Spontaneous Processes and Entropy?

Water in the waterfall flows spontaneously to a lower elevation, where the potential energy is lower. Water cannot run uphill without an input of energy, therefore it is not spontaneous - this cannot occur on its own naturally. Water freezing and melting on its own is a spontaneous process because energy was neither used or taken away. Other examples include iron rusting when exposed to air and water.

In conclusion:

1) Spontaneous?: naturally occurs under a given set of conditions.
2) Nonspontaneous: does not occur under a given set of conditions.
If a process is spontaneous in the forward direction, then the reverse process will be
nonspontaneous.

What makes a process spontaneous?

• Tendency for a system to go to a lower state of energy.
Many spontaneous processes are exothermic (i.e. combustion reactions have deltaH < 0),
but some spontaneous processes are endothermic (i.e. ice melting has deltaH > 0).
• Tendency for a system to become more disordered or random.
Entropy of a system typically increases, deltaS > 0 (+deltaS: increase in disorder)

Thus, spontaneity is a balance between enthalpy and entropy.