalS is negative, and at equilibrium if ΔtotalS is zero.
Since, TΔS=ΔG and since ΔG and ΔS have opposite signs, we can restate the Thermodynamic criterion for the spontaneity of a reaction carried out at a constant temperature and pressure.
If ΔG<0, the reaction is spontaneous
If ΔG>0, the reaction is non-spontaneous
If ΔG=0, the reaction is at equilibrium
Read the above paragraph carefully and answer the following questions based on the above comprehension.
If an endothermic reaction is non-spontaneous at freezing point of water and becomes feasible at its boiling point, then:

A
ΔH and ΔS both are (+ve)
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B
ΔH is (ve) and ΔS is (+ve)
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C
ΔH and ΔS both are (ve)
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D
ΔH is +ve and ΔS is (ve)
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Solution

The correct option is C ΔH and ΔS both are (+ve)
For endothermic reaction, ΔH=+ve as heat is absorbed during the reaction. As ΔG=ΔHTΔS, is +ve at low temperature if ΔS is positive but TΔS<ΔH.
At high temperature, TΔS>ΔH and ΔG become ve.
Thus at low temperature (freezing point), the process is non spontaneous (free energy change is positive) and at high temperature (boiling point), the process is spontaneous (free energy change is negative).

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