At equilibrium under constant temperature and pressure, what is the value of dG?

At equilibrium, the change in Gibbs free energy (dG) is equal to zero. This is a fundamental principle in thermodynamics. When a system is at equilibrium, it indicates that there is no net change occurring; the forward and reverse reactions are happening at equal rates, leading to a stable mixture of reactants and products.

The Gibbs free energy change (dG) serves as an indicator of spontaneity of a process. When dG is negative, the process can occur spontaneously. When dG is positive, the process is non-spontaneous. At equilibrium, dG reaches a point of zero, indicating that the system is in a stable state and no driving force exists to shift the reaction in either direction.

While the other options mention concepts related to free energy, enthalpy, entropy, and work, they do not directly capture the specific condition at equilibrium, which is characterized by dG being equal to zero. Understanding this concept is key to analyzing phase changes, chemical reactions, and other processes in physical chemistry.