In thermodynamics, understanding entropy and spontaneity is crucial for determining whether a chemical process will occur naturally. This section begins with a definition of spontaneity, emphasizing that spontaneous processes occur without external input, even if the reaction rate is slow. Spontaneity is not strictly determined by energy changes since reactions can be spontaneous even when they absorb heat (endothermic).
The text highlights that entropy (S) signifies disorder in a system. For a spontaneous process, the total change in entropy (∆S) for a system and its surroundings increases. This is articulated through the Second Law of Thermodynamics, which asserts that the entropy of an isolated system tends to increase over time.
The relationship between enthalpy (H) and spontaneity is discussed; while spontaneous reactions often feature a decrease in enthalpy, it is not an exclusive criterion — some endothermic reactions are spontaneous due to significant increases in entropy. Additionally, Gibbs free energy (G), defined as G = H - TS (where T is temperature), serves as a useful measure for predicting spontaneity. A negative Gibbs free energy change (∆G < 0) indicates that a process can proceed spontaneously. Finally, the section touches on the third law of thermodynamics, asserting that the entropy of a perfect crystal approaches zero as temperature approaches absolute zero.