Question

state and explain Gibbs free energy

Answer 1

Definition
Gibbs free energy is a measure of the potential for reversible or maximum work that may be done by a system at constant temperature and pressure. It is a thermodynamic property that was defined in 1876 by Josiah Willard Gibbs to predict whether a process will occur spontaneously at constant temperature and pressure. Gibbs free energy G is defined as G = H - TS where H, T, and S are the enthalpy, temperature, and entropy.
The SI unit for Gibbs energy is the kilojoule (kJ).

Changes in the Gibbs free energy G correspond to changes in free energy for processes at constant temperature and pressure. The change in Gibbs free energy change is the maximum non-expansion work obtainable under these conditions in a closed system. ΔG is negative for spontaneous processes, positive for nonspontaneous processes and zero for processes at equilibrium.