Spontaneity and entropy

A spontaneous process can proceed on its own without outside help once started; spontaneity says nothing about how fast it happens.

A negative ΔH favours spontaneity but does not guarantee it: some endothermic processes, such as dissolving certain salts, are spontaneous.

Entropy S measures the degree of randomness or disorder of a system; it is a state function.

Entropy increases from solid to liquid to gas, on dissolving a solid, and generally when a reaction produces more gas molecules.

For a reversible transfer of heat q_rev at temperature T, ΔS = q_rev/T; the same amount of heat raises entropy more at low temperature than at high temperature.

A process is spontaneous when the total entropy change is positive: ΔS_total = ΔS_sys + ΔS_surr > 0.

At equilibrium ΔS_total = 0.

For an isolated system ΔU = 0, and entropy is the driving force: ΔS > 0 for a spontaneous change.

Third law: as the temperature approaches 0 K, the entropy of a pure, perfectly crystalline substance approaches zero. Solutions and supercooled liquids keep some entropy at 0 K. The law lets absolute entropies of pure substances be worked out from thermal data.