Chapter 20: Spontaneous Change: Entropy and Free Energy Introduction

There is nothing harder than keeping one's room (or desk) neat and orderly. Mess just seems to happen spontaneously. The tendency for systems to become disordered is an aspect of the second law of thermodynamics. We say that the entropy of the system has increased.

When we apply this to chemical systems, we find that some reactions tend to proceed in a direction that increases the entropy of the system. This is true, for example, in combustion, where solid compounds react to form gaseous compounds. Gases have a much higher entropy than solids.

If entropy were the only important factor, how could we understand the formation of precipitates? Precipitation is clearly a reaction from higher entropy to lower entropy situations ... or is it?

In this chapter, we will explore both the second and third laws of thermodynamics. We will examine the concept of entropy and its role in determining the spontaneity of reactions. We will also define a new energy term, the free energy change. This concept relates enthalpy change (H) and entropy change (S) to help us understand under what conditions a given reaction will occur spontaneously.

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