Entropy

A spontaneous change is one that occurs within a system by itself without a continous input of energy from outside the system. The melting of ice at 1 atm and temperatures above 0^oC is an example of a spontaneous reaction. Non-spontaneous systems require the addition of energy. For example, a book may fall of a table spontaneously but energy must be applied to lift it back onto the table. It follows then, that if a process is spontaneous in one direction it is not spontaneous in the reverse direction.

An implication of this, the 2nd law of thermodynamics, is that systems become disordered spontaneously all around you. Entropy is the chemists measure of the amount of disorder in a system. If you have children (or an office), you are quite familiar with entropy.  Without intervention, your child's room and/or your office will increase in entropy (i.e., get messier) if no work (cleaning up) is done to contain the disorder. Work must be done to keep the entropy of the system low.  All spontaneous changes in an isolated chemical system result in an increase in entropy.  Entropy, like temperature, pressure, and enthalphy, is a state function. It is represented in the literature by the symbol "S".  Entropy increases as a substance changes state from a solid to a liquid to a gas, that is, as temperature rises, so does entropy.