Final answer:
The criterion of equilibrium at constant temperature and volume is that ∆G (Delta G) must equal zero, indicating that the system is in a state where there is no net change in Gibbs free energy.
Step-by-step explanation:
The criterion of equilibrium at constant temperature and volume is ∆G = 0 (Delta G equals zero). This condition for equilibrium corresponds to the Gibbs free energy of the system, which combines enthalpy (∆H), entropy (∆S), and temperature (T) in the relationship G = H - TS. When ∆G is zero, the system is at equilibrium and there is no net change in the free energy; reactions are reversible, and there is no tendency for the system to change state. This is a foundational concept in thermodynamics and is directly connected to the prediction of whether a process will occur spontaneously at constant temperature and volume.