Criteria of Spontaneity





Some important criteria of spontaneous processes are as follows:

(i)     A spontaneous change is one way or unidirectional. For reverse change to occur, work has to be done.

(ii)     For a spontaneous change to occur, time is no factor. A spontaneous reaction may take place rapidly or very slowly.

(iii)    If the system is not in equilibrium state, a spontaneous change is inevitable. The change will continue till the system attains the state of equilibrium.

(iv)      Once a system is in equilibrium state, it does not undergo any further spontaneous change in state if left undisturbed. To take the system away from equilibrium, some external work must be done on the system.

A spontaneous change is accompanied by decrease of internal energy or enthalpy (\Delta H). It implises that not only such reactions will occur which are exothermic, but the melting of ice and evaporation of rain water are endothermic processes which proceed spontaneously.

Clearly, there is some other factor in addition to AH which governs spontaneity. It is the second law of thermodynamics which introduces the new factor which is called Entropy.



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