Gibb's free energy (∆G) and Equilibrium constant (Kc):

The part f energy which is converted into usefull work called Gibb's free energy or Gibb's function.

 

Energy (H) = Useful work (G) + Non useful (TS)

We van not calculate absolute value of 'G' so we calculate change in Gibb's free energy. 

             ∆G = ∆H -∆TS

       ∆G = ∆H - (∆TS + T∆S). ......(1)

Standard Gibb's energy (∆G°) change at  standard condition is 1 bar  and 298 K

             ∆G° = ∆H° -∆TS°  .......(2) 

Relation between ∆G° and Equilibrium constant (K):

(2) For the equilibrium, PCl 5​ (g)⇌PCl 3​ (g)+Cl 2​ (g) at 298K, K=1.8×10 −7 .Calculate ΔG 0 for the reaction (R=8.314JK −1 mol −1 ).

(2) Equilibrium constant for the reaction given below is 2.0 × 10^-7 at 300 K. calculate standard Gibb's free energy change for the reaction PCl5 (g) --=--PCl3 (g) + Cl2 (g) also calculate standard entropy change if ∆H° = 28.40 Kj mole-1.