Potential energy and mechanical processes

In purely mechanical systems, a basic law governs behavior. The state of equilibrium is the position of minimum potential energy, and everything that happens is an approach to this state. For example, a book falling from a height to a table represents an approach to a minimum of potential energy. 

All physical processes and chemical reactions are an approach to some equilibrium state. The question arises Does this same principle of minimum potential energy apply to chemical reactions or physical processes  

However, some reactions occur spontaneously with an absorption of heat—that is, an increase of enthalpy. An example of such a reaction that absorbs a large amount of heat and therefore must go to a higher enthalpy state is given at the end of this chapter. In such endothermic reactions, we can presume that the potential energy increases. It can therefore be concluded that at least one other factor needs to be considered. 

We thus see that two basic drives cause a process/reaction to proceed toward equilibrium  

The question then arises as to what happens in a real situation when enthalpy and entropy are forced to compromise. 

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