ABSTRACT
A mug of hot coffee will cool to room temperature. Gas in a popped balloon will escape. Carbonated drinks, if left open to the air, will become flat. This tendency for matter and energy to disperse and spread out is what is called entropy (S). The second law of thermodynamics defines the natural direction, entropy increases, of a process: “It is impossible for a room temperature object, on its own, to become hotter (or to become colder).” With added heat or work, the entropy of a system can decrease, which is how heat pumps and refrigeration work. In this chapter, readers will be introduced to the concept of bidirectionality in reactions (reactants can convert to products and vice versa) and to the quantification of changes in entropy. Entropy, along with enthalpy, then provides a means of determining whether a reaction is favored in the forward direction or in the reverse direction.
