Absolute Entropy and Entropy Change

Entropy tracks the number of microstates available to a system, the ways energy and position can be spread among particles. More microstates means more disorder in the useful sense: more ways to arrange the same energy. A gas molecule can move in three dimensions with huge freedom, a liquid molecule has less room to roam, and a solid molecule barely moves beyond vibrating in place. So gases almost always carry far more entropy than liquids or solids of similar size, and a reaction that increases moles of gas almost always has a positive delta S.

To predict the sign of delta S deg, compare the physical states and particle counts on both sides of the equation. Going from solid to liquid to gas raises entropy. Increasing the number of gas molecules raises entropy, even if mass is conserved, because more separate particles means more ways to arrange them. Dissolving a solid into ions in solution usually raises entropy too. For an exact value, use delta S deg = sum of S deg(products) - sum of S deg(reactants), each multiplied by its coefficient, using tabulated standard molar entropies in J/(mol K).

This sits right next to Gibbs free energy, delta G = delta H - T delta S, which combines this entropy change with the enthalpy change to decide whether a reaction is spontaneous. A reaction can have unfavorable delta H and still run forward if delta S is positive enough, especially at high temperature. That same free energy will reappear measured in volts once the unit turns to electrochemistry, so getting delta S right here sets up everything that follows.