CBSE Class 11-science Answered

Inside an isolated system, in order for a process to proceed, Δ S > 0. Such a process is said to be spontaneous. A process for which ΔS < 0 is called non-spontaneous and is impossible for an isolated system.

1. Freezing of water is exothermic (?Hsys < 0) but it is not spontaneous at T > 0 °C.

2. Melting of ice is endothermic (?Hsys > 0) but it is spontaneous at T > 0 °C.

3. Gas expanding to vacuum under isothermal conditions does  not change gas’ internal energy (?Hsys = 0) but it is  spontaneous.

Conclusion: The thermodynamic function of enthalpy, H, does not define the spontaneity of a process. The first two examples show that spontaneity depends on the ability of  heat exchange at different temperatures of  surroundings. Spontaneity depends on the Ability of Heat Exchange between the System and Surroundings.