Entropy
Definition and meaning of Entropy in chemistry.
Entropy is a thermodynamic state function, symbol S, that measures the number of ways energy and matter can be arranged among a system's microstates, informally, a measure of disorder or dispersal.
In more detail
Statistically, entropy is defined by Boltzmann's equation, S = k_B ln W, where W is the number of accessible microstates for a given macrostate; more microstates means higher entropy. The second law of thermodynamics states that the total entropy of an isolated system (system plus surroundings) never decreases in a spontaneous process, which is why heat flows from hot to cold and gases expand to fill available volume. A system's own entropy can decrease during a process, but only if the surroundings' entropy increases by at least as much, keeping the universe's total entropy constant or rising. Entropy changes, combined with enthalpy changes, determine reaction spontaneity through the Gibbs free energy equation, ΔG = ΔH − TΔS.
Key facts
| Symbol | S |
|---|---|
| SI Unit | joules per kelvin (J/K) |
| Key equation | S = k_B ln W (Boltzmann); ΔS = q_rev/T (Clausius) |
| Field | Physical Chemistry |
When ice melts at 0 °C, water molecules go from a rigid, ordered crystal lattice to a more randomly arranged liquid, so the entropy of the H2O increases (ΔS > 0), even though the process absorbs heat.
Frequently asked questions
Does entropy always increase?
The total entropy of an isolated system (or the universe as a whole) increases for spontaneous processes, per the second law of thermodynamics. A localized system's entropy can decrease, but only if a compensating, larger increase occurs in its surroundings.
What is standard molar entropy?
Standard molar entropy, S°, is the entropy of one mole of a substance in its standard state at a specified temperature (usually 298 K), tabulated in J/(mol·K) and used to calculate ΔS° for reactions.