Characteristics of Free energy 'G'
i) G is defined as (H-TS) where H and S are the enthalpy and entropy of the system respectively. T = temperature. Since H and S are state functions, G is a state function.
ii) G is an extensive property while DG = (G2-G1) which is the free energy change between the initial (1) and final (2) states of the system becomes the intensive property when mass remains constant between initial and final states (or) when the system is a closed system.
iii) G has a single value for the thermodynamic state of the system.
iv) G and DG values correspond to the system only. There are three cases of DG in predicting the nature of the process. When, DG<0 (negative), the process is spontaneous and feasible; DG = 0. The process is in equilibrium and DG > 0 (positive), the process is nonspontaneous and not feasible.
v) DG = DH - TDS. But according to I law of thermodynamics,
DH = DE + PDV and DE = q - w.
∴ DG = q - w + PDV - TDS
But DS = q/T and TDS = q = heat involved in the process.
∴ DG = q - w + PDV - q = -w + PDV
(or) -DG = w - PDV = network.
The decrease in free energy -