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.

**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 -