Spontaneity, Randomness & Nature of driving force





Spontaneity and Randomness

Careful examination shows that in each of the processes E. g. melting of ice and evaporation of rain water, there is an increase in randomness or disorder of the system. The water molecules in ice are arranged in a highly organised crystal pattern which permits little movement. As the ice melts, the water molecules become disorganised and can move more freely.

The movement of molecules becomes free still when the water evaporates into space as now they can roam about throughout the entire atmosphere. In other words, it may be said that randomness of the water molecules increases as ice melts into water or water evaporates into vapour.

 

Nature of driving force

For many years scientists believed that only exothermic changes resulting in a lowering of internal energy or enthalpy could occur spontaneously. But melting of ice is an endothermic process and yet occurs spontaneously. On a warm day, ice melts by itself. The evaporation of water is another example of spontaneous endothermic process.

Thus arose the need of finding another driving force that affects the spontaneity. This was known as the entropy change, \Delta S Also the increase in randomness favours a spontaneous change.



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