Key Terms

entropy (S)

state function that is a measure of the matter and/or energy dispersal within a system, determined by the number of system microstates often described as a measure of the disorder of the system

Gibbs free energy change (G)

thermodynamic property defined in terms of system enthalpy and entropy; all spontaneous processes involve a decrease in G

microstate (W)

possible configuration or arrangement of matter and energy within a system

nonspontaneous process

process that requires continual input of energy from an external source

reversible process

process that takes place so slowly as to be capable of reversing direction in response to an infinitesimally small change in conditions; hypothetical construct that can only be approximated by real processes removed

second law of thermodynamics

entropy of the universe increases for a spontaneous process

spontaneous change

process that takes place without a continuous input of energy from an external source

standard entropy (S°)

entropy for a substance at 1 bar pressure; tabulated values are usually determined at 298.15 K and denoted $S_{298}^{°}$

standard entropy change (ΔS°)

change in entropy for a reaction calculated using the standard entropies, usually at room temperature and denoted $\text{Δ}{S}_{298}^{°}$

standard free energy change (ΔG°)

change in free energy for a process occurring under standard conditions (1 bar pressure for gases, 1 M concentration for solutions)

standard free energy of formation $(\text{Δ}{G}_{\text{f}}^{°})$

change in free energy accompanying the formation of one mole of substance from its elements in their standard states

third law of thermodynamics

entropy of a perfect crystal at absolute zero (0 K) is zero