Define Gibb's free energy .

Explanation

Characteristics of Gibbs free energy:
(i) Gibbs free energy is defined as the part of total energy of a system that can be converted (or) available for conversion into work .
G=H-TS ….(1)
Where H =enthalpy, T = temperature and S =entropy
(ii) G is a state function and is a single value function.
(iii)G is an extensive property, whereas `DeltaG` becomes intensive property for a closed system. Both G and `DeltaG` values correspond to the system only.
(iv) `DeltaG` gives a criteria for spontaneity at constant pressure and temperature.
If `DeltaG` is negative (`DeltaG lt0`), the process is spontaneous.
If `DeltaG` is positive (`DeltaG > 0`), the process is non-spontaneous
If `DeltaG` is zero (`DeltaG` ), the process is equilibrium.
(v) For any system at constant pressure and temperature,
`DeltaG=DeltaH-TDeltaS` ...(2)
We know `DeltaH=DeltaU+PDeltaV`
`therefore DeltaG=DeltaU+PDeltaV-TDeltaS`...(3)
(iv)For the first law of thermodynamics, `DeltaU=q+w`
`DeltaG=q+w+PDeltaV-TDeltaS`...(4)
For second law of thermodyanmics, `DeltaS=q/T`
`therefore DeltaG=q+w+PDeltaV-T(q/T)`
`therefore DeltaG=w+PDeltaV` ...(5)
`-DeltaG=-w-PDeltaV`....(6)
(vii) `-PDeltaV` represent the work done due to expansion against a constant external pressure.Therefore, it is clear that the decrease in free energy`(-DeltaG)` accompanying a process taking place at constant temperature and pressure is equal to the maximum work obtainable from the system other than the work of expansion.
(viii) Unit of Gibb.s free energy is `"J mol"^(-1)`