Entropy and Gibbs Free Energy (HL)

Whether a reaction happens spontaneously is not determined by enthalpy alone. Some endothermic processes, such as the dissolving of ammonium nitrate or the melting of ice above $0\,^\circ\text{C}$, occur readily even though they absorb heat. To predict spontaneity we need a second quantity, entropy, symbolized $S$, which measures the dispersal of energy and matter, often described loosely as the degree of disorder in a system. A system with many accessible microscopic arrangements has high entropy; a highly ordered system has low entropy.

Entropy increases when matter becomes more spread out or more randomly arranged. Gases have much higher entropy than liquids, which in turn have higher entropy than solids, because particles in a gas move freely through a large volume. Consequently, entro