Editing Spontaneous process (section)

==Using entropy to determine spontaneity==
When using the entropy change of a process to assess spontaneity, it is important to carefully consider the definition of the system and surroundings. The [[second law of thermodynamics]] states that a process involving an isolated system will be spontaneous if the entropy of the system increases over time. For open or closed systems, however, the statement must be modified to say that the total entropy of the ''combined'' system and surroundings must increase, or,
<math display="block">\Delta S_\text{total} = \Delta S_\text{system} + \Delta S_\text{surroundings} \ge 0 \,.</math>

This criterion can then be used to explain how it is possible for the entropy of an open or closed system to decrease during a spontaneous process. A decrease in system entropy can only occur spontaneously if the entropy change of the surroundings is both positive in sign and has a larger magnitude than the entropy change of the system:
<math display="block">\Delta S_\text{surroundings} > 0 </math>
and
<math display="block">\left|\Delta S_\text{surroundings}\right| > \left|\Delta S_\text{system}\right| </math>

In many processes, the increase in entropy of the surroundings is accomplished via heat transfer from the system to the surroundings (i.e. an exothermic process).