Let us suppose we have a suit of playing cards arranged numerically (Fig. Increase in Randomness Favors a Spontaneous ChangeĪ change that brings about randomness is more likely to occur than one that brings about order. In other words, we can say that the randomness of the water molecules increases as ice melts into water or water evaporates into space. The movement of molecules becomes freer still when the water evaporates into space as now they can roam about throughout the entire atmosphere. As the ice melts, the water molecules become disorganized and can move more freely. The water molecules in ice are arranged in a highly organized crystal pattern which permits little movement. Spontaneity and RandomnessĬareful examination shows that in each of the processes viz., melting of ice and evaporation of water, there is an increase in randomness or disorder of the system. This was known as the entropy change, Δ S. Thus arose the need of inventing another driving force that affects the spontaneity. The evaporation of water is another example of a spontaneous endothermic process. It is the second law of thermodynamics which introduces this new factor that is called entropy. But the melting of ice and evaporation of rain water are endothermic processes which proceed spontaneously.Ĭlearly, there is some other factor in addition to ΔH which governs spontaneity. It implies that only such reactions will occur which are exothermic. (5) A spontaneous change is accompanied by decrease of internal energy or enthalpy (ΔH). To take the system away from equilibrium, some external work must be done on the system (4) Once a system is in equilibrium state, it does not undergo any further spontaneous change in state if left undisturbed. The change will continue till the system attains the state of equilibrium. (3) If the system is not in equilibrium state (unstable), a spontaneous change is inevitable. (2) For a spontaneous change to occur, time is no factor.Ī spontaneous reaction may take place rapidly or very slowly. (1) A spontaneous change is one-way or unidirectional.įor reverse change to occur, work has to be done. Some important criteria of spontaneous physical and chemical changes are listed below.